aaron_do wrote on Jul 29th, 2014, 7:45pm:Anyway for the opamp, the gain needs to be even higher to take into account the closed-loop gain right?
A bit higher so that isn't the limiting factor, but the loop gain doesn't figure into it (to first order). For example, suppose you had a 72dB open loop gain opamp. This is (barely) good enough for a 12 bit ADC. (Actually I guess it might need to be 78dB to get within 1/2 LSB.) If you wanted to get 1.5 bits in the first stage your gain would be two and your loop gain would be 66dB. This isn't a problem because the output only has to be good to 11 bits. Similarly, if you were doing 3.5 bits in the first stage you'd want a gain of 8, resulting in a loop gain of 54dB. Again, this is ok since the output only has to be accurate to 9 bits. Does that make sense?
In the real world you'd want more margin... I take as much as is easily achievable so I don't have to worry about it over corners. Or whether it is 72dB or 78dB
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. Also, the input capacitance of the opamp reduces the loop gain.