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ADS1293: Common mode amp BW

BRIAN
Prodigy 130 points
Part Number: ADS1293

In the attached reference design for the TI ADS1293 found in your app note, it would appear the BW of the common mode amplifier is 16 Hz  (1/(6.28*10 M * 1000 pf). This can't be right. What am I missing? Seems like the BW should be > 180 Hz. 1293 reference design.pdf

  • 0
    TI__Guru 55152 points

    Hi Brian,

    This is a mistake and you are correct. We are working on getting this revised. Decreasing the size of the resistor to 1Mohm moves the cutoff frequency to 159Hz, which is what it should be. 

  • 0 in reply to Alexander Smith
    Prodigy 130 points

    Hi Alexander,

    Thanks for getting back to me. As an aside, what process would you recommend for optimizing the value of R15 in the reference design - or what value do you recommend for R15 given that the resistor is 1 Mohm?

    Brian

  • 0 in reply to BRIAN
    TI__Guru 55152 points

    Hi Brian,

    I apologize for the delay here.

    R15 is the gain resistor, Rg, so it depends on how much gain of the signal you would like. This can be simulated using the attached file. 

    Upon further inspection of the original circuit, I think using 10Mohm is acceptable. While the cutoff frequency is indeed ~16Hz, (shows 14.96 in the simulation), the gain at 60Hz is still ~47dB, which is sufficient. 

    You can play with the file by changing the values of the resistor, voltage sources, etc, and running an AC simulation by going to Analysis -> AC Analysis -> AC Transfer Characteristic...

    3323.RLD_Loop.TSC