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Design >> Analog Design >> Delta-Sigma 3rd harmonic problem
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Message started by georgejor on Nov 10^th, 2003, 7:52am

Title: Delta-Sigma 3rd harmonic problem
Post by georgejor on Nov 10^th, 2003, 7:52am

Dear All,
I have designed a third order delta sigma modulator with switching mixer. They are implemented by switched capacitor.
The FFT result shows there is a 3rd harmonic. I have tried to triple the biasing current of OpAmp to 800u and confirm there is 60degree phase margin. But the 3rd harmonic still at -75dB level.
Could anyone suggest some idea for me to minimize the 3rd harmonic and tell me where it may come from?
THANK YOU VERY MUCH!

George

Title: Re: Delta-Sigma 3rd harmonic problem
Post by wolf on Jun 30^th, 2004, 2:07am

First you have to verify by matlab scripts with the same input signal if your 3rd harmonic is a spurious coming from the SD loop. If not, you could verify the linearity of your S&H which should probably be the issue. An unsufficient bw in your ota will more rise your inband-noise than produce harmonics.