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LMP91200: Faulty VOCM and VOUT signal, seems random

Part Number: LMP91200
Other Parts Discussed in Thread: REF3120, REF3220


I have a weird problem where both the VOCM and VOUT signal seems completely random, single-ended and differential.

The differential can range from -0,55 - 0,2 when I'm measuring calibration fluid at pH 7

The signal is not stable at all. The signal jumps +- 100mv every time my analog fluke multimeter (115) is updating. 


What I have tried:


This behavior is occurring on 9/9 LMP91200 chips I have tried out.



In the start with the first AFE's I did my own circuit and got this strange behavior. Later on I tried the exact same circuit as in the, "Wireless pH Sensor Transmitter (DevPack for SensorTag) Reference Design" project to see if it would make any changes in the outcome. Even with the same circuit I got the same strange behavior. The only difference I have between the reference design and the one I replicated is that,

1. My circuit is on a breadboard

2. I'm using hole mounted components instead of SMT, however, with the same characteristics. 

3. For REF I am using REF3120 instead of REF3220. (verified with LMP91200EVAL board and it works fine)

4. 3.3V source i am using MAX882CPA instead. (Verified with LMP91200EVAL board and it works fine)

5. LMP91200, REF and 3.3V source are SMT components, so I have used SMT adapters from to make them hole mounted.

6. BNC connector I have just jacked the pins into the breadboard as close to the LMP91200 as I could get it.



I have purchased the LMP91200EVAL board and that piece works completely fine in the same environment  with a very stable signal. I Used this board to verify that my REF signal and 3.3V source is good, which it was.


4. (to be tested)

I have now also purchased additionally 2x LMP91200 from another vendor to rule out the potential of a faulty batch. I have not yet received it so I will try at as soon as I get the package.


So basically, what I'm asking is, what is causing my irregularity signal? I believe I have tried out everything and I am out of ideas. It feels like I have missed something in the datasheet but I have read it through several times and cannot find anything that I have missed.


I have attached the schematic below, I just made it quickly in Visio and yes, I agree with you that it looks shitty but hopefully it is simple enough for you to understand my circuit :)

  • A ***, I see a mistake on the Schematic, of course the VDD on LMP91200 PIN 14 is connected directly to the 3.3V source

  • I am glad you found the issue. Thank you for sharing the problem and solution. I like to see what others have found and this information may help someone else.

    Good luck with your design.

  • I think there is a misunderstanding here. The issue is not solved. I was just stating that the schematic I attached does not correspond to my design. In the schematic the pin14 is not connected to the 3.3V source but in my design it is connected. I just missed to add the line in the schematic. The edit button was not there before when I posted but it is now. So the schematic is now corrected to mirror the actual design.

  • A few comments that may help.

    1) The probe is very high impedance (Ultra low current) What this means is that you will not be able to connect a multimeter to any of the probe connections.

    2) The probe INP is so sensitive that it requires a Guard band to protect it from outside influences. (Humidity, Flux,  Ionic contamination of any kind)

    a) Clean the board and dry it completely before use. (This includes No Clean Flux. It must be cleaned and removed. Bake after cleaning air drying is not enough)

    3) This circuit is nearly impossible to build and test on a breadboard. (If you must, keep all wires as short as possible)

    4) More than 90% of our customers having erratic probe issues or no measurements from the probe is due to incorrect settings for the probe.

    a) I recommend that you down load the EVM GUI  and run the program to find a probe close to your probe, to use as a beginning point for the settings.

    Let me know what you find.

  • Thank you.

    Washing the LMP91200 with soap and dry it completely solved the problem. Now I am getting stable signals.

  • Awesome, I'm glad it helped.