Very low DC gain of pstb simulation

Hi:

I am simulating a folded-cascode amplifier with chopper, based on SPECTRE 18.

The DC gain without chopper is about 70dB (simulated with ac and stb).

When I use pss+pac or pss-tran to simulate or derive the open-loop gain, the result is also about 70dB.

But, pss+pstb shows different DC gain, which is only 20 dB. Meanwhile, the GBW and high-frequency Bode Plot are still same to ac and stb results.

Thanks!

Teddy