Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss
PT1000 circuit
477 views
Skip to first unread message
jmariano
May 13, 2014, 5:41:00 PM5/13/14
to
Hi all,
Need your advice....
I'm building a prototype of a bench-top instrument. I need to measure absolute temperature using a RTD pt1000 sensor with short lead wires (<50 cm). I intend to use two wires and I want to go all analog, from sensor to the 12 bit ADC - a kind of analog design exercise, so no all-integrated solutions loke ADuC834 or ADS1247/8.
The temp range is -20 C to 150 C and my initial target was a overall precision of +/-0.1 C, but after reading some posts in this group I'll settle for the best I can get.
As far as I know, the 3 most common (DC) ways to signal-conditioning a pt100(0) are simple voltage divider, constant current driving and bridge. For my needs, it seems like a voltage divider, eventually polarized from a precision reference, and a buffer amplifier will do the trick, but i'm not sure i'm not missing something.
Would someone care to elaborate on this.
Best regards
José Mariano
============
Physics Department
University of the Algarve
Portugal
Need your advice....
I'm building a prototype of a bench-top instrument. I need to measure absolute temperature using a RTD pt1000 sensor with short lead wires (<50 cm). I intend to use two wires and I want to go all analog, from sensor to the 12 bit ADC - a kind of analog design exercise, so no all-integrated solutions loke ADuC834 or ADS1247/8.
The temp range is -20 C to 150 C and my initial target was a overall precision of +/-0.1 C, but after reading some posts in this group I'll settle for the best I can get.
As far as I know, the 3 most common (DC) ways to signal-conditioning a pt100(0) are simple voltage divider, constant current driving and bridge. For my needs, it seems like a voltage divider, eventually polarized from a precision reference, and a buffer amplifier will do the trick, but i'm not sure i'm not missing something.
Would someone care to elaborate on this.
Best regards
José Mariano
============
Physics Department
University of the Algarve
Portugal
Spehro Pefhany
May 13, 2014, 6:37:11 PM5/13/14
to
Put a series resistor from your sensor to a positive reference.
Something like 250uA is appropriate.
Build a positive gain stage with appropriate output for your ADC (zero
and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
the output fed back to the non-inverting input of the op-amp. Select
theat resistor (should be hunreds of K ohms) so that the output
voltage is half-scale at the resistance corresponding to 65�C
(mid-scale)
The resistor linearizes the non-linear response of the sensor, which
is slightly more than a constant current.
The resistor values interact, so it's a PITA to change ranges, but
this is all do-able with ordinary parts from an online supplier like
Digikey.
>Best regards
>
>Jos� Mariano
>============
>Physics Department
>University of the Algarve
>Portugal
Algarve eh? I'd like to pop over and show you on a whiteboard.
>Physics Department
>University of the Algarve
>Portugal
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
jmariano
May 14, 2014, 7:45:17 AM5/14/14
to
Hi Spehro
Thanks for your input.
>
> You can build a signal conditioning stage with a single op-amp.
> Put a series resistor from your sensor to a positive reference.
> Something like 250uA is appropriate.
You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
> You can build a signal conditioning stage with a single op-amp.
> Put a series resistor from your sensor to a positive reference.
> Something like 250uA is appropriate.
On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>
>
> Build a positive gain stage with appropriate output for your ADC (zero
> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
>
> Build a positive gain stage with appropriate output for your ADC (zero
> the output fed back to the non-inverting input of the op-amp. Select
> theat resistor (should be hunreds of K ohms) so that the output
> voltage is half-scale at the resistance corresponding to 65�C
> theat resistor (should be hunreds of K ohms) so that the output
> (mid-scale)
> The resistor linearizes the non-linear response of the sensor, which
> is slightly more than a constant current.
> The resistor values interact, so it's a PITA to change ranges, but
> this is all do-able with ordinary parts from an online supplier like
> Digikey.
Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
> The resistor linearizes the non-linear response of the sensor, which
> is slightly more than a constant current.
> The resistor values interact, so it's a PITA to change ranges, but
> this is all do-able with ordinary parts from an online supplier like
> Digikey.
> Algarve eh? I'd like to pop over and show you on a whiteboard.
Regards
Lasse Langwadt Christensen
May 14, 2014, 8:26:23 AM5/14/14
to
if you drive it with a voltage source and a resistor your voltage is no longer
linear with resistance, you can of course fix it in software but you'll need more bits in the ADC
-Lasse
Phil Hobbs
May 14, 2014, 9:09:58 AM5/14/14
to
On 5/14/2014 8:26 AM, Lasse Langwadt Christensen wrote:
> Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
>> Hi Spehro
>>
>>
>>
>> Thanks for your input.
>>
>>>
>>
>>> You can build a signal conditioning stage with a single op-amp.
>>
>>> Put a series resistor from your sensor to a positive reference.
>>
>>> Something like 250uA is appropriate.
>>
>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>>
>> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>>
>>
>>>
>>
>>> Build a positive gain stage with appropriate output for your ADC (zero
>>
>>> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
> Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
>> Hi Spehro
>>
>>
>>
>> Thanks for your input.
>>
>>>
>>
>>> You can build a signal conditioning stage with a single op-amp.
>>
>>> Put a series resistor from your sensor to a positive reference.
>>
>>> Something like 250uA is appropriate.
