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ADS1114: Measuring different voltage than actual input voltage on A0 pin

Part Number: ADS1114
Other Parts Discussed in Thread: OPA387,

I have an ADS1114 receiving an input signal between 0V - 4V, the signal goes through an OPA387 set up as a buffer, just before the A0 input of the ADC i have a low pass filter for 80hz made from a 200ohm resistor and a 10uF ceramic SMD capacitor. Sometimes the setup works great and other times i have this weird issue where the voltage drops and varies wildly from the actual input signal voltage, i can measure this voltage drop just after the 80hz filter resistor, one end of the resistor has the actual voltage and the other end has a dropped voltage that keeps jumping around, always lower than the actual signal voltage though. The ADC is making correct measurements of said varying voltage. I have no idea where this could be coming from. I have tried to remove the 80hz filter capacitor and now there is just a 200ohm resistor in line with the buffer, nothing has changed though. The ADC does this in both single ended & differential modes. The input signal does not vary at all when the issue appears and keeps steady. Again this system has worked on occasions and there is no significant voltage drop from the 200ohm resistor when this system works as designed. Following is the schematic.

  • Hi Valerio,

    10uF is quite a large capacitive load for an amplifier to drive. What seems most likely from your description is that this circuit is having op amp stability issues, and the output is oscillating. The system may be marginally stable, which is why sometimes it works, but can easily start oscillating. 

    This is more an issue with the buffer than with the ADC, as you said, the ADC is making correct measurements of said varying voltage. 

    I would suggest using an oscilloscope rather than a DMM to be able to confirm if there's oscillations. 

    If you are unfamiliar with this concept, I would suggest firstly to take a look at our video series on this topic:  Precision labs series: Op amps | TI.com, in particular the stability section. This explains this concept in detail and will give you a better idea of what causes the behavior you are seeing. 

    You might have to change the ADC input filter to something the buffer can drive, by lowering the capacitance, increasing the resistance, or looking into compensation schemes to make this circuit stable if possible. 

    You may also find these resources helpful:

    Do-it-yourself: Three ways to stabilize op amp capacitive loads

    How to resolve Op-Amp stability issues using SPICE simulations | Video | TI.com

    Best Regards,

    Angel

  • Hi Angel, i will study the material you sent because it looks super useful, but do note that i still had similar issues even after i removed the 10uF capacitor from the circuit, the op amp mentions in its datasheet that its cool beans with being a buffer for ADCs so i hope i wont have to change my op amp. In other notes my teacher says this may be a power supply issue and i will test that theory also and hope for the best. My issue remains unresolved so any ideas are very very welcome, i will get back to this post when i have more information.