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OPA187: Regarding Input bias current

Daisuke Higa
Genius 4765 points

Part Number: OPA187
Other Parts Discussed in Thread: OPA188,

Tool/software:

Hi support team.

Could you please provide some information about the method and measurement environment for measuring the input bias current of the OPA188 introduced in the application note below?

https://www.ti.com/lit/wp/sboa586/sboa586.pdf

Currently, we have confirmed that the input bias current increases several times when the ambient temperature is between 30°C and 40°C, and we are trying to measure it.

Best regards,

Higa

  • 0
    TI__Mastermind 29138 points

    Higa,

    1. The application note considers the effects DC bias current and bias current transients.  However, the measurements from the app-note were designed to look at the transient behavior.
    2. Traditionally, a simple approach to measure IB is to place a large resistor on the amplifier's non-inverting input.  You would measure the output offset with and without this resistor.  That is basically what is done for the application note (see slide 2).  However, in this case the offset at the output of the op amp is from both IB x Rin, and the settling of the bias current transients.  The main message of the app note is that chopper transients can cause an additional offset that is not accounted for by IB x Rin.  This is why the app note recommends a maximum input impedance ( Table 5-1).
    3. The OPA187 is a good choice for chopper applications that require a large input impedance.  This device can tolerate up to 500k and will have minimal transient introduced offset.
    4. The DC bias current should double every 10C.  That is the normal expected behavior for CMOS amplifiers.  The bias current transients will change a little over temperature, but the DC bias current will change much more over temperature.
      1. Thus, you can select a source impedance like 100kΩ.  This should typically produce an offset change at 25C of IB x Rin = (100pA)(100kΩ) = 10μV.  My measured results were much lower than that.  Note that you should look with Rin = 0 and Rin = 100k and look at the offset shift between the two cases.
      2. Increasing the temperature 10C should double the IB, so at 35C you should measure approximately 20μV.
      3. The rule-of-thumb of IB doubling every 10C is an approximation.  You will not see exactly a factor of 2 but it should be close.  If you see a factor of 5 or 10, than something is likely incorrect in your setup.  If this is the case, please send me details on how you are doing the measurement.
    5. Below is the power point summarizing the measurement circuit and results for the OPA187 used in the app note.

    chopper measurements - opa187.pdf

    best regards, Art