>>
>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>>
>> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>>
>>
>>>
>>
>>> Build a positive gain stage with appropriate output for your ADC (zero
>>
>>
>>> the output fed back to the non-inverting input of the op-amp. Select
>>
>>> theat resistor (should be hunreds of K ohms) so that the output
>>
>>> voltage is half-scale at the resistance corresponding to 65�C
>>> the output fed back to the non-inverting input of the op-amp. Select
>>
>>> theat resistor (should be hunreds of K ohms) so that the output
>>
>>
>>> (mid-scale)
>>
>>> The resistor linearizes the non-linear response of the sensor, which
>>
>>> is slightly more than a constant current.
>>
>>> The resistor values interact, so it's a PITA to change ranges, but
>>
>>> this is all do-able with ordinary parts from an online supplier like
>>
>>> Digikey.
>>
>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>>
>>
>>
>>> Algarve eh? I'd like to pop over and show you on a whiteboard.
>>
>> Please do that, you'll be very welcome here. It will be a pressure to talk to a fellow engineer. No such a thing were, only golf courses, beautiful beaches and girls in bikinis. Very boring.... :-)
>>
>>
>>
>> Regards
>
> if you drive it with a current source you get a voltage that is linear with resistance thus (almost) linear with temperature
>
> if you drive it with a voltage source and a resistor your voltage is no longer
> linear with resistance, you can of course fix it in software but you'll need more bits in the ADC
>
>
> -Lasse
>
You can make it really linear by using a negative resistance, iirc about
>>> (mid-scale)
>>
>>> The resistor linearizes the non-linear response of the sensor, which
>>
>>> is slightly more than a constant current.
>>
>>> The resistor values interact, so it's a PITA to change ranges, but
>>
>>> this is all do-able with ordinary parts from an online supplier like
>>
>>> Digikey.
>>
>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>>
>>
>>
>>> Algarve eh? I'd like to pop over and show you on a whiteboard.
>>
>> Please do that, you'll be very welcome here. It will be a pressure to talk to a fellow engineer. No such a thing were, only golf courses, beautiful beaches and girls in bikinis. Very boring.... :-)
>>
>>
>>
>> Regards
>
> if you drive it with a current source you get a voltage that is linear with resistance thus (almost) linear with temperature
>
> if you drive it with a voltage source and a resistor your voltage is no longer
> linear with resistance, you can of course fix it in software but you'll need more bits in the ADC
>
>
> -Lasse
>
-25 times the 0 C resistance of the sensor for -150C to +500C. (The
exact optimum depends a bit on the range you want it to operate over,
but it's easy to find the right number.)
Also make sure to use a reference, resistors, and ADC with a low enough
tempco--the RTD only moves 3500 ppm/K, so if your resistors are 200 ppm,
you'll be pretty sensitive to the temperature of the instrument.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Tauno Voipio
May 14, 2014, 10:39:25 AM5/14/14
to
On 14.5.14 14:45, jmariano wrote:
>
> Hi Spehro
>
> Thanks for your input.
>>
>> You can build a signal conditioning stage with a single op-amp.
>> Put a series resistor from your sensor to a positive reference.
>> Something like 250uA is appropriate.
> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>
>> Build a positive gain stage with appropriate output for your ADC (zero
>> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
>
> Hi Spehro
>
> Thanks for your input.
>>
>> You can build a signal conditioning stage with a single op-amp.
>> Put a series resistor from your sensor to a positive reference.
>> Something like 250uA is appropriate.
> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>
>> Build a positive gain stage with appropriate output for your ADC (zero
>> the output fed back to the non-inverting input of the op-amp. Select
>> theat resistor (should be hunreds of K ohms) so that the output
>> voltage is half-scale at the resistance corresponding to 65�C
>> theat resistor (should be hunreds of K ohms) so that the output
>> (mid-scale)
>> The resistor linearizes the non-linear response of the sensor, which
>> is slightly more than a constant current.
>> The resistor values interact, so it's a PITA to change ranges, but
>> this is all do-able with ordinary parts from an online supplier like
>> Digikey.
> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>
>> Algarve eh? I'd like to pop over and show you on a whiteboard.
> Please do that, you'll be very welcome here. It will be a pressure to talk to a fellow engineer. No such a thing were, only golf courses, beautiful beaches and girls in bikinis. Very boring.... :-)
>
> Regards
>
Have a look at you gmail mailbox.
>> The resistor linearizes the non-linear response of the sensor, which
>> is slightly more than a constant current.
>> The resistor values interact, so it's a PITA to change ranges, but
>> this is all do-able with ordinary parts from an online supplier like
>> Digikey.
> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>
>> Algarve eh? I'd like to pop over and show you on a whiteboard.
> Please do that, you'll be very welcome here. It will be a pressure to talk to a fellow engineer. No such a thing were, only golf courses, beautiful beaches and girls in bikinis. Very boring.... :-)
>
> Regards
>
--
-TV
Spehro Pefhany
May 14, 2014, 11:05:19 AM5/14/14
to
On Wed, 14 May 2014 05:26:23 -0700 (PDT), Lasse Langwadt Christensen
<lang...@fonz.dk> wrote:
>Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
>> Hi Spehro
>>
>>
>>
>> Thanks for your input.
>>
>> >
>>
>> > You can build a signal conditioning stage with a single op-amp.
>>
>> > Put a series resistor from your sensor to a positive reference.
>>
>> > Something like 250uA is appropriate.
>>
>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
One way uses a few more parts (a current source and a separate
<lang...@fonz.dk> wrote:
>Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
>> Hi Spehro
>>
>>
>>
>> Thanks for your input.
>>
>> >
>>
>> > You can build a signal conditioning stage with a single op-amp.
>>
>> > Put a series resistor from your sensor to a positive reference.
>>
>> > Something like 250uA is appropriate.
