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OPA376: OPA376 non-inverting connection problem

Sergay Malkin
Prodigy 70 points

Part Number: OPA376

I try to use OPA376AIDBVR as high impedance non-inverting amplifier with gain=1 (repeater).

Inverting input is connected to output and non-inverting – to ground through resistor 500 Mega ohm. Power supply is +- 2.5 Volt.

Without any input signal the output voltage is about -1000 mV.

Short circuit current between non-inverting input and ground is about 150 microamperes instead of the expected femtoampers. I checked several copies of this chip and got exactly the same results on all copies.

Please explain what could be causing this problem and how to fix it.

Best regards.

Sergay.

  • 0
    Guru 140200 points
    Hi Sergay,

    500M source resistance is very high. This can pick up noise and EMI which can be demodulated at the input circuitry of OPAmp and converted into an offset voltage. The OPAmp can also oscillate.

    Does the output offset voltage disappear when you connect a 10...100nF cap from the +input of OPA376 to GND, in parallel to the 500M resistor?

    Another issue is capacitive load. Touching the output of OPA376 with a DVM can introduce a too high capacitive load which can make the OPA376 oscillate. Section 7.3.3 of datasheet tells how to isolate the output of OPA376 from capacitive loads and by this keeping the OPAmp stable.

    One indicator for OPAmp oscillation is an increased supply current. You can check the supply current with a 1R shunt in the supply line by measuring the voltage drop across it.

    I hope you have decoupling caps from both supply pins to GND? See section 10.1 of datasheet.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 70 points
    Hi Kai.
    Thank you for fast and informative response.
    I know that high input impedance may cause a noise, but it’s not problem in this case, because the signal source have internal impedance only 1..2M and it will be connected in parallel to 500M input resistor.
    I tried to connect 1M resistor or/and 100 nf capacitor, as you suggested, to parallel with 500M resistor, but it didn’t solve the problem.
    Yes, there are decoupling capacitors for both supply voltages.
    The op amp is disconnected from any external circuits and so it has no a capacitive load.
    Besides, I do not observe any oscillations in the output. It’s clear DC with value -1080 mv.
    Is it possible that I just received a defective batch of op amps from Digikey? I just don’t see any another explanation for such a simple case.
    Sergay.
  • 0 in reply to kai klaas69
    Prodigy 70 points
    I forgot to write what the amplifier transfers an input AC signal, but has a very low input impedance and the offset.
  • 0 in reply to Sergay Malkin
    Guru 140200 points
    Hi Sergay,

    hmm, have you swapped the pins? What package do you use? Take care, the SOT23 package and SC70 package have different pin configurations.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 70 points
    Of course I checked the pinout very carefully.
    I also measured all the voltages directly on the chip pins.
    Now I want to try other op amp models on the same board. It will take some time.

    Sergay.