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OPA838: Unexpected Large Offset Voltage

Pengkun Liu
Prodigy 10 points
Part Number: OPA838
Other Parts Discussed in Thread: TLV3541

Hi,

I'm using TI's op-amps (OPA838 7V/V + TLV3541 1.3V/V) as two-stage amplifier for a current signal. The schematic is shown as following. The amplified signal goes to a MAX comparator.

However, during the hardware test, we notice there's an unexpected 30mV offset at the OPA838 output side which is much larger than datasheet value (worst case <300uV at input side). The offset makes this zero-current detection very bad. Could anyone help me to see what's wrong here?

Best,

Patrick

  • 0
    Guru 140200 points
    Hi Pengkun,

    input bias current of OPA838 is typically 1.5µA.This will cause a voltage drop of 4.5mV across R31 (3k). This voltage drop is amplified by the noise gain of your circuit ((R32+R33)/R32=8) and appears as 36mV offset voltage at output of OPA838.

    Taking into account also the input bias current flowing out of the - input you finally have an offset voltage of about 34mV at output of OPA838. Just what you observed.

    If you decrease R31 to 180R you can nearly eliminate this unwanted offset voltage.

    Kai