Hi,
I would like to seek some clarifications on the problem that i’m having with ADS7871. I noticed that for few chips that i’m working on, there are some weird issues that seems odd. First of all, i’ll like to point out that these issues do not normally occur with “good” chips but only happens with a few “bad” chips. These boards are machine soldered.
The ADS7871 is operating in single-ended input configuration. The gain is set to 1 and reference is set to internal (2.5V). I have verified that the reference voltage with a calibrated Fluke multimeter. The issue that i’m having is that when i try to sample 2.4V, i noticed that on the “bad” chips, i see frequent occurence of OVR bit set to 1 in Register 0. This error means something is wrong with the PGA side according to the datasheet. A check on the Register 2 reveals that PGA Valid 5 bit is triggered (Register 2 value = 0x20). This seems odd because i have verified that the configuration is indeed set to single-ended input. I have also measured the voltage on the ADC channel pin to make sure it is not hardware issue. It is just that the issue usually alternates between good reading (OVR bit is not set) and maybe bad reading (OVR bit is set) which makes it troublesome. Because it seems like at some time, the chips work okay but other times it just goes funky (PGA Valid 5 bit is set and it got nothing to do with the intended application which mainly deals with positive range).
Any idea as to why this is likely to occur and under what circumstances ? Take note that once i replace the “bad” chip with a new chip from the parts bin on the same board, the problem goes away.
Much appreciated if anyone could shed some light on this problem. Thank you.