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LMH5401: CMRR measure for LMH5401

Mickey Zhang
Genius 12080 points
Part Number: LMH5401
Other Parts Discussed in Thread: TINA-TI

Hi team,

The customer is using LMH5401. For the Figure 50 of the datasheet, when the frequency is close to 1GHz,

CMRR is -50dB. The customer would like to measure the -50dB CMRR value when the frequency is close to 1GHz.

Would you explain how to measure CMRR value? 

Would you send me a refer circuit or a TINA circuit for LMH5401 to measure LMH5401?

What is the common voltage range for the CMRR measurement?

 has replied to me in my previous post. But I would like to know more about this. 

My previous post is : 

I note a CMRR measure post：

In this post, our engineer provide a TINA circuit to measure CMRR.

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

  • 0
    TI__Genius 15975 points

    Hi Mickey,

    The CMRR measurement on the LMH5401 is done using a four-port VNA, with the setup diagram similar to that's shown in Figure 57. The CMRR is mainly the ratio of differential signal measured at the amplifier output to the common-mode stimulus at the differential input. The basic idea is to look for differential signal rejection at the output for a common mode stimulus and lower the value, the better is the amplifier in rejecting the common mode signal at the output. So, once you do the four-port analysis of the LMH5401, the VNA usually gives output in individual S-parameter format (S11,S12,..Sxy). The individual S-parameters can then be extracted to give mixed-mode (Sdc21) parameter, which is the differential response to common mode stimulus (or CMRR) as noted in Figure 50 of the datasheet.

    You could simulate CMRR in TINA-TI and attached is the reference circuit for that. Note that the CMRR at lower frequencies is usually limited by the matching of the external feedback and gain setting resistors. I believe the CMRR plot taken in the datasheet is at an output common mode voltage close to mid-supply or GND for +/-2.5V supplies.

    Best Regards,

    Rohit

    LMH5401_CMRR.TSC