>>
>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
amplifier) but is a bit simpler to calculate and will have fewer weird
values of resistors. So you have a current source (transistor, op-amp,
reference, say) and an amplifier.
The other way uses only one op-amp and feeds back the output to
maintain either a constant current, or a bit more than that, which
linearizes an RTD pretty well, and you can probably avoid the
equations to the accuracy of the RTD.
>> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
has another, so any drift in one relative to the other results in
errors. I like converting the reference to a current and bouncing it
off the positive rail with a precision current mirror, which is a dual
op-amp, two precision resistors (can be the same value or a matched
pair) and two transistors. More parts, but it can be very precise.
I would not use an LM334.
>> >
>>
>> >
>>
>> > Build a positive gain stage with appropriate output for your ADC (zero
>>
>>
>> > the output fed back to the non-inverting input of the op-amp. Select
>>
>> > theat resistor (should be hunreds of K ohms) so that the output
>>
>> > voltage is half-scale at the resistance corresponding to 65�C
>> > the output fed back to the non-inverting input of the op-amp. Select
>>
>> > theat resistor (should be hunreds of K ohms) so that the output
>>
>>
>> > (mid-scale)
>>
>> > The resistor linearizes the non-linear response of the sensor, which
>>
>> > is slightly more than a constant current.
>>
>> > The resistor values interact, so it's a PITA to change ranges, but
>>
>> > this is all do-able with ordinary parts from an online supplier like
>>
>> > Digikey.
>>
>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
If you want to use the equations you can use a constant current. The
>> > (mid-scale)
>>
>> > The resistor linearizes the non-linear response of the sensor, which
>>
>> > is slightly more than a constant current.
>>
>> > The resistor values interact, so it's a PITA to change ranges, but
>>
>> > this is all do-able with ordinary parts from an online supplier like
>>
>> > Digikey.
>>
>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
analog linearization was common back in the day when the equations
were difficult to do in a microcontroller. The RTD is almost linear
itself when excited with a constant current so there is no appreciable
downside to doing the linearization in the micro (no loss of
resolution as there would be with a really nonlinear sensor such as a
thermistor over a wide temperature range).
John Larkin
May 14, 2014, 11:02:59 AM5/14/14
to
On Wed, 14 May 2014 04:45:17 -0700 (PDT), jmariano <jmari...@gmail.com> wrote:
>
>Hi Spehro
>
>Thanks for your input.
>>
>> You can build a signal conditioning stage with a single op-amp.
>> Put a series resistor from your sensor to a positive reference.
>> Something like 250uA is appropriate.
>You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>
>> Build a positive gain stage with appropriate output for your ADC (zero
>> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
>
>Hi Spehro
>
>Thanks for your input.
>>
>> You can build a signal conditioning stage with a single op-amp.
>> Put a series resistor from your sensor to a positive reference.
>> Something like 250uA is appropriate.
>You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>
>> Build a positive gain stage with appropriate output for your ADC (zero
>> the output fed back to the non-inverting input of the op-amp. Select
>> theat resistor (should be hunreds of K ohms) so that the output
>> voltage is half-scale at the resistance corresponding to 65�C
>> theat resistor (should be hunreds of K ohms) so that the output
>> (mid-scale)
>> The resistor linearizes the non-linear response of the sensor, which
>> is slightly more than a constant current.
>> The resistor values interact, so it's a PITA to change ranges, but
>> this is all do-able with ordinary parts from an online supplier like
>> Digikey.
>Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
If you don't mind doing some math, put the RTD in series with a good resistor,
>> The resistor linearizes the non-linear response of the sensor, which
>> is slightly more than a constant current.
>> The resistor values interact, so it's a PITA to change ranges, but
>> this is all do-able with ordinary parts from an online supplier like
>> Digikey.
>Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
the pair across a reasonably stable power supply. Measure the voltage across the
RTD and the voltage across the resistor (or the supply voltage, if that's
easier), and divide to compute the RTD resistance, then do the CVD equation. The
measurement is ratiomatric on the reference resistor. Use a Susumu 0.05%
thinfilm maybe. This works great with a dual-channel, diff-input delta-sigma
ADC.
>
>> Algarve eh? I'd like to pop over and show you on a whiteboard.
>Please do that, you'll be very welcome here. It will be a pressure to talk to a fellow engineer. No such a thing were, only golf courses, beautiful beaches and girls in bikinis. Very boring.... :-)
>
>Regards
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Bill Sloman
May 14, 2014, 11:13:51 AM5/14/14
to
On Wednesday, 14 May 2014 23:09:58 UTC+10, Phil Hobbs wrote:
> On 5/14/2014 8:26 AM, Lasse Langwadt Christensen wrote:
> > Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
> >> Hi Spehro
>> Thanks for your input.
>>
> >>> You can build a signal conditioning stage with a single op-amp.
> >>>
> >>> Put a series resistor from your sensor to a positive reference.
> >>
> >>> Something like 250uA is appropriate.
> >>
> >> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
> >>
> >> On the other hand NS suggest using a 2 op amp configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
> On 5/14/2014 8:26 AM, Lasse Langwadt Christensen wrote:
> > Den onsdag den 14. maj 2014 13.45.17 UTC+2 skrev jmariano:
> >> Hi Spehro
>> Thanks for your input.
>>
> >>> You can build a signal conditioning stage with a single op-amp.
> >>>
> >>> Put a series resistor from your sensor to a positive reference.
> >>
> >>> Something like 250uA is appropriate.
> >>
> >> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
> >>
The LM334 is good, but not quite up to PT100/Pt1000 bridge standards.
