Hi,

I'm designing a Sigma Delta modulator with programmable input gain.
The circuit for the first stage is attached herewith.
The default cap ratios are:
C3/C1 = 4/1
C3/C2 = 4/1

With default cap ratios and full scale input(say Vf), the SNR at the output is coming OK.
When I change the cap ratios as under:
C3/C1 = 4/0.8
C3/C2 = 4/1
and increase the input to 1.25 x Vf, the SNR degrades drastically by almost 40dB.

Could anyone explain me this behavior?

Thanks in advance for your time and comments.

Best Regards,
ADesign