Re: about offset of amp
Classical chopper means that you "chop" you input signal by multipying it with another signal which changes its value periodically from "1" to "-1" and then you do the same with your output signal. As offset internally to your system doesn't change its sign, the output signal (after the output chopper) is the signal you should have ideally, plus the system ofset multiplied by a an alternating series of +1 and -1. So, by filtering out the varying offset signal, you can recover the output signal with no offset. The multiplying operation by +1 and -1 is performed by reversing the polarity of the input and output signals by means of switches.
Another technique also called chopper stabilizations consists in having two amplifiers, each one with an anciliary input to cancell offset. One amplifier, called the main amplifier is always connected to the signal path. The other amplifier, called nulling, makes an auto-zeroing step to cancell out its own offset, it store it into a capacitos and feeds this offset to its own anciliary nulling input. Then it looks for the signal at the input of the main amplifier and apply a correcting voltage at the nulling input of the main amplifier, storing also this voltage into another capacitor. The auto-nulling and main-nulling steps are repeated periodically. You can look for information about this method by searching "zero offset amplifiers". This is the method used by commercial amplifiers called, zero-offset or zero-drift by burr-brown, analog devices, etc. As you always cacell the offset at any moment, the offset drift with temperature (maybe the most anoying fact of offset) is removed. Properly implement this method requires a lot of analysis, due to there are many tradoffs between gains, bandwidths, chopping frequency, etc.
Ping-pong consists in having two identical amplifiers which, each one at its turn, connects to the signal path, while the other is in the auto-zero stage. Then the amplifier which was at AZ, connects to the signal path, while the first one makes its AZ and so on.