> >>> Build a positive gain stage with appropriate output for your ADC (zero
> >>> the output fed back to the non-inverting input of the op-amp. Select
> >>> that resistor (should be hunreds of K ohms) so that the output
> >>> voltage is half-scale at the resistance corresponding to 65C
> >>> (mid-scale)
> >>
> >>> The resistor linearizes the non-linear response of the sensor, which is slightly more than a constant current.
> >>
> >>> The resistor values interact, so it's a PITA to change ranges, but
> >>> this is all do-able with ordinary parts from an online supplier like
> >>> Digikey.
> >
> >> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
It's all in the hardware, which is cheaper and quicker and harder for the programmer to mess up.
> >>
> >>> The resistor linearizes the non-linear response of the sensor, which is slightly more than a constant current.
> >>
> >>> The resistor values interact, so it's a PITA to change ranges, but
> >>> this is all do-able with ordinary parts from an online supplier like
> >>> Digikey.
> >
> >> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
> >>> Algarve eh? I'd like to pop over and show you on a whiteboard.
> >>
>
> > if you drive it with a current source you get a voltage that is linear with resistance thus (almost) linear with temperature
> >
> > if you drive it with a voltage source and a resistor your voltage is no longer linear with resistance, you can of course fix it in software but you'll need more bits in the ADC.
> > if you drive it with a current source you get a voltage that is linear with resistance thus (almost) linear with temperature
> >
>
> You can make it really linear by using a negative resistance, iirc about
> -25 times the 0 C resistance of the sensor for -150C to +500C. (The
> exact optimum depends a bit on the range you want it to operate over,
> but it's easy to find the right number.)
>
> Also make sure to use a reference, resistors, and ADC with a low enough
> tempco--the RTD only moves 3500 ppm/K, so if your resistors are 200 ppm,
> you'll be pretty sensitive to the temperature of the instrument.
All perfectly correct. I worked out the linearising trick for myself back in 1979, only to have some half-wit dump it because it was "positive feedback" and thus "made the circuit unstable", rather neglecting that it was gain of 1.04, and it takes a positive feedback of 2 to get instability.
> You can make it really linear by using a negative resistance, iirc about
> -25 times the 0 C resistance of the sensor for -150C to +500C. (The
> exact optimum depends a bit on the range you want it to operate over,
> but it's easy to find the right number.)
>
> Also make sure to use a reference, resistors, and ADC with a low enough
> tempco--the RTD only moves 3500 ppm/K, so if your resistors are 200 ppm,
> you'll be pretty sensitive to the temperature of the instrument.
Honeywell put the same scheme into their Pt100 sensing modules a few years later. It doesn't do a perfect job of linearisation, but it's pretty good.
I ended up with a four op amp circuit - which my boss hated - so I had to draw the one, two and three op amp versions, showing how the extra op amps could be progressively cheaper and let you get away with progressively cheaper passive components.
One of the cuter aspects of my circuit was the use of a four 10k (IIRR) tight tolerance metal film resistors on a common substrate. They tracked to about 5ppm/C, which helped. That didn't make it into production either.
--
Bill Sloman, Sydney
rickman
May 14, 2014, 11:59:41 AM5/14/14
to
eliminate the error of *both* references? Add a low tempco resistance
to the constant current path between the PT1000 and ground. Use this
voltage as your reference for the ADC. Then the only temperature
related errors are the reference resistance and any other parts used to
get this voltage to an appropriate level for the ADC.
--
Rick
Spehro Pefhany
May 14, 2014, 1:14:51 PM5/14/14
to
On Wed, 14 May 2014 11:59:41 -0400, rickman <gnu...@gmail.com> wrote:
>
>
>Hmmm, wouldn't it make sense to use a ratiometric measurement to
>eliminate the error of *both* references? Add a low tempco resistance
>to the constant current path between the PT1000 and ground. Use this
>voltage as your reference for the ADC. Then the only temperature
>related errors are the reference resistance and any other parts used to
>get this voltage to an appropriate level for the ADC.
>
>--
>
>Rick
Yes, you can do that, but it can be challenging to keep the noise down
>
>
>Hmmm, wouldn't it make sense to use a ratiometric measurement to
>eliminate the error of *both* references? Add a low tempco resistance
>to the constant current path between the PT1000 and ground. Use this
>voltage as your reference for the ADC. Then the only temperature
>related errors are the reference resistance and any other parts used to
>get this voltage to an appropriate level for the ADC.
>
>--
>
>Rick
when you attach an antenna to the reference. I've used that method for
relatively low performance setups.
The method I suggest is also dependent only on one absolute resistor
value and resistor ratios.
Best regards
Spehro Pefhany
Phil Hobbs
May 14, 2014, 1:49:13 PM5/14/14
to
The one time I used the negative-resistance trick, I used a ratiometric
A/D, one resistor in series with the RTD, and hung the positive-FB op
amp off to one side, i.e. I didn't use it for the measurement, just the
correction. That made me 25 times less sensitive to its offset. The
negative resistance also cancelled the top resistor in the bridge.
Lasse Langwadt Christensen
May 14, 2014, 2:19:11 PM5/14/14
to
Den onsdag den 14. maj 2014 17.02.59 UTC+2 skrev John Larkin:
> On Wed, 14 May 2014 04:45:17 -0700 (PDT), jmariano <jmari...@gmail.com> wrote:
>
>
>
> >
>
> >Hi Spehro
>
> >
>
> >Thanks for your input.
>
> >>
>
> >> You can build a signal conditioning stage with a single op-amp.
>
> >> Put a series resistor from your sensor to a positive reference.
>
> >> Something like 250uA is appropriate.
