Thermocouple op amp circuit for type K
Andrew
I'm trying to build a temperature logger for very high temperatures
(1500F-1900F). I'd like to build an amplifier circuit for a type K to
convert the 0-2000F into 0 to 2V for my data logger. This seems like
a very simple project for an op amp with a low pass filter. But when
I look on line I find that analog devices makes a thermocouple
amplifier for like $30. So, what am I missing?
I am planning on using a single supply op amp (LM324) with a gain of
200 and a first order low pass filter with a corner frequency of
30Hz. Does anyone have experience with this? Approximately how much
error should I expect from this circuit?
thanks
Bill Sloman
The LM324 is a very poor choice of amplifier; it has a high input off-
set voltage and the temperature coefficient of input offset voltage is
on the high side. Moreover, the bipolar input transistors effectively
rectify any high frequency noise on the input (all frequencies above
the - low- bandwidth of the LM324). It takes some noise to drive the
input into non-linearity - the "rectification" becomes progressively
less efficiently as the noise swings less than 52mV - but it can still
be a problem with much smaller swings.
FET-input amplifiers are an order of magnitude or two less sensitive
to high frequency noise.
If your first order low pass filter keeps on rolling off steadily up
into the radio spectrum (which is unlikely - most capacitors
eventually look like resistors or inductors at sufficiently high
frequencies) you wouldn't need to worry about this, but in practice it
is usually a problem that needs to be dealt with.
The Analog devices AD595 circuit includes cold junction compensation
and laser-trimmed non-linearity corrections
http://www.analog.com/static/imported-files/data_sheets/AD594_595.pdf
so you get quite a lot more bang for your $30.
The application note doesn't seem to mention the noise-rectification
problem, but it probably wasn't written by anybody who had used the
circuit in the field.
The Linear Technology application note on thermocouple measurements
AN-28 was written by Jim Williams, who is pretty good, but it still
only mentions noise in passing
http://www.linear.com/pc/downloadDocument.do?id=4121
--
Bill Sloman, Nijmegen
Spehro Pefhany
<andrewk...@gmail.com> wrote:
>Hello,
>I'm trying to build a temperature logger for very high temperatures
>(1500F-1900F). I'd like to build an amplifier circuit for a type K to
>convert the 0-2000F into 0 to 2V for my data logger. This seems like
>a very simple project for an op amp with a low pass filter. But when
>I look on line I find that analog devices makes a thermocouple
>amplifier for like $30. So, what am I missing?
An accuracy specification. All else will follow from that. The 10-cent
LM324 is not a very good low-level amplifier, but it's not ridiculous
in this application either, if your requirements are very loose.
>I am planning on using a single supply op amp (LM324) with a gain of
>200 and a first order low pass filter with a corner frequency of
>30Hz. Does anyone have experience with this? Approximately how much
>error should I expect from this circuit?
>
>thanks
The typical DC drift is 7uV/K, 30uV/K maximum so the zero drift due to
op-amp TCVos will be around 0.2~0.8 �F/�F. You need a gain of around
40 to cover the 0-2000F range you mention-- 200 is too high.
Add onto that the error due to whatever you are using as a
cold-junction sensor, gain error, and whatever error exists in your
calculations for thermocouple linearization and cold-junction
compensation (to do the best job possible you need two roughly 10th
order polynomials- one for forward linearization and one for reverse).
Older (and new lower performance) designs such as the AD595 tend to
use a linear approximation for one of the polynomials, which works
okay in your type of situation if the cold junction is not too far
from room temperature and mostly on the + side.
Also there will typically be a small error due to bias current change
vs. sensor resistance (which also changes due to temperature).
You should be able to 'typically' get within 10-20F over a narrow
ambient range, with care, a decent cold-junction sensor, and good
calibration. A better op-amp will easily cut that in half, but your
cold junction compensation will begin to dominate. I'm not a fan of
expensive chips like the '595 that require you to closely thermally
couple a DIP to a terminal strip.
Your easiest (and best for a one-off) solution is to go buy a
thermocouple transmitter that will provide galvanic isolation,
filtering, cold-junction compensation and linearization for perhaps
$200 off the shelf- less than an hour or two of engineering.
Andy
wrote:
> On Tue, 21 Jul 2009 19:04:36 -0700 (PDT), Andrew
>
Thanks for your advice. I really like the LM324 because it has a
single supply voltage, even though it's not the best of op amps. Dual
supply voltages are fine in a lab, but in field instruments they are a
pain. There has to be a good way of taking a single 9V battery and
creating a pos and neg supply. Anyway, I'll probably go with the
AD595 due to the fact that I need about 6 of these. And I can make
the whole package, including data collection, for less than $400.
