Problem with sampling low voltages by using AD7674

[djnik1362]

hi guys

we have a serious problem with using AD7674 A/D converter.

we use this device with ADR435 reference voltage.

we have a DC signal to sample between 0-5V differentially.

our problem is in voltages between 0.8 V ~ 5 V the A/D works good
but in voltages below 0.8V it shows the signal is zero or near zero.

in our PCB there is no separate digital and analog ground.

we use some Power-One DC/DC to produce our supply voltages.

Can you help us.
We need your help.

Thanks for your support.

 

[dick_freebird]

I'd begin by putting a 'scope to the ADC input, returned to ADC
analog ground, and triggered off the DC-DC sync and any other
clocks you can find. You're looking for a synchronous input blip,
perhaps coupled in through the input filter (if any).

Does "works good" mean you are actually meeting accuracy
specs across the remainder of the input band? That you have
taken this data?