Instrumentation Amplifier

Thread Starter

Danlar81

Joined Apr 19, 2019
87
Hi Ian,
As this was an assignment task, I did not get to choose the Op-Amp to use. It was a specification that needed to be met. A further specification was having to introduce DC offset. Without OA5 and OA6 I would not be able to control the zero to five FSO.
 

BitsNBytes

Joined Mar 22, 2021
35
Hi Ian,
As this was an assignment task, I did not get to choose the Op-Amp to use. It was a specification that needed to be met. A further specification was having to introduce DC offset. Without OA5 and OA6 I would not be able to control the zero to five FSO.
Hopefully the main goal is that you learned something from the circuit and suggestions here.

A commercial solution would only use 2 or 3 op-amps as others have shown already.
 

MrAl

Joined Jun 17, 2014
11,263
Hi,
I am hoping I may be able to get some guidance and or ideas in identifying a problem I have with an instrumentation amplifier I have designed.

I have simulated the design in Circuit Lab, however I am not getting the expected output as desired during a DC sweep.
Essentially, I have a voltage difference test circuit which simulates a transducer. On the high side there is 1.4615 volts and the the low side 1.4585 volts, therefore there is a difference of three milliVolts (0.003 mV). I am trying to to achieve five volts output from the IA, therefore I have a gain of 1666.666. I have performed the amplification in two stages, the amplifier with a gain of 1000 and the remainder handled by the input circuit and RG.
I have introduced two summing amplifiers to remove the DC offset coming from the input stage.
When I run a time domain simulation I achieve the 5 Volt output as desired, however If I perform a DC sweep across the resistor in my test circuit R2 Vs the amplifier output, the output voltage does not begin from zero volts, rather the output only drops as low as 4.664 Volts, at R2 equal to zero ohms and 5 volts at 43 ohms.

I am wondering if there is problems with my DC offsets, however I am unsure what to expect as I have not implemented summing amplifiers in the instrumentation circuit before. They may be misplaced.
Initially what I did was set the DC offset at V4 for zero volts and V3 for 0.003 mV, but I was unable to see a change in the output. After further adjustments I set V3 for 2.48 mV which gave me a 5 Volt output when I performed a time domain simulation, but still have problems with the DC sweep.

Anyway that is a fairly big description which I hope is clear. I have attached my schematic, and have provided the voltages of various test points below, along with some graphed simulations. I would appreciate some feedback so I can get the circuit working as expected.

V1 = 1.4615 V
V2 = 1.4585 V
V3 = 2.480 mV (When Supply V3 is adjusted for -1.456743 V)
V4 = 0 Volts (When Supply V3 is adjusted for -1.456825 V)
V5 = -40.73μV
V6 = -45.78μV
Output = 5V

I'm not sure what is going on with the meter in the circuit VM1 as it is displaying 8.911 V even after a time domain simulation graphs a 5 V.

Regards

Hello,

What is it you are trying to measure.

It looks like you could benefit from using a chopper stabilized op amp. That will get you very low offset right from the start.
 
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