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| ADC MCP3204 measuring completely off?! |
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| nemail:
Hi I'm still working on my self made Lab PSU which was inspired by Daves µSupply. Most things work quite nice and I'm making good progress, however the ADC MCP3204 was giving me quite a hard time recently. I had this issue before but thought i fried something by back-EMF (is it called so?) by a high rpm DC motor when I didn't have a protection diode yet in my circuit. But now I got a new revision PCB from JLCPCB with a protection diode and some other improvements, soldered everything together and tested it and everything was good until suddenly the ADC was off again (don't know what i could possibly have done to trigger that). Ok, now some hard facts. - Schematic and board layout are attached - Oscilloscope shows some SPI noise from the display and the ADC on the ADC VREF and VCC but only a couple of mV peak to peak. - Oscilloscope and multimeter measure e.g. 5.7V on the output (bang on like set via the DAC/opamp control loop) and the ADC measures 8.295V through the opamp and voltage divider. - this seems to disappear when the circuit wasn't used for a couple of hours. after like 5 minutes the voltage which the ADC reads rises again. - it does not matter what voltage I set, the ADC is almost always way off. 3.3V show 4.955V on the ADC, 10.2V show 10.215V(! this works?!) but 12.285V show 17.7V on the ADC (wtf). 300 mV measure 700mV on the ADC. I always looked at the output voltage with the multimeter and the scope, it was always bang on and clean as a whistle so definitely the ADC is measuring garbage - i already tried to slow down the measurement to 1 measurement per second because I read that polling the ADC too often makes the readings go bonkers. Does anyone got an idea what's going on here? I have another version of this board on the way to me where I got rid of the external DAC and ADC and where I'm using the Teensy integrated ones, just for kicks to look what performance I'll get out of that. naturally this design got much cleaner and more slick so maybe I got rid of some noise problems there as well but nonetheless I'd love to find out whats going on here. If I missed something, if you need any more details, let me know. Kind of tired and it is likely that I missed something to point out. Thanks! EDIT!!! LOL I definitely forgot to mention where the signal the ADC measures comes from. It is the signals coming from IC11A and IC11B. Basically ALL ADC channels are off, they all measure wrong values so there seems to be some issue with the ADC itself. I didn't socket it and I don't think I have a spare (new) one here but I bet if I'd swap it out, the issue would be gone for some time. |
| MasterT:
I wouldn't drive adc inputs directly from +15V powered OPA, capable to supply +-65 mA. |
| nemail2:
sorry, goofed up my other account (mail verification not coming). not even with the voltage dividers? any suggestions how to solve this? thanks! |
| MasterT:
Resistive voltage divider is o'k, below 10k and you are safe and have accurate results with low sampling rate like 100ksps. If error still concern, keep sampling rate constant, though voltage drop caused by internal S/H of the adc 'd be constant and easily calibrated out. For perfectionists, than resistive dividers you simply buffer with OPA powered by same PS line as adc. |
| Dave:
This may be a really dumb suggestion, but you might as well check it just in case: Check if the IC is properly soldered to the board. I once spent a day and a half dicking around an ADC that would spew erratic measurements, only to find that the QFN chip wasn't sitting quite as flat as it should have been. There was the slightest gap between that particular input's pad and its respective land on the PCB, just enough to let the surface contamination let *something* through and give me seemingly random results. Learn from my mistakes! |
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