Hi all,
I was wondering about how well matched the PSRR is between the two channels of a dual opamp.
Background:
I have a physics package that has two outputs. Each output can be modelled as a floating voltage source with about 1Mohm source resistance.
The voltages are tiny (uV), about the same amplitude as the Johnson noise of the source. The bandwidth is DC to about 50kHz. I want to amplify each output voltage by about x 100 before passing to a two channel 16 or 24 bit ADC.
One of the measurements is differential - I'll do this by subtraction in the digital domain after the ADC.
Back to the question...
I was thinking of using a regular MOS-input opamp in the regular non- inverting amplifier configuration to amplify each channel before the ADC. I'll bias the inputs to 1-2V above gnd to match the ADC input requirements.
I can also use the opamp datasheet PSRR to work back to a noise specification for the opamp positive power supply rail.
If I'm only interested in the noise when I'm taking the differential measurement, can I relax the power supply noise requirement? This would rely on the transfer function from the opamp +ve rail to the output to be well matched between the two halves of the dual opamp.
In practice, how well matched are they?
I haven't chosen an opamp yet. I have some MAX4477 in the junkbox, but I could potentially use anything under a few dollars.
Thanks, Allan