Thanks again for your advice.
oopere
>
> Thanks for your advice. I really like the LM324 because it has a
> single supply voltage, even though it's not the best of op amps. Dual
> supply voltages are fine in a lab, but in field instruments they are a
> pain. There has to be a good way of taking a single 9V battery and
> creating a pos and neg supply.
I have not played with them seriously, but you may have a look at Texas
Instruments' TLE2426CLPE3, which provides a virtual ground with quite
low output impedance.
Pere
Jan Panteltje
<andrewk...@gmail.com> wrote in
<74d8408e-8566-44a3...@y10g2000prf.googlegroups.com>:
>There has to be a good way of taking a single 9V battery and
>creating a pos and neg supply.
Split it in half with a LM380 :-)
You can also use a simple LC potcore + 1 transistor + 2 diodes + 5 caps + 1 resistor.
2 x 9 turns gives you plus and minus 8 V.
+--------------------
| | |
| |10u e
| --- -b PNP BC558
9V |+ === | c ------
| | | |
| | | ----)--------a diode k --------- + 12V
| | | |. | | |
| | | ||(5t | | |
=== |+ |_1t |||----| | |
--- |_________)||(. | | |+
|10u | ||(9t === | === 10u
| R 4k7 ||( |22n | ---
| | | | | |
- ---------------------------------)-----------------------
potcore | |+
This is a sine wave generator | === 10u
| ---
|___ k diode a____|_____ - 12 V
Original hand painted ASCII
whit3rd
> I'm trying to build a temperature logger for very high temperatures
> (1500F-1900F). I'd like to build an amplifier circuit for a type K to
> convert the 0-2000F into 0 to 2V for my data logger. This seems like
> a very simple project for an op amp with a low pass filter. But when
> I look on line I find that analog devices makes a thermocouple
> amplifier for like $30. So, what am I missing?
Thermocouples always come in pairs (a complete electric
circuit has a hot junction and a cold junction), so you'll need to
digitize both the loop voltage and the cold junction temperature,
OR use a functional module that sums in a cold-junction compensation
and maybe does some linearizing of the voltage/temperature
curve. The 'thermocouple amplifier' is the full functional
module, and saves you from dedicating two datalogger channels.
You can do your own temperature compensation with a diode-connected
transistor, if there's any kind of regulated power available... but
your calculations will get somewhat messy, and calibration is
up to you. And, it DOES take some kind of reference source,
not just op amps.
Bill Sloman
There have been a few more single supply op amps developed since the
LM324 hit the market.
The Linear Technology LT1006 is almost as old, and has much better off-
set and drift specifications. Linear Technology has introduced a lot
of other precision single supply op amps since then; the LTC1050 and
its successors might be worth a look, and the Analog Devices
equivalents are also interesting.
--
Bill Sloman, Nijmegen
Spehro Pefhany
LT1013 is a nice dual for under a dollar US in small quantities.
Rene Tschaggelar
Beside the poor choice of an OpAmp, I'd skip the OpAmp
altogether and have a sufficiently good ADC to measure
the thermocouple. A K type does in the order of 40uV
per degree Celsius, making in the order of 25uV per
Fahrenheit, and the 1500 F thus give in the order of
40mV, a useable input for a standard 20bit ADC. Some
may even have selectable amplifier stages built in.
Rene
Winfield Hill
>
> You can also use a simple LC potcore + 1 transistor + 2 diodes + 5 caps + 1 resistor.
> 2 x 9 turns gives you plus and minus 8 V.
>
> +--------------------
> | | |
> | |10u e
> | --- -b PNP BC558
> 9V |+ === | c ------
> | | | |
> | | | ----)--------a diode k --------- + 12V
> | | | |. | | |
> | | | ||(5t | | |
> === |+ |_1t |||----| | |
> --- |_________)||(. | | |+
> |10u | ||(9t === | === 10u
> | R 4k7 ||( |22n | ---
> | | | | | |
> - ---------------------------------)-----------------------
> potcore | |+
> This is a sine wave generator | === 10u
> | ---
> |___ k diode a____|_____ - 12 V
> Original hand painted ASCII
Interesting circuit. What frequency does it run at?
What's with the 2nd elec's polarity? I'm puzzled by the
PNP's 2mA base current, seems rather high. Can you
talk us through the circuit's operation?
MooseFET
Once the circuit is running the PNP's eb junction is back biased most
of the time. 9 turns will have 9V peak = 18V p-p on it so one turn
has 2Vp-p.
> I'm puzzled by the
> PNP's 2mA base current, seems rather high.
It is running as a class-C1 stage. During saturation, the hfe of the
transistor goes nearly to zero. A lot of the base drive current is
taken up during that time.
I am sort of surprised it doesn't squeg. The 10uF gives a time
constant way longer than the ring up time of the tuned circuit.