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THS4304 input biasing

Xiaobo Li
Prodigy 50 points

Other Parts Discussed in Thread: THS4304

Does the opamp (THS4304) have internal bias at its input?

I designed the opamp as an unity-gain buffer with 3.3V power supply. Then I disconnet its input from 1.5V common-mode. Then I find the input voltage goes up to about 2.2V.

  • 0
    TI__Expert 3030 points

    The THS4304, like any op-amp, has a input bias current. This part is bipolar technology and specifies a 7uA (typ) input bias current but can be as high as 12uA at room temperature. If there is no resistance to a reference point (such as your 1.5V common-mode), then the bias current has no where to flow. The end result is the input will float one way or another until it saturates or reaches equilibrium. In the case of the THS4304, it will float up until the Rail-to-Rail Input stages self equalizes and on the part you have it is 2.2V. This will vary with temperature and will have part-to-part variations.

    It is necessary to provide a path for this input bias current to flow, and must also be within the common-mode input voltage range - which for this part is not an issue as it is RRI. Remember that an offset will also occur due to this bias current and the resistance you connect to it. Ex: if you connect 1k to GND, then there will be 7uA X 1k = 7mV of offset at the THS4304 input pin.

    Cheers,

    Randy