>
> >You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>
> >On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>
> >>
>
> >>
>
> >> Build a positive gain stage with appropriate output for your ADC (zero
>
> >> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
> On Wed, 14 May 2014 04:45:17 -0700 (PDT), jmariano <jmari...@gmail.com> wrote:
>
>
>
> >
>
> >Hi Spehro
>
> >
>
> >Thanks for your input.
>
> >>
>
> >> You can build a signal conditioning stage with a single op-amp.
>
> >> Put a series resistor from your sensor to a positive reference.
>
> >> Something like 250uA is appropriate.
>
> >You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>
> >On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>
> >>
>
> >>
>
> >> Build a positive gain stage with appropriate output for your ADC (zero
>
>
> >> the output fed back to the non-inverting input of the op-amp. Select
>
> >> theat resistor (should be hunreds of K ohms) so that the output
>
> >> voltage is half-scale at the resistance corresponding to 65�C
> >> the output fed back to the non-inverting input of the op-amp. Select
>
> >> theat resistor (should be hunreds of K ohms) so that the output
>
>
> >> (mid-scale)
>
> >> The resistor linearizes the non-linear response of the sensor, which
>
> >> is slightly more than a constant current.
>
> >> The resistor values interact, so it's a PITA to change ranges, but
>
> >> this is all do-able with ordinary parts from an online supplier like
>
> >> Digikey.
>
> >Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>
>
>
> If you don't mind doing some math, put the RTD in series with a good resistor,
>
> the pair across a reasonably stable power supply. Measure the voltage across the
>
> RTD and the voltage across the resistor (or the supply voltage, if that's
>
> easier), and divide to compute the RTD resistance, then do the CVD equation. The
>
> measurement is ratiomatric on the reference resistor. Use a Susumu 0.05%
>
> thinfilm maybe. This works great with a dual-channel, diff-input delta-sigma
>
> ADC.
>
yes it works well, but you need quite a few more bits that 12 if you want to do the linearizing in software and get 0.1'C
> >> (mid-scale)
>
> >> The resistor linearizes the non-linear response of the sensor, which
>
> >> is slightly more than a constant current.
>
> >> The resistor values interact, so it's a PITA to change ranges, but
>
> >> this is all do-able with ordinary parts from an online supplier like
>
> >> Digikey.
>
> >Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>
>
>
> If you don't mind doing some math, put the RTD in series with a good resistor,
>
> the pair across a reasonably stable power supply. Measure the voltage across the
>
> RTD and the voltage across the resistor (or the supply voltage, if that's
>
> easier), and divide to compute the RTD resistance, then do the CVD equation. The
>
> measurement is ratiomatric on the reference resistor. Use a Susumu 0.05%
>
> thinfilm maybe. This works great with a dual-channel, diff-input delta-sigma
>
> ADC.
>
-Lasse
Tauno Voipio
May 14, 2014, 3:47:00 PM5/14/14
to
On 14.5.14 21:19, Lasse Langwadt Christensen wrote:
> Den onsdag den 14. maj 2014 17.02.59 UTC+2 skrev John Larkin:
>> On Wed, 14 May 2014 04:45:17 -0700 (PDT), jmariano <jmari...@gmail.com> wrote:
>>
>>
>>
>>>
>>
>>> Hi Spehro
>>
>>>
>>
>>> Thanks for your input.
>>
>>>>
>>
>>>> You can build a signal conditioning stage with a single op-amp.
>>
>>>> Put a series resistor from your sensor to a positive reference.
>>
>>>> Something like 250uA is appropriate.
>>
>>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>>
>>> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>>>
>>
>>>>
>>
>>>> Build a positive gain stage with appropriate output for your ADC (zero
>>
>>>> and span- say 0 V out at -20�C and 2.5V out at 150�C), with a bit of
> Den onsdag den 14. maj 2014 17.02.59 UTC+2 skrev John Larkin:
>> On Wed, 14 May 2014 04:45:17 -0700 (PDT), jmariano <jmari...@gmail.com> wrote:
>>
>>
>>
>>>
>>
>>> Hi Spehro
>>
>>>
>>
>>> Thanks for your input.
>>
>>>>
>>
>>>> You can build a signal conditioning stage with a single op-amp.
>>
>>>> Put a series resistor from your sensor to a positive reference.
>>
>>>> Something like 250uA is appropriate.
>>
>>> You mean voltage reference? How about current excitation? In this particular case I don't get the pros and cons of using one or the other...
>>
>>> On the other hand NS suggest using a 2 ampop configuration to built a floating current source (AN1559) but LM334 claims to be floating. So, why should I built one if I can use one of the shelf?
>>
>>>>
>>
>>>>
>>
>>>> Build a positive gain stage with appropriate output for your ADC (zero
>>
>>
>>>> the output fed back to the non-inverting input of the op-amp. Select
>>
>>>> theat resistor (should be hunreds of K ohms) so that the output
>>
>>>> voltage is half-scale at the resistance corresponding to 65�C
>>>> the output fed back to the non-inverting input of the op-amp. Select
>>
>>>> theat resistor (should be hunreds of K ohms) so that the output
>>
>>
>>>> (mid-scale)
>>
>>>> The resistor linearizes the non-linear response of the sensor, which
>>
>>>> is slightly more than a constant current.
>>
>>>> The resistor values interact, so it's a PITA to change ranges, but
>>
>>>> this is all do-able with ordinary parts from an online supplier like
>>
>>>> Digikey.
