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OPA2192: CMRR test for AC signal

yashodhar kadakeri
Prodigy 20 points

Part Number: OPA2192
Other Parts Discussed in Thread: OPA192

Hi,

As per customer requirement , we have to measure CMRR for IC OPA2192 for AC signal @ different frequencies like 100Khz, 330KHz, 1Mhz.

Kindly provide procedure along with test conditions for the same.

  • 0
    Guru 140200 points

    Hi,

    there's no need to measure the common mode rejection becasue this measurement has already be done by the manufacturer and the result can be seen in figure 21 of datasheet.

    If you are interested in the common mode rejection in your application, then keep in mind that the result heavily depends on your actual circuit. Even the least imbalance can totally ruin the common mode rejection, especially at high frequencies.

    You may find this video helpful:

    https://www.youtube.com/watch?v=MwxyNmW--70

    Can you post the circuit from which you want to know the common mode rejection?

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Kada-san,

    Kai covered this OPA192 CMRR test issue nicely. His point about "the least imbalance can totally ruin the common mode rejection, especially at high frequencies" can't be emphasized enough. TI has been producing op amps for more than 50 years and has perfected the lab characterization techniques used to characterize a parameter such as CMRR and PSRR vs. frequency. The information about the methods applied are TI Company Proprietary and we would not be able to share them with you. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hi,

    Thank you for the reply.

    Actually we have done the CMRR test for DC signal with the schematic provided in below link (time stamp 3.14 in video) and results are within the range.

    https://www.youtube.com/watch?v=MwxyNmW--70

    As given in video, using same schematic we would like to do CMRR test with AC signal(time stamp 3.14 in video). Kindly provide input ac signal specification with some measurement samples to calcuate CMRR with frequency 100KHz.

    Regards

    Yashodhar

  • 0 in reply to yashodhar kadakeri
    Guru 140200 points

    Hi Yashodhar,

    the circuits shown in the video are test circuits for the simulation of CMRR. These are no test circuits for the measurement. Please watch the video at 3:38.

    This link shows a similar measuring method. But as mentioned there you will need precision resistors which must match within 1ppm (0.0001%) to measure CMRR >100dB:

    https://www.analogictips.com/when-is-a-high-cmrr-needed-in-an-op-amp-faq/

    And at 1MHz a stray capacitance mismatch of about 0.1pF between the inputs of OPAmp causes an impedance mismatch of about 1ppm in combination with 2k resistors.

    So, it's really really hard to even only approximately confirm the datasheet values with this simple method. To get way more precise results TI is measuring the AC CMRR in a totally different way.

    Also see this nice presentation:

    slideplayer.com/.../

    Kai 

  • 0 in reply to yashodhar kadakeri
    TI__Guru* 93105 points

    Hello Yashida,

    TI does not provide custom electrical characterization services for our products. We will not be able to provide OPA192 samples with measured 100 kHz CMRR. TI does test and assure that the CMRR will be within the limits specified in the Electrical Characteristics table. And do observe that the Figure 21 CMRR and PSRR vs. Frequency graph represents a "Typical" device. There are no assurances made that the CMRR vs. Frequency of a particular OPA192 will be the same, higher or lower than the device(s) used to create the graph.

    Regards, Thomas

    Precision Amplifiers Applications Engineering