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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
 

ericgibbs

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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
 

MikeMl

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To approach the performance of integrated Instrumentation Amps, you would need to match resistors to better than 0.01%
 

crutschow

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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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ericgibbs

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spec

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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 ATM,

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

spec
 
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