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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.
Hi Rahul,
Thank you for the reply. Let me try to answer your questions:
1) Yes.
2) No because it happens below 2.4V (down to about 2.35V).
3) Yes (Scope capture of LN6 = 2.414V [Measured with Fluke DMM])
I would like add that the chip is operating at VDD = 3.3V. I apologize for not adding this in the first post.
Regards,
Sharma
Hi Sharma,
Thanks for the additional details.
Figure 10 in the data-sheet shows the transfer characteristics of the PGA. The PGA has upper and lower compliance limits beyond which its operation is non-linear. If the output of the PGA exceeds these compliance limits, the corresponding valid bit is set.
At 5V VDD, the lower compliance limit is approximately 3.8V which is almost 75% of VDD. Hence at 3.3V VDD in your system the lower compliance limit is 2.475V which is very much marginal for the input you are sampling.
To confirm if this is the root cause of the problem you are seeing, could you please increase device VDD to 3.6V and sample [2.5V > Vin > 2.35V]?
Thanks.
Regards,
Rahul
Hi Rahul,
Thank you for your help and sorry for the delayed reply. Yes, i have tested with higher VDD as suggested and got the chip to work as expected. However, since you have brought up Figure 10, we are now curious:
1. How did you apply that to Single Ended configuration (what about the upper compliance?)?
2. Is the compliance percentage relates to the references used by the comparators for fault checking ? Is there a typical value or min\max range to consider or the 75% based on the graph is a good rule of thumb?
3. Any input as to why does the replacement chip solves the problem even at the same VDD ? Is the compliance level checked by the comparators may be affected by temperature (resistive divider?) ?
Sorry for all these questions. We are a bit confused by the compliance limit and couldn't find much information on the fault detecting comparators. But, up to this point you have helped a lot and we are able to eliminate some doubts.
Thank you.
Regards,
Sharma