>>
>>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>>
>>
>>
>> If you don't mind doing some math, put the RTD in series with a good resistor,
>>
>> the pair across a reasonably stable power supply. Measure the voltage across the
>>
>> RTD and the voltage across the resistor (or the supply voltage, if that's
>>
>> easier), and divide to compute the RTD resistance, then do the CVD equation. The
>>
>> measurement is ratiomatric on the reference resistor. Use a Susumu 0.05%
>>
>> thinfilm maybe. This works great with a dual-channel, diff-input delta-sigma
>>
>> ADC.
>>
>
> yes it works well, but you need quite a few more bits that 12 if you want to do the linearizing in software and get 0.1'C
>
> -Lasse
>
20 bits (e.g. LT2420) is more than enough.
>>>> (mid-scale)
>>
>>>> The resistor linearizes the non-linear response of the sensor, which
>>
>>>> is slightly more than a constant current.
>>
>>>> The resistor values interact, so it's a PITA to change ranges, but
>>
>>>> this is all do-able with ordinary parts from an online supplier like
>>
>>>> Digikey.
>>
>>> Linearize the transfer function of the sensor or the voltage divider equation? I will be using the Callender-Van Dusen equation(s) on the microcontroller to get T from R so I don't understand the need to linearize the sensor output.
>>
>>
>>
>> If you don't mind doing some math, put the RTD in series with a good resistor,
>>
>> the pair across a reasonably stable power supply. Measure the voltage across the
>>
>> RTD and the voltage across the resistor (or the supply voltage, if that's
>>
>> easier), and divide to compute the RTD resistance, then do the CVD equation. The
>>
>> measurement is ratiomatric on the reference resistor. Use a Susumu 0.05%
>>
>> thinfilm maybe. This works great with a dual-channel, diff-input delta-sigma
>>
>> ADC.
>>
>
> yes it works well, but you need quite a few more bits that 12 if you want to do the linearizing in software and get 0.1'C
>
> -Lasse
>
0.1 C will be better than Class A sensing element at its best (0.15 C at
0 C).
--
Tauno Voipio
rickman
May 14, 2014, 5:26:17 PM5/14/14
to
--
Rick
Lasse Langwadt Christensen
May 14, 2014, 5:39:55 PM5/14/14
to
I prefer a resistor to both ends to keep things symmetric.
I've seen one design with an instrumentation amp and a current source feeding the sensor begin very sensitive to noise, with one wire a high impedance
current source the other hard ground the common mode rejection sucked
-Lasse
John Larkin
May 14, 2014, 7:31:03 PM5/14/14
to
https://dl.dropboxusercontent.com/u/53724080/Thermal/RTD_Ckt.jpg
The upper voltage can also go into the ADC reference, instead of
CH1,which has its good and bad points. The usual delta-sigma ADC can
be programmed to have a notch at 50 or 60 Hz, which keeps the noise
down.
The V+ reference needn't be stable; the measurement is ratiometric on
Rref.
--
John Larkin Highland Technology, Inc
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Spehro Pefhany
May 14, 2014, 10:37:30 PM5/14/14
to
Don't do much four-wire, usually three wire sensors.
George Herold
May 15, 2014, 10:44:42 AM5/15/14
to
On Wednesday, May 14, 2014 10:37:30 PM UTC-4, Spehro Pefhany wrote:
> On Wed, 14 May 2014 16:31:03 -0700, the renowned John Larkin
>
> <jla...@highlandtechnology.com> wrote:
> >Something like this maybe.
> >
> >https://dl.dropboxusercontent.com/u/53724080/Thermal/RTD_Ckt.jpg
> >
> >The upper voltage can also go into the ADC reference, instead of
> >CH1,which has its good and bad points. The usual delta-sigma ADC can
> >be programmed to have a notch at 50 or 60 Hz, which keeps the noise
> >down.
> >
>
> >The V+ reference needn't be stable; the measurement is ratiometric on
>
> >Rref.
>
>
>
> Yup, can do that, and put a low(er) pass filter on Rref voltage.
>
> Don't do much four-wire, usually three wire sensors.
>
Three wires? So a drive, upper voltage sense, and then ground.
> On Wed, 14 May 2014 16:31:03 -0700, the renowned John Larkin
>
> <jla...@highlandtechnology.com> wrote:
> >Something like this maybe.
> >
> >https://dl.dropboxusercontent.com/u/53724080/Thermal/RTD_Ckt.jpg
> >
> >The upper voltage can also go into the ADC reference, instead of
> >CH1,which has its good and bad points. The usual delta-sigma ADC can
> >be programmed to have a notch at 50 or 60 Hz, which keeps the noise
> >down.
> >
>
> >The V+ reference needn't be stable; the measurement is ratiometric on
>
> >Rref.
>
>
>
> Yup, can do that, and put a low(er) pass filter on Rref voltage.
>
> Don't do much four-wire, usually three wire sensors.
>
(one sensor end is tied to a *hard* ground.)
I've done that, but didn't know it was common.
If the OP has limited bits, then why not a bridge..
centered in the middle of his temperature range.
George H.
Spehro Pefhany
May 15, 2014, 2:22:11 PM5/15/14
to
On Thu, 15 May 2014 07:44:42 -0700 (PDT), George Herold
<ghe...@teachspin.com> wrote:
>
>Three wires? So a drive, upper voltage sense, and then ground.
>(one sensor end is tied to a *hard* ground.)
>I've done that, but didn't know it was common.
Yes, and two or three of the wires have current going through them. If
<ghe...@teachspin.com> wrote:
>
>Three wires? So a drive, upper voltage sense, and then ground.
>(one sensor end is tied to a *hard* ground.)
