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.
No excerpt attached??
I suspected a pc board layout problem. See the "headphone amplifier" section of my article:
Planet Analog - Articles - Op amps: to dual or not to dual? (Part 1 of 2)
Planet Analog - Articles - Op amps: to dual or not to dual? (Part 2 of 2)
I'm also surprised that the ADA4000-1 and PGA204 go directly to the ADC with no small R-C
anti-aliasing filter. Even though the AD7862 input is rather benign compared to a traditional
SAR ADC, theory says for any ADC you need an AAF ahead of it. Just good engineering
practice. You can always remove the cap and replace the R with a zero ohm resistor after
evalatuation in the lab. See:
Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter | Analog Devices