Multiplex sensors... how to ground?
mooseo
I am trying to multiplex several (8-24) temperature sensors (LM35) into
an A/D card. I am playing around with them running into a 4051 MUX,
then to my DAQ system. The DAQ system has 4 channels of analog in, plus
digital output ( board specs here: http://tinyurl.com/bo7hx ). I am
using the digital output port on the DAQ board to switch the MUX
channel, then read the differential signal on the A/D channel.
The problem (one of them, anyway) that is puzzling me is how to ground
the DAQ card and my multiplex circuitry. The DAQ has separate analog
and digital grounds, but since I am using the digital output to control
the mux, they end up being connected at that point. The problem is that
they are at different levels, so when I connect the two grounds, I end
up shifting my signal by about 0.5V.
The circuit setup is basically: LM35s into 4051 mux... output of the
mux goes through a simple lowpass RC filter to get rid of noise (I'm
sampling each channel at about 1Hz)... after the filter, i have a
voltage follower to prevent voltage drop into the A/D.
I welcome any suggestions. or questions about my circuit. I'm somewhat
of a newbie, so it is entirely likely that I am unaware of something
simple that I am supposed to ground.
Thanks!
mike
bill....@ieee.org
You shouldn't be using the 4051, but the 4052, and selecting the local
ground at each LM35 as well as the LM35 output pin. The differential
output for the mux should then be fed through a differential amplifier
- either a subtractor-with-gain or an instrumentation amplifier - to
amplify the signal to fill the A/D converter range, and convert it
from a differential to a single-ended signal referred to the ground
voltage at the common point of the DAQ analog and digital grounds.
If you could arrange things so that the wasn't any current running
through the 8-24 tracks connecting your AD35 grounds to DAQ "star"
ground point, you mightn't need to bother with this. The 60uA drawn by
the LM35 isn't going to produce a 0.5V offset in any remotely likely
length of track, and 0.5V is large - but by no means unknown - voltage
drop across a ground track, though it would take a pretty thoughtless
layout to generate it.
Since that level of error voltage is probably not being generated by a
DC voltage drop, but by some kind of rectification of hgh frequency
interference at the LM35, the buffer amplfier or the analog input to
the A/D converter input, the differential multiplexer and the
differential amplifier is probably a safer route to follow. It has
worked well for me in similar situations.
--
Bill Sloman, Nijmegen
mooseo
Thanks a lot for the reply. From this and other things I've read, I'm
realizing that I am being very sloppy about my grounds. Right now, this
circuit is just laid out on a breadboard, and I am just plugging
anything that needs grounding to the nearest available bus. I am going
to rip the circuit out and start over, keeping things separate and see
if this helps. I'll also order up some dual muxes (I think that I will
go with some 8x2 like an ADG707 rather than a 4052 just so I don't have
to re-write the software. I only need 3 of them, so the cost isn't
really important) and a differential amp.
Is there a standard differential instrumentation amp that anyone can
recommend? This is another of those devices where a quick search at
Digikey turns up 100s of items with similar looking specs and prices
from $0.40 - $40.
As for the source of the offset voltage, it seems like it is coming
from the digital side of the mux, regardless of whether the LM35s are
connected. Don't know if that means anything.
Thanks again,
mike
Mark
think about where the current is flowing through the ground and
creating the 0.5 volt diff..also try to keep you low level signal
differential., even if one side is actually ground, you can "pretend"
it is differential and connect it to ground at a place where it is the
"correct" ground...
Mark
mooseo
Thanks again for the replies. I actually found that I had everything I
needed in my Misc. Stuff box, so I was able to wire it up last night.
Keeping the signal differential has fixed the ground shifting problems,
and everything seems to be working... at least, I'm getting reasonable
readings out. I'm still puzzled slightly by the behavior of this
circuit, and I wonder if anyone can shed light on it.
I'm running both the signal and the LM35 gnd wires through the 4052 and
from there to the instrumentation amp (I had a burr brown INA122
around). The data sheet on this recommends using 47K resistors from
each input to gnd to dissipate input bias currents. My actual output
seems to shift if I vary the size of this resistor, but 47K seems to
give me exactly the right value.
