Hello Chungmnig,

You are right: the low pass filter is meant to remove spikes.
One thing you should aware of: even if spikes are removed,
they contribute to the offset. The offset is strictly proportional
to the frequency (that is why high precision chopper use low
chopping frequency).
I got a chopper back from the fab last year. It is paced at
2MHz (a bit like yours). I have measured an offset 150uV
due to charge injection. This offset was strictly proportional
to the chopping frequency (<100uV at 1MHz).

If I were you, you would try to decrease the amplitude and
the duration of the spike.

Good luck, Jerome.