>I've done that, but didn't know it was common.
the wires are matched in resistance you can make the leadwire
resistance cancel 100%. It's still not as good as a Kelvin connection,
but plenty good enough for most control systems.
>If the OP has limited bits, then why not a bridge..
>centered in the middle of his temperature range.
>
>George H.
George Herold
May 15, 2014, 3:17:15 PM5/15/14
to
On Thursday, May 15, 2014 2:22:11 PM UTC-4, Spehro Pefhany wrote:
> On Thu, 15 May 2014 07:44:42 -0700 (PDT), George Herold
>
> <ghe...@teachspin.com> wrote:
>
>
>
> >
>
> >Three wires? So a drive, upper voltage sense, and then ground.
> >(one sensor end is tied to a *hard* ground.)
> >I've done that, but didn't know it was common.
>
> Yes, and two or three of the wires have current going through them. If
> the wires are matched in resistance you can make the leadwire
> resistance cancel 100%. It's still not as good as a Kelvin connection,
> but plenty good enough for most control systems.
Oh that's not what I did... This was down a probe and I wanted to save
> On Thu, 15 May 2014 07:44:42 -0700 (PDT), George Herold
>
> <ghe...@teachspin.com> wrote:
>
>
>
> >
>
> >Three wires? So a drive, upper voltage sense, and then ground.
> >(one sensor end is tied to a *hard* ground.)
> >I've done that, but didn't know it was common.
>
> Yes, and two or three of the wires have current going through them. If
> the wires are matched in resistance you can make the leadwire
> resistance cancel 100%. It's still not as good as a Kelvin connection,
> but plenty good enough for most control systems.
wires so one end was tied to the probe case (ground).
And I just ignored the voltage drop of the current along ground.
It sounds like you subtracted the lead wire resistance.
>
>
> >If the OP has limited bits, then why not a bridge..
> >centered in the middle of his temperature range.
>
>
>
> >If the OP has limited bits, then why not a bridge..
> >centered in the middle of his temperature range.
>
>
> I think of the ADC as the missing half of the bridge.
Well the bridge can get rid of some of the DC offset.
So he (the OP) would only be looking at the changes in resistance.
He'd could get more out of his limited bits.
(I realize you know all this, so that last bit (NPI) was directed at the OP.)
George H.
Spehro Pefhany
May 15, 2014, 4:39:02 PM5/15/14
to
On Thu, 15 May 2014 12:17:15 -0700 (PDT), George Herold
day.
<ghe...@teachspin.com> wrote:
>On Thursday, May 15, 2014 2:22:11 PM UTC-4, Spehro Pefhany wrote:
>> On Thu, 15 May 2014 07:44:42 -0700 (PDT), George Herold
>>
>> <ghe...@teachspin.com> wrote:
>>
>>
>>
>> >
>>
>> >Three wires? So a drive, upper voltage sense, and then ground.
>> >(one sensor end is tied to a *hard* ground.)
>> >I've done that, but didn't know it was common.
>>
>> Yes, and two or three of the wires have current going through them. If
>> the wires are matched in resistance you can make the leadwire
>> resistance cancel 100%. It's still not as good as a Kelvin connection,
>> but plenty good enough for most control systems.
>
>Oh that's not what I did... This was down a probe and I wanted to save
>wires so one end was tied to the probe case (ground).
>And I just ignored the voltage drop of the current along ground.
>
>It sounds like you subtracted the lead wire resistance.
Basically subtract twice the resistance of *one* wire and call it a
>On Thursday, May 15, 2014 2:22:11 PM UTC-4, Spehro Pefhany wrote:
>> On Thu, 15 May 2014 07:44:42 -0700 (PDT), George Herold
>>
>> <ghe...@teachspin.com> wrote:
>>
>>
>>
>> >
>>
>> >Three wires? So a drive, upper voltage sense, and then ground.
>> >(one sensor end is tied to a *hard* ground.)
>> >I've done that, but didn't know it was common.
>>
>> Yes, and two or three of the wires have current going through them. If
>> the wires are matched in resistance you can make the leadwire
>> resistance cancel 100%. It's still not as good as a Kelvin connection,
>> but plenty good enough for most control systems.
>
>Oh that's not what I did... This was down a probe and I wanted to save
>wires so one end was tied to the probe case (ground).
>And I just ignored the voltage drop of the current along ground.
>
>It sounds like you subtracted the lead wire resistance.
day.
josephkk
May 17, 2014, 6:49:04 AM5/17/14
to
On Tue, 13 May 2014 14:41:00 -0700 (PDT), jmariano <jmari...@gmail.com>
wrote:
First off why are you not interested in the rather significant advantages
of 4-terminal (Kelvin clips) measurement?
Second why only 12 bits? 16 bits will gain you a lot performance wise.
Finally, how are you dealing with calibration? 0.1 degrees C is readily
achievable but requires calibration.
?-)
wrote:
of 4-terminal (Kelvin clips) measurement?
Second why only 12 bits? 16 bits will gain you a lot performance wise.
Finally, how are you dealing with calibration? 0.1 degrees C is readily
achievable but requires calibration.
?-)
Lasse Langwadt Christensen
May 17, 2014, 7:43:11 AM5/17/14
to
Den lørdag den 17. maj 2014 12.49.04 UTC+2 skrev josephkk:
> On Tue, 13 May 2014 14:41:00 -0700 (PDT), jmariano <jmari...@gmail.com>
>
> wrote:
>
>
>
> >Hi all,
>
> >
>
> >Need your advice....
