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AD7862 conversion noise at sample rate

I am diagnosing noise on a legacy design that has an AD7862 ADC.  Typical signal is 2 to 5 Hz sinusoid.  Sample rate is ca 7500 Hz.  Problem is that converted digital signal is affected by at least +/- 5% by the sample rate.  If I do an FFT I see the sinusoid peak and I see the 7500 peak about 20% of the former's magnitude.  Otherwise very clean.  ADC has 100 nF cer bypass cap to Vcc. Vref has 100 nF to ground. Vb1 = Vb2 = AGnd.  /CS = AO = DGnd.  Not sure what is wrong with this setup, but the noise is unacceptable.

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