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Design >> Analog Design >> Programmable gain variation in Sigma Delta modulator
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Message started by adesign on Sep 25^th, 2009, 3:09am

Title: Programmable gain variation in Sigma Delta modulator
Post by adesign on Sep 25^th, 2009, 3:09am

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