chopping frequency and bandgap bandwidth selection issue

chopping frequency

Hi all,
I'm designing a chopping-op based bandgap such as attachment and I encounter an issue about the selection of chopping frequency and loop bandwidth:

In attachment, bandgap itself's Loop 1 can suppress the high frequency parts if its bandwidth (open loop unity-gain bandwidth) is lower than chopping frequency. But it will result in bad respondence of chopping signal.

The other way is use Node 1 to suppress high frequency parts with large cap. It seems better.

Which way is better? How to select chopping frequency and bandgap bandwidth? Thanks!

chopping frequency usage

Some preliminaries:

1. If the opamp have some mV offset and the diff-stage act as a gm-cell you have a differential switched current.

2. If you crossswitch these differential current in synchronism with the input voltage switch to an output current mirror the offset of the mirror does not get chopped out!

Because of 2. you need after the differential current switch a less offset contributing component. For instance an active PMOS load with high Vdsat and big area or a poly resistor.

Because of 1. the differential current should be filtered by a big cap. Then the dominant pole is set by this cap. Setting another cap as dominant is overkill.

From this cap voltage you need only a low voltage gain to the gate drive of the PTAT current sources.

The next spin of circuit desgin excellence is a AC-amplifier which detect the differential current, amplifies to a good level, synchron demodulate, filter and feed back to the differential input stage so to minimize the offset current ripple. That is the chopper noise regulator.

Have fun!