AD7862 conversion noise at sample rate

Question

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.

FormerMember · Accepted Answer

Michael, 
 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 
 Harry

EZAdministrator · Answer

This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,
EZ Admin

Electronic Design Services - PartnerZone

Products In This Thread

AD7862 conversion noise at sample rate