Instrumentation amplifier problems

[Simon]

I tried building a circuit with an instrumentation amplifier. The intention was that I would take a signal from a thermistor and remove the DC voltage I did not require and then amplify what is left to make full use of my voltage rail as I am then feeding into an ADC as I don't want to be wasting bits. However despite a voltage of just over 3 V this circuit outputted nothing. Well a very very low voltage practically zero. What did I get so wrong?

[bktemp]

What is the voltage between pin 1 and 4?
In your configuration pin 4 must always be more positive than pin 1, otherwise the output will be stuck at 0V.
Did you swap pin 1+4? The datasheet says pin 4 is the non inverting input.

[StillTrying]

Yep, looks like you may have 1,4 reversed.

And just for good measure, if you have Vs=5V, Ref=0V and a CM voltage about 3V, then you might still have another problem, it looks like you have to keep the CMV at 1.9V. A 'normal' op amp might be easier.

[danadak]

Page 5 shows a thermistor setup

http://www.ti.com/lit/ug/tidu491/tidu491.pdf


Regards, Dana.

[Simon]

ok yes, sounds like I have inverted the inputs, that would completely explain it. Yes I found the input/output graphs a bit confusing, it almost sounds like they say it can do one thing and then say something else.

What would be a good rail to rail opamp to do this with, that's rail to rail on inout and output. Fortunately I didn't want much gain and still get a good reading bypassing the amp for the sake of a prototype.

[Simon]

I could run it at 24V if that helps

[rstofer]

This sounds a lot like 'offset and scale' - remove the DC offset and scale the desired signal.  If so, Chapter 4, Section 4.3 of "Op Amps For Everyone" deals with this.  They also discuss it in terms of a single supply rail-to-rail op amp.

http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

[Kalvin]

Please, show the schematics how you have connected the thermistor and how you have biased the thermistor.

[Zero999]

If you want to run off 24V, yet interface with an ADC, then you don't need a rail-to-rail op-amp, just one which will operate at the negative rail. There are plenty of op-amps which will do this and have a low offset voltage, which I presume is important: OPA234, LT1006, LT1013. If all you need is something cheap, then there's LM358 but the offset voltagemay be too high for your application.

[Simon]

If you want to run off 24V, yet interface with an ADC, then you don't need a rail-to-rail op-amp, just one which will operate at the negative rail. There are plenty of op-amps which will do this and have a low offset voltage, which I presume is important: OPA234, LT1006, LT1013. If all you need is something cheap, then there's LM358 but the offset voltagemay be too high for your application.

Indeed that is what I'm starting to think.

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[Simon]

Please, show the schematics how you have connected the thermistor and how you have biased the thermistor.

The thermistor goes from input to GND with a bypass cap and then pulled up to 5V

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