This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA4197: Some question about the parameters...

Part Number: OPA4197

Dears,

I have questions about the datasheet parameters about OPA4197 as below.

1. What is the input resistance? (The data in datasheet is input impedance)
2. What is the test condiction for input impedance measurement?

Thanks.

  • Hi Peter,

    it's there, please look again into the datasheet: 100MOhm in parallel with 1.6pF between the inputs (differential) and 10TOhm in parallel with 6.4pF from each input to signal ground (common mode) :-)

    Kai

  • Dear Kai,

    May I think the answer is for question2?

    And how about question1?

    Thanks.

  • Hello Peter,

    Kai answered the first question. The OPA4197 Input Impedance (Z) specification provides both the differential and common-mode input resistances and input reactances, which in the case of the latter portion of Z is capacitance.

    For example, the resistive portion of the differential input impedance is approximately 100 Megohms as listed for ZID in the table. The input resistance of voltage feedback op amps such as the OPA4197 is extremely high and is usually of no substantial consequence in most applications.

    The differential input resistance is the effective resistance between the two op amp inputs when operating open loop. Therefore, its characterization can be performed in conjunction with an open loop gain measurement setup. We do not reveal the details of our op amp characterization circuits, because company proprietary techniques are employed.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Dear Sir,

    As my customer feedback, the input resistance is around G-ohm, not 100M ohm.

    They use OPA4197 as uni-gain buffer with input is 10V, and the measured current is less than 5nA.

    Is there any suggestion about it?

    Thanks. 

  • Hi Peter,

    the common mode input resistance is what counts in a unity-gain buffer.

    The common mode input resistance is defined as the relationship R = dU / dI. From figure 12 of datasheet the common mode input resistance over the common mode input voltage range between -18V and +15V can be estimated:

    R ~ 40V / 25pA = 1.6 x 10^12 Ohm.

    The common mode input resistance of 10^13 Ohm specified in the datasheet, on the other hand, is given for a common mode input voltage equal to middsupply :-)

    Kai

  • Hi Peter,

    To add to Kai's comments:

    The OPA4197 input bias current is specified with a maximum of +/-20 pA, at TA = 25°C, and is tested to that maximum in production. An input current approaching 5 nA would be about 250x too high. The only way the OPA4197 input bias current might be that high in use is if the TA was about +125°C.

    The other possibility is leakage current adding to the OPA4197 input bias current due to a poorly cleaned PC board on which the op amp is mounted. Flux residue remining after PC board cleaning can set up leakage current paths on the PC board that corrupt the very small, normally very low level input bias currents. This is a very common issue had with very low bias current, JFET and CMOS input op amp circuits.

    We suggest using solder that has water soluble flux to solder down the components to the PC board. Then, run the board through two separate ultrasonic cleanings in DI water. Change the water for each cleaning. The board should then be dried with forced hot air, or by being placed in an oven to dry.

    Regards, Thomas

    Precision Amplifiers Applications Engineering