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THS4131 Vocm external driving from ADS8422

Uenal Dogan
Prodigy 20 points
Other Parts Discussed in Thread: THS4131, ADS8422, OPA320, OPA336, OPA340, OPA344, OPA363

Hi

in a new design i like to use the THS4131 to drive the ADS8422 very similiar to what the ADS8422 datasheet proposes for "Fully Differential Input driver..." in Figure 29. However, i do not like to use dual supply voltage which seems, for whatever reason, mandatory in this case. At least from the THS4131 datasheet i can not find a good reason for that so i guess it is because of "insufficient" driving capabilities of the ADