Question on Sigma-Delta ADC

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Rainleo

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Hi, guys,
I am working on a receiver design. My sigma-delta ADC is running with a clock at 26MHz. Now I have a problem with the ADC output. There is always a tone located at around 13MHz. The tone is radiating into everywhere and destroy my noise figure at frf=n*13MHz. Anyone know what's happened?
 

If 26 MHz is the sampling clock, 13 MHz should be far out of signal band. It guess, you are observing normal operation with mid scale bias point.
 

Do you mean that delta-sigma ADC will always have a tone at around fclk/2 at its output? Thx.
 

the origin of the tone is various. they can be from pattern noise or mismatch of feedback dac components.
 

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