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PGA281: PGA281 CMRR Test condition

Yunjing Wang1
Expert 5280 points

Part Number: PGA281

Hi Team,

Can you share more information about the test condition of PGA281 CMRR?

Below information in datasheet only shows power supply condition, Gain setting, Vcm=0 and temperature and CMRR vs signal frequency.

But customer need more information, can you share below info:

1.What about input voltage(INN and INP voltage) when test CMRR?

2.What about the CMRR test result when Vcm is not 0?

Thank you 

YJ

  • 0
    Guru 140200 points

    Hi Yunjing,

    the CMRR mainly depends on gain and frequency. As long as the input voltage is within the linear common mode input voltage range which is from "(VSN) + 2.5V" to "(VSP) - 2.5V" here, the influence of common mode input voltage on CMRR is only marginal.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 77501 points

    Hi Yunjing,

    The Figure 14, CMRR vs. frequency is captured the  IA's frequency Rejection characteristics. From Gain=0.125V/V to G=128V/V, the roll off AC signal rejection characteristics are very similar at -20dB/decade. For G=128, there appears to have two- zero occurred at 30kHz , followed by another pole at approx. 60kHz, (one zero at ~39kHz will generate a flat in CMRR response from -20dB/decade, two-zeros will have gain rise at +20dB/decade). 

    Here is the calculation example, where CMRR = 20log (Vin/Vo)

    Assume G=128 as example 1: The typical CMRR=142dB at 10Hz. If we connected Vin-p and Vin-n together and inject 100mV or 0.1V common mode input sine wave frequency at 10Hz at the input, the PGA281 IA 's output will see  0.1/10^(142/20) = 1e(-8)V or 10nV sine wave frequency at 10Hz. 

    Any signals appear in phase on the both inputs will be rejected by the IA amplifier by a significant in amplitude (DC and AC input signals that is less than BW in frequency. The ability of reject the common mode signals start to diminish as the input frequency is increasing, which it rolls off at -20dB/decade after the BW. 

    If you need further help or simulation, please let us know. 

    Best,

    Raymond

     

     

  • 0 in reply to Raymond Zhang1
    TI__Expert 5280 points

    Raymond, Kai

    Got it and thanks very much for your reply.  

    Regards 

    YJ

  • 0 in reply to Yunjing Wang1
    Guru 140200 points

    You are welcome :-)

    Kai