I am diagnosing a legacy design that uses an AD7862 SAR ADC.

• Input is 500 Hz, 2.4 Vpp sinusoid, relatively low noise.
• Digitized output adds a noise component waveform +/- ca 0.25 Vpp.
• The FFT spectral peak of the noise is exactly the sample rate, 7500 Hz. I expect a small amount of sample rate noise, but not this extreme amount.
• Of course I can FIR it after the fact, but would rather find root cause.
• The digital control signals to ADC are ringing to the point where they almost reach V(INH) and V(INL) thresholds. Could be false triggering? Affecting the SAR process? Otherwise digital signals are nominal.
• Happy to share diagrams, etc.

I am diagnosing a legacy design that uses an AD7862 SAR ADC.

• Input is 500 Hz, 2.4 Vpp sinusoid, relatively low noise.
• Digitized output adds a noise component waveform +/- ca 0.25 Vpp.
• The FFT spectral peak (7484 Hz) of the noise is close to the sample rate, 7782 Hz. I expect a small amount of sample rate noise, but not this extreme amount. The noise waveform appears sinusoidal as well, not as a typical quantization.
• Of course I can FIR it after the fact, but would rather find root cause.
• The digital control signals to ADC are ringing to the point where they almost reach V(INH) and V(INL) thresholds. Could be false triggering? Affecting the SAR process? Otherwise digital signals are nominal.
• Happy to share diagrams, etc.