Common Mode Voltage

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Hi All
I have designned differential amplifiers using seperated op-amps and external resistors (in retrospect unfortunatley I should have used either instrumentation amplifiers or difference amplifiers with laser trimmed internal resistors). I am using 1% resistors and have noticed that the common mode is having an effect on the gain that I wish it changes i.e. changes depending on the input voltage. In the application space I am working the difference in voltage between inputs will be between 0.1-1V. I was wondering if anyone could suggest a more suitable op-amp for this application, I am also going to try switching to 0.1% resistors to see how much of an effect that has but any help would be greatly appreciated.
Kind Regards
Art
 
hi Art,
I use the AD623 IA, it works on a single supply.
Do you have a LTS sim or diagram of your differential amp.?
E
 
To approach the performance of integrated Instrumentation Amps, you would need to match resistors to better than 0.01%
 
1% resistors will give a worst-case common-mode gain of .02 for a differential gain of 1 amp.
 
This is the simulation of the PCB I have designned, the voltages that are mentioned match what I am after I have made the inputs sinusoidal just to see what kind of response I would get if there was a variation in voltages. I also test CM and there is an obvious effect from the resistors. I am just wondering if my choice of op amp was also poor? (I know now in the future I should just use an instrumentation amplifier or difference amplifier), so if the choice of op-amp was poor can anyone suggest a standard op-amp that I could use that would not be effect by common mode as much, I will be changing resistors to 0.1% or even less. Currently I am using the following op amp: STMicro MC33174. I also tried this with Texas Instrument TL074ACDR.
 

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Hi ATM,

In addition to the points made by the other members above, I would recommend changing to a Texas Instruments OPA2197 opamp: https://www.ti.com/lit/ds/symlink/opa2197.pdf

spec
 
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