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Precision Data Converters
Precision Data Converters Forum
I have been using this device for over two years with perfect results. However, in my most recent productin run, I have come to a problem that I can't seem to solve. When requesting a reading, 25-30% of the time a Large Positive Offset appears, apparently added to my reading. This problem persists for as long as I request readings without shutting off power, indicating some form of latching. If power is removed from the board, the error generally clears itself, although occasionally, recycling power more than once is required. My measurement routine is to turn power ON to AVdd (+5.0V) and DVdd (+3.3V), take a temperaure reading, turn power ON to the Amplifier and reference followed by 16 voltage readings, and remove power from the Amplifier and reference.. I then sum all 16 readings, shift 12 places right and display. My scale factor is -100 counts per pound, so if I am weighing 10 pounds, I should expect a raw reading of about FF,FC08hex or -1,000dec. When the problem occurs, all 16 readings will be about 7F,Fxxxhex, a fairly large positive number. At least on some occasions, when the problem appears, the temperature reads 80,000hex . I have not yet investgated if this temperature reading occurs every time the the Large Positive Offset appears, or only occasionally..
Here are some of the tests I have made to identify or correct the problem:
1. In order to allow the Amplifier (OPA2333) and ADS1225 to reach full scale, power to the amplifier and to ADS1225 pin 15 (AVdd) are both at +5.0V, while the reference is +4.096V, VrefP, pin 14, with respect to VrefN, pin 13, and VrefN is +0.452V with respect to ground. I have tried moving the reference voltage down to +4.096V with respect to ground, (connected VrefN to ground.) This has no effect.
2. Normally, I turn the power to the amplifier and reference ON near the end of the temperature measurement, so I tried turning them ON at the beginning of the temperature measurement to no avail.
3. I tried leaving the power to the amplifier ON continuously, again to no avail.
4. I have tried to widen the start pulse, with no effect.
5 I have slowed down the data retrieval clock rate, with no effect.
6. I have monitored the voltages very carefully, and especially the Reference Voltage, since if there were a dip in the reference voltage that took it close to groundm the reading could be very strange, but all the voltages look fine, as does the output from the amplifiers.
7. Power sequencing can occasionally cause a latch-up, and I have compared the sequencing of power from my current production with previousl production, but see no difference.
8. I have tried ADS1225 from an older lot with no effect.
9. There is no tag for the ADS1225.
Suggestions are gratefully received, and appreciated.
Welcome to the forum! Even though you have been descriptive with your design , it would be helpful to see a schematic. Might it be possible that the controlling processor might be powering one of the digital pins? Another possibility is differing charge rates on capacitors connected to the supplies or other inputs. This can affect the sequencing.
How is your debug going? Have you found anything? Are you able to share the details of your connection to the ADS1225?
Turns out that the ADS was fine, but the uP firmware made it appear so. The resident programmer has apparently found his problem, and though is was intermittent, I believe we have done enough testing to confirm that the problem is solved.
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