HdrChopper
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Demodulating inside the opamp is a technique that has been used recently and successfully. It has two main advantages:
1) It allows for chopping SE amplifiers as you already suggested 2) It allows for chopping at high frequency, which usually cannot be done if the amplifier is chopped outside the feedback loop.
#1 works since demodulation is still done differential, prior to SE conversion. Although the second opamp stage is not included in the chopper - thus contributing with offset - its offset contribution is small enough such that it can be neglected assuming the chopped first stage gain is high enough. This is the case for most opamp designs. For example assume your AOL is 80dB = G1[dB] + G2[dB], where G1 is 60 and G2 is 20, and your closed loop is 20dB. If the second stage gain generates 1mv of offset, such offset (which is NOT chopped) is referred to the input by dividing it by 1000 (60dB). Thus the 2nd stage contributes with 1uv input offset. That will translate into 10uv output offset (20dB closed loop gain). From this example you can see that generally speaking there is no need for chopping a second gain stage.
As for #2, just consider the demodulation is performed within the first stage, which is a [b]wideband[b] gain stage. Such wide BW allows for faster chopping compared to the case where the demodulation is performed outside the loop: in that case the BW is the closed loop BW which is ACL times smaller than the GBW of the opamp (ACL is closed loop gain).
In order to check if the amplifier is offset compensated, assuming you chop the amplifier, just add an ideal voltage source in series with one of the opamp inputs - but after the input modulation switches - and without applied signal. By running the transient analysis you should see a triangular-like ripple (modulated offset) around 0v (or input CM in the case of a buffer). If that output signal is perfectly centered around 0v (or input CM) that means your amplifier is offset-cancelled.
Hope this helps Tosei
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