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Unexpected offset at input and output when driven from opamp and DC-blocking cap

Thread Summary

The user observed a several-volt offset at the input and output of the LT1010 when using it as a cable driver. The final answer identified the input bias current flowing through the 47kΩ resistor as the cause, resulting in an expected -4.23V offset. The solution is to remove the DC blocking capacitor and drive the LT1010 input directly from the previous opamp output.
AI Generated Content
Category: Hardware
Product Number: LT1010

Greetings,

we are using the LT1010 as a cable driver for fairly low frequency signals (so yes, probably overkill). Out set up is straightforward:

General-purpose opamp driving a 1.5 uF coupling cap, then a 47k resistor to ground, and then connected to the input of the LT1010. The LT1010 is not in any feedback loop.

We are observing several volts of offset at the input and output of the LT1010 with or without a signal.

Q: is there anything in the LT1010 that would explain this?  Do we need to drive the input from an opamp without DC blocking cap?

Thanks,

- Lambert

  • Hi  

    The input bias current likely flows through the 47k resistor from the input node to ground.

    Based on the LT1010 datasheet input bias current plot, the input bias current is approximately 90 µA at room temperature. With a 47 kΩ resistor providing the DC return path, the expected voltage at the LT1010 input node is approximately: 47k × -90µA = -4.23V.

     

    Since the LT1010 is being used as a buffer outside of any feedback loop, the output will follow this input offset. Can you confirm that the offset at the input is around this level?

    Best Regards,
    Ian 

  • Thanks, that matches exactly what we observe. Guess we need to drop the DC blocking cap and drive the input directly from the previous opamp output.

    Thank you so much!

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