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OPA2388: Drifts in High temperature (85C)

leonid Furman
Prodigy 40 points
Part Number: OPA2388

Hi,
We are using the OPA2388 as a buffer for a differential output sensor. The opamps connected as buffers, with a 100 Ohm resistor in feedback. Each opamp buffers one of the differential outputs. We get good results, besides a drift we see at 85C. We've tested the output of the sensor before the buffers and after the buffers, and clearly see that the signal is ok before the buffers, but drifts after it. in all other temperatures, we don't see any drift (-40~85C). This drift stays when the temperature goes back to room temperature, and we get a hysteresis.
The drift looks exponential, like capacitor-discharge, and it drifts about 0.4mV in 45 minutes. Do you have any suggestions or insight on what may cause it?
Thanks,
Leonid.

  • 0
    TI__Guru* 93105 points

    Hello Leonid,

    The drift you are observing would be a very uncharacteristic behavior for the OPA2388. The operation of a properly functioning chopper op amp should automatically work to reduce its voltage offset and drift to a level approaching zero volts. Therefore, if these are correctly functioning OPA2388 op amps something else must be going on in the circuit to produce the said high drift at high temperature.

    I have some questions about the OPA2388 application circuit:

    1. Do all of the OPA2388 op amps tested in this circuit exhibit the very high drift at high temperature?
    2. Does this drift occur on all PC boards, or certain individual boards?
    3. What kind of supply or battery is being used to power the OPA2388?
    4. What is the input voltage source and what load is applied to the output of each OPA2388 amplifier section?
    5. What are the resistor characteristics including rated temperature coefficient?
    6. Is the OPA2388 PC board assembly fully cleaned after soldering?
    7. Were the OPA2388 op amps obtained directly from TI, or one of TI's authorized distributors, or from another source?

    I suggest trying these things and determining if they make a difference or reveal a problem:

    1. Connect a DSO with a 10x probe to each OPA2388 output while drifting the device. See if there is any evidence of an unexpected waveform, or oscillation occurring at any temperature.
    2. Remove the 100 Ohm resistors and replace them with 0 Ohm shunts. See if that makes any difference in the drift.
    3. If you don't have a rigorous cleaning procedure for the PC board do clean the board as recommended here and then retest. Note that this is for water based solder flux. Subject the PC board to an ultrasonic cleaner using clean DI water as the cleaning medium. After cleaning, dry the board in a bake out over at reasonable temperature to drive out any remaining moisture. It is recommend that the board be cleaned twice in succession to fully remove any residual solder flux.

    Let us know what you find.

    Regards, Thomas

    Precision Amplifiers Applications Engineering