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Input bias current versus input differential voltage for the OPA316IDCKR

Brett Gidge
Intellectual 370 points
Other Parts Discussed in Thread: OPA378, OPA316

I have a laser driver current clamp circuit utilizing the OPA378. During DVT testing I discovered this part suffers from an increased input bias current (increases into the uA range) when the input differential voltage exceeds roughly 750mA. This is a definite problem for my design because I can have an input differential voltage as high as 1.75VDC. The prototype boards are done so I'm trying to find a package and pin compatible drop in replacement part that doesn't have this issue. I think I have found a part that might work - OPA316IDCKR. However, the datasheet doesn't give any information about the input bias current versus differential voltage. Can anyone tell me this information? The OPA378 shows it in the application section of the datasheet but I missed it during my initial design. The fact that this type of information is missing in the OPA316 datasheet could mean (and I hope it means) that this part doesn't suffer from this issue. The input bias current for the OPA316 is listed in the electrical characteristics section as +/-15nA over the entire temp operating range which would be good enough for me. Thanks for any help!

  • 0
    TI__Guru* 93105 points

    Hi Brett,

    The reason the input current increased with increased differential voltage for the OPA378 is because it has back-to-back input diodes between its inputs. The differential voltage will forward bias one diode, or the other, if the voltage becomes large enough. The OPA378 has internal 1.5 k series input resistors as its datasheet Figure 28 shows so the current will be limited, but can be too high for your application if the differential voltage is made large.

    The OPA316 datasheet does not include an input current vs differential voltage graph. That really isn't necessary for a CMOS op amp such as the OPA316 that doesn't use an input-to-input diode clamp. It uses complementary P-channel, N-channel differential pairs in the front end, and their inherent gate current is in the tens of femptoamps. However, the overwhelming majority of the input current is contributed by the leakage current of the input protection ESD diodes. There is a one from the input pin to each power supply rail, and they are reverse biased and off under normal circuit operating conditions. They intended to just "go along for the ride" once the op amp is installed and soldered on to the PC board.

    The ESD diode leakage current makes up most of the +/-5 pA "25 C typical" input specification. The voltage across each input ESD diode changes with the applied common-mode voltage (VCM), but their leakage current doesn't change radically with that change. The leakage current follows the "Reverse Current vs. Reverse Voltage" behavior of a silicon diode. It increases by a small multiple with increased reverse voltage. So if the "typical" input current is +/-5 pA, and it increased by a factor of 5x, then the input current will be in the range of 25 pA. That is much, much less than the input current associated with a forward biased input clamp diode.

    Do be aware that the input bias current of op amps is a function of temperature. OPA316 Figure 10 shows an example of how it behaves. This is the typical behavior with the VCM set to Vs/2, mid supply.

    Regards, Thomas

    PA - Linear Applications Engineering