I want to know total noise I am feeding into ADC and I have learnt about Opamp Noise and resistor noise thanks to this forum.
Now I want to know the contribution of resistor in my overall noise due to 'current noise' of Opamp and for that I need to find equivalent source resistance which I can just multiply with Current noise to get voltage noise (am I right) and also the equivalent feedback resistance through which this noise current is flowing to get voltage noise due to it.
I read somewhere that equivalent resistance in a wheatstone bridge is nothing but the value of a single resistor but only if all resistor are of same value and my case doesn't seem to fit the criteria, the wheatstone bridge I am using is SM5652 pressure sensor.
what will the equivalent source and feedback resistances in this case?
Best Answer
A link to your opamps.
First I'm guessing the R of the pressure sensor is less than the 2k ohm you have in series.
If so you can mostly ignore the R of the sensor.
The opamp has very small current noise.. (see page 4.)
i-noise < 1 fA/rtHz, so even with 1 Meg of resistance that's still ~1nV/rtHz of voltage noise. (Just ignore the current noise.)
The voltage noise of that part (~80 nV/rtHz) seems high to me, but maybe that is typical for autozero (chopper stabilized?) opamps. Or perhaps someone else can recommend a better part.
Edit: about RC filters. Since you are filtering the input to ~1kHz (2k ohm and 0.1uF) you might as well do that on the output too. (bump up R13 and 14)