The most puzzling thing is that if I measure the voltage across the 2
outputs of the 4052, I get a value exactly 2x what I expect. This turns
out to be fortuitous, because the amp has a minimum gain of 5x, so
between the two of those, I get 10x gain. Again, the voltage between
the two channels is dependent on the size of the input bias resistors,
which I guess makes sense because the V- side eventually has another
path to ground. So, my question is, is the 47K resistor a magic number
that matches the internal circuitry of the amp? At this point, should
I just relax and enjoy the fact that it is actually working?
Thanks again for all the help. Rewiring this whole circuit from
scratch has helped in many ways, especially in clearing up noise... the
new circuit is much cleaner (I remembered to tie the shield from the
sensor cables to ground, which helped a lot) so I don't even think I
need the low-pass filter.
cheers
mike
bill....@ieee.org
mooseo wrote:
> Mark and Bill,
> Thanks again for the replies. I actually found that I had everything I
> needed in my Misc. Stuff box, so I was able to wire it up last night.
> Keeping the signal differential has fixed the ground shifting problems,
> and everything seems to be working... at least, I'm getting reasonable
> readings out. I'm still puzzled slightly by the behavior of this
> circuit, and I wonder if anyone can shed light on it.
>
> I'm running both the signal and the LM35 gnd wires through the 4052 and
> from there to the instrumentation amp (I had a burr brown INA122
> around). The data sheet on this recommends using 47K resistors from
> each input to gnd to dissipate input bias currents. My actual output
> seems to shift if I vary the size of this resistor, but 47K seems to
> give me exactly the right value.
>
> The most puzzling thing is that if I measure the voltage across the 2
> outputs of the 4052, I get a value exactly 2x what I expect. This turns
> out to be fortuitous, because the amp has a minimum gain of 5x, so
> between the two of those, I get 10x gain. Again, the voltage between
> the two channels is dependent on the size of the input bias resistors,
> which I guess makes sense because the V- side eventually has another
> path to ground. So, my question is, is the 47K resistor a magic number
> that matches the internal circuitry of the amp? At this point, should
> I just relax and enjoy the fact that it is actually working?
Without a complete circuit diagram, all I can say is that this is
weird. The LM35 is a relatively low-impedance voltage source (at least
for small currents), and the "on" resistance of the 4052 ought not to
be high enough to do anything interesting, so the odds are that what
you are reading doesn't mean what you think it means.
> Thanks again for all the help. Rewiring this whole circuit from
> scratch has helped in many ways, especially in clearing up noise... the
> new circuit is much cleaner (I remembered to tie the shield from the
> sensor cables to ground, which helped a lot) so I don't even think I
> need the low-pass filter.
You always need the low pass filter - you just don't know when ...
--
Bill Sloman, Nijmegen
mooseo
Yes, that is my impression too. When I get back to the lab on Sunday
or Monday, I'll upload a schematic. I realized that it isn't working
as well as I thought... the values are great in a narrow range around
room temp, but when I bring the probe down to 0, it is still reading 12
C. I notice that there is a big voltage offset, too.
Hopefully this will make more sense in a new week. I'll put up the
diagram to help others.
Thanks again.
mike
mooseo
http://www.moosecraft.org/TURD/mux_board_w_mp.jpg
The ground is connected to the digital and analog ground of the DAQ
card after pin 5 on the amp. Thus, the only path for current through
the LM35 leads through the amp... is that a poor design?
Thanks,
mike
If anyone has suggestions of things I should check, I'd appreciate it.
Walter Harley
news:1138642726.2...@g43g2000cwa.googlegroups.com...
The schematic helps. Your path to ground isn't "through the amp" (since the
input impedance of the INA is up in the gigohms, that wouldn't be much of a
path); it's through the 47k resistor.
The LM35 draws nominally 60uA of supply current. That current has to flow
to ground (since it came from Vcc). In your case, it has to flow through a
47k resistor. 47k * 60uA = 2.8V. So the ground of the LM35 is 2.8V above
actual ground. But, since Vcc is 5V, that means you're powering the LM35
with 2.2V. It requires 4V, minimum, to work.
To fix it: Instead of connecting the - input of the INA to ground through
47k, just connect it directly to ground. And don't bother with the 47k from
the + input to ground; that is only necessary when the input is floating
with respect to ground.
mooseo
Thanks a bunch, that solved it! The explanation makes perfect enough
sense that I might not even make that mistake again, now that I
understand it.
Bill and Mark and everyone who looked at this, thanks for your insight.
I'm really happy to have this device working and to have made some
progress learning about circuit design.
cheers,
mike