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OPA2191: Estimating maximum input bias current at 85C

Part Number: OPA2191

I am using the OPA2191.  The datasheet lists the maximum input bias at 125C as being 9nA.

The device is a CMOS type Op-Amp so I believe the input bias should double for every 10C rise.

Therefore I assume that the maximum input bias at 85C should be 9nA / 2^((125-85)/10) = 563pA.

Is this calculation correct, or is the worst case input bias significantly more or less than that at 85C?

  • Hi Andrew,

    have you seen figure 17 of datasheet? It shows the typical input bias current.

  • Unfortunately I can't really design based on a typical current.  If I don't design based on the worst case then I can't guarantee the accuracy of the circuit to my customers.  Unless I characterize a large number of units at high temperature to determine a bound, or else individually testing each unit at high temperature.

  • Hi Andrew,

    figure 17 shows a typical input bias current of about 400pA at 85°C. So, your estimated 563pA maximum input bias current might be a bit too optimistic. I would assume about 2nA.

  • You are probably right. Worst case is usually 3x to 5x higher than typical. So 2nA seems right.

    I hope that someone has some verified numbers.
  • Hi Andrew,

    I found some limited OPAx191 characterization data. It has curves of Ib vs temperature for several devices and some tabularized listing of the Ib numbers at specific temperatures.

    The information is considered TI Confidential so I am unable to share the report. However, I can tell you there is a readpoint at 90°C and the highest Ib device is just under 2 nA at that temperature. The Ib would be a little lower at 85°C.

    I think Kai's suggestion of ±2 nA is a good estimate of what the maximum Ib will be at 85°C. Keep in mind that TI makes no assurances of a maximum Ib at 85°C, because it isn't listed in the OPAx191 datasheet Electrical Characteristics table. It could be higher with a different production lot. And that is only bounded by the TA = –40°C to +125°C Ib limit of ±9 nA, which ordinarily will be at the highest temperature.

    I hope this helps.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thanks. That was exactly what I was looking for. I will use the 2nA number, though not guaranteed its at least based on some measured data and is consistent with the other data.

    The ratio between the max and typical quoted at 25C is 4:1 (20pA vs 5pA). The ratio between 2nA and 500pA is also 4:1. So it seems for this device the ratio between typical and max is about 4:1 even over temperature. The fact that the two ratios agree at two very different temperatures adds to my level of confidence in the 2nA figure.

    Just as a side note,

    Having guaranteed bias numbers at 25C and 125C is great but there are times when I want to operate hotter than room temperature but lower than boiling (for example anything that goes outside that is handled by people). As long as it doesn't drive up cost too much it only benefits TI to put more guaranteed limits in the datasheet, because it allows me to design the amplifier into products that have stricter requirements.