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OPA810: Input Bias Current

Liu Yang
Expert 5945 points
Part Number: OPA810

Hi Team,

I am designing a voltage follower circuit using OPA810.
There are two graphs for input bias current in the datasheet. Figure 7-53 and Figure 7-54.
I believe that Fig. 7-53 Input Bias Current vs Input Common Mode Voltage applies to the voltage follower, am I correct?

Also, in the case of Differential Input, can you tell me why the input bias current increases so much?


Best Regards,
Tom

  • 0
    Guru 259470 points

    Yes, 7-53 was measured with a differential input voltage of 0 V.

    For larger differential input voltages, a current flows because there are diodes between the two inputs.

  • +1
    TI__Prodigy 525 points

    Hi Liu,

    You're right. Figure 7-53 for input bias current vs input common-mode voltage is applicable in your unity-gain follower circuit. This plot shows the very small (and pA range) variation in bias current with change in amplifier's input common-mode voltage, which is the main benefit of using FET-input amplifiers.

    Regarding the case with differential input voltage, typically amplifiers are supposed to be used in closed loop which ensures that the differential input voltage in steady state condition is zero (virtual ground as we say it). It is only in transient conditions, for example to list a few possibilities-

    1. MUX switching at amplifier's input

    2. Sudden change in output voltage due to output load transient

    3. Transient step input signal were the amplifier's output lags due to limited slew rate causing an instantaneous differential input voltage until steady state is reached

    4. Using the amplifier as a comparator in open-loop (not recommended usually with devices which don't offer input protection)

    For cases like above, a differential voltage will appear on amplifier's input and the OPA810 has clamping circuit between its two inputs, which can tolerate upto +/-7V differential), to protect from any irreversible damage without the need for series current limiting resistors. There are no back-to-back diodes at OPA810's inputs, unlike bipolar amplifiers. This makes the OPA810 MUX-friendly. It is this input clamping circuit which shows this increase in input bias current with Vin,diff > +/-1.5V, as shown in Figure 54, but protects from any damage to the amplifier's input transistors.

    Thanks,

    Bharat Agrawal