>
> >
>
> >I'm building a prototype of a bench-top instrument. I need to measure absolute temperature using a RTD pt1000 sensor with short lead wires (<50 cm). I intend to use two wires and I want to go all analog, from sensor to the 12 bit ADC - a kind of analog design exercise, so no all-integrated solutions loke ADuC834 or ADS1247/8.
>
> >
>
> >The temp range is -20 C to 150 C and my initial target was a overall precision of +/-0.1 C, but after reading some posts in this group I'll settle for the best I can get.
>
> >
>
> >As far as I know, the 3 most common (DC) ways to signal-conditioning a pt100(0) are simple voltage divider, constant current driving and bridge. For my needs, it seems like a voltage divider, eventually polarized from a precision reference, and a buffer amplifier will do the trick, but i'm not sure i'm not missing something.
>
> >
>
> >Would someone care to elaborate on this.
>
> >
>
> >Best regards
>
> >
>
> >José Mariano
>
> >============
>
> >Physics Department
>
> >University of the Algarve
>
> >Portugal
>
>
>
> First off why are you not interested in the rather significant advantages
>
> of 4-terminal (Kelvin clips) measurement?
>
he's using pt1000 and <50cm wire
> On Tue, 13 May 2014 14:41:00 -0700 (PDT), jmariano <jmari...@gmail.com>
>
> wrote:
>
>
>
> >Hi all,
>
> >
>
> >Need your advice....
>
> >
>
> >I'm building a prototype of a bench-top instrument. I need to measure absolute temperature using a RTD pt1000 sensor with short lead wires (<50 cm). I intend to use two wires and I want to go all analog, from sensor to the 12 bit ADC - a kind of analog design exercise, so no all-integrated solutions loke ADuC834 or ADS1247/8.
>
> >
>
> >The temp range is -20 C to 150 C and my initial target was a overall precision of +/-0.1 C, but after reading some posts in this group I'll settle for the best I can get.
>
> >
>
> >As far as I know, the 3 most common (DC) ways to signal-conditioning a pt100(0) are simple voltage divider, constant current driving and bridge. For my needs, it seems like a voltage divider, eventually polarized from a precision reference, and a buffer amplifier will do the trick, but i'm not sure i'm not missing something.
>
> >
>
> >Would someone care to elaborate on this.
>
> >
>
> >Best regards
>
> >
>
> >José Mariano
>
> >============
>
> >Physics Department
>
> >University of the Algarve
>
> >Portugal
>
>
>
> First off why are you not interested in the rather significant advantages
>
> of 4-terminal (Kelvin clips) measurement?
>
PT1000 is ~4 Ohm/C, 1 meter of wire will be maybe 0.4 Ohm so ~0.1C
so 4-wire is probably not worth the effort
>
>
> Second why only 12 bits? 16 bits will gain you a lot performance wise.
>
>
>
> Finally, how are you dealing with calibration? 0.1 degrees C is readily
>
> achievable but requires calibration.
>
>
>
> ?-)
Bill Sloman
May 17, 2014, 9:40:49 AM5/17/14
to
On Saturday, 17 May 2014 20:49:04 UTC+10, josephkk wrote:
> On Tue, 13 May 2014 14:41:00 -0700 (PDT), jmariano <jmari...@gmail.com>
> wrote:
<snip>
> On Tue, 13 May 2014 14:41:00 -0700 (PDT), jmariano <jmari...@gmail.com>
> wrote:
> Finally, how are you dealing with calibration? 0.1 degrees C is readily
> achievable but requires calibration.
I'm happier with a well-stirred crushed-ice-water bath - the water has to be kept moving through the ice, and spinning magnet stirrer seems to work fine.
It's good to close to couple of millidegrees Kelvin.
--
Bill Sloman, Sydney
jmariano
May 17, 2014, 6:57:42 PM5/17/14
to
>
> >
>
> > First off why are you not interested in the rather significant advantages
> > of 4-terminal (Kelvin clips) measurement?
>
> he's using pt1000 and <50cm wire
> PT1000 is ~4 Ohm/C, 1 meter of wire will be maybe 0.4 Ohm so ~0.1C
>
> so 4-wire is probably not worth the effort
>
Exactly Lasse, more precisely AWG23 is 0.067 ohm/m, so well below 0.1C
> >
>
> > First off why are you not interested in the rather significant advantages
> > of 4-terminal (Kelvin clips) measurement?
>
> he's using pt1000 and <50cm wire
> PT1000 is ~4 Ohm/C, 1 meter of wire will be maybe 0.4 Ohm so ~0.1C
>
> so 4-wire is probably not worth the effort
>
>
>
> >
>
> >
>
> > Second why only 12 bits? 16 bits will gain you a lot performance wise.
>
> I'm guessing that is what the MCU he's using has on chip
>
>
Josephkk, in fact I'm using a TI launchpad with a ARM Cortex uP. This is a one-off design to be used in a research environment. Using the evaluation board saves me the effort of designing the digital part of the project (I hope!). On the other hand 12 bit with a voltage span of 3.3 V gives me a resolution of 0.8 mV or, with a 170C of temp spam, a 0.04C resolution. So, if I can keep all the errors of the analog part below the 0.8mV threshold I'll be OK with 12 bits, but if I can't (most probably) I don't see the advantage of using more bits unless I use an all digital solution.... right?
> > Finally, how are you dealing with calibration? 0.1 degrees C is readily
> > achievable but requires calibration.
> > ?-)
Regards
Mariano
John Larkin
May 17, 2014, 7:03:24 PM5/17/14
to
analog noise, which will improve the DNL of the ADC. One way to get the noise is
to stream random bits out a parallel port pin and RC lowpass filter.
The uP on-chip ADCs tend to be mediocre.
0 new messages