Hello,
I’m testing a custom PCB with an ADS1282 that I did which is plugged to the ADS1282EVM motherboard. My PCB replaces the ADS1282EVM daughter board equipping the ADC. I first tested it without the resistors at CAPP and CAPN to push the idle tones out of the passband. (ADS1282 Datasheet, section 9 and applications information). Indeed, I found some tones that looked like idle at frequencies 3.2Hz and a few harmonics.
I later added the two 75kΩ ±0.1% resistors between CAP and Ref as suggested in the application information. The tones disappeared but a new one arose at 1461Hz.
Red plot: without offset-inducing 75kΩ ±0.1% external resistors.
Blue plot: with offset-inducing 75kΩ ±0.1% external resistors.
I’m using the internally shorted (400Ω) input channel in the plots to show the problem doesn’t come from my input stage or connections. Nonetheless, the same behavior is repeated when I use Ch1 or Ch2 short circuited externally.
When exploring the blue signal in the time domain the 20mV offset is there.
The 1461Hz peak disappears when injecting a signal using a precision function generator.
I tested with PGA=1 and PGA=64. I only found this peak with PGA=1.
I have two units of this PCB and just one of them exhibits this peak at 1461Hz. The other one also showed idle tones and they just disappeared after adding the offset resistors, with no other peak appearing in return.
Do you know where this peak may be coming from?
Would you have any suggestion on how to make it go away? Just decreasing the 75kΩ ±0.1% resistors to 50kΩ ±0.1% maybe?
Could it be the symptom of a malfunction somewhere else?