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sigma delta adc problem

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hoka_89

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i am working on a sigma delta adc second order three bit and i do the schematic design on cadence and i get 100 db and it is ok when i make the layout and make post layout it become 93db i found in the drawing of fft a pulse at another frequency 200hz(my input frequency is 100hz and band width is 200)
and this is the picture
 

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philcorb

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That is the 2nd harmonic. Is it a differential design? If so, the layout is not symmetric and introducing differences between the positive and negative sides.
 

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