• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 48 subscribers
  • Views 186 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA928EVM: OPA928EVM

Andrew Livesey
Prodigy 10 points
Part Number: OPA928EVM
Other Parts Discussed in Thread: LMP7721, OPA928,

We are evaluating the OPA928 in Trans impedance Mode using the Evaluation board, using a 2T glass feedback resistor mounted above the pcb enclosed by the screening can. The board is supplied from a clean +/- 2.5V, the input to the Op amp is connected to Ground via a 22pF capacitor to simulate our Electrometer assembly, the output is being logged to quantify the noise levels. We have found the base noise to be in spec but we are seeing occasional random splikes ( approx 5 in 10 mins) with amplitude between 5 and 10 times the average base noise. The spikes seem to be unique to this board as we have run similar test using an LMP7721 device without seeing them.

Has this feature been observed before? We have tried all sorts of shielding different power supplies etc with no difference

Any help or ideas appreciated

driftNC 290823 plot7.xlsx

  • 0
    TI__Expert 8145 points

    Hi Andrew,

    I have seen a similar behavior using the OPA928EVM, typically caused by electrostatic coupling from the environment. Paul Grohe's article Design femtoampere circuits with low leakage Part 2 discusses this effect in detail. You can monitor this effect in real time by simply waving your hand around the assembly during a measurement.

    You can expect to see noise like this if the assembly or connected cables are moved or bumped during the measurement. I have even noticed this effect by moving around or walking too close to the measurement assembly. I believe your specific application circuit may be even more sensitive to electrostatic coupling due to the extremely large (in both magnitude and physical size) feedback resistor which tends to act like an antenna for electrostatic coupling.

    When taking low-level measurements with the OPA928EVM, I typically place the EVM in an additional shielded enclosure connected to ground and ensure that no bodies are moving near the assembly. If the assembly is placed on a large table or bench around other lab users, someone working on the other end may bump or shake the bench causing electrostatic coupling.

    One thing to be aware of, is that the shield clips do not always provide a stable mechanical connection to mount the small RF shields to the EVM. If the shields are frequently removed and replaced, the spring-action of the shield clips tends to wear out which will allow the small RF shields more freedom to move or vibrate in response to acoustic or mechanical disturbances. This is the "electrostatic microphone" effect that Paul discusses in his article linked above. I recommend inspecting the mechanical stability of the mounted RF shields. If the shield clips are loose/damaged, you may remove these shields as long as the assembly is placed in an additional shielded enclosure during measurement. Be very careful not to allow contamination of the sensitive input nodes when these shields are removed.

    Please ensure that the mounted RF shields, any connected cables, and the entire measurement assembly are mechanically stable without any movement or vibrations during your measurement. Refrain from moving or walking near the test assembly during measurement and prevent other lab users from doing the same. You may compare the measurement results under these conditions to another test in which the assembly is frequently bumped or moved to verify that the spikes are caused by electrostatic coupling.

    Best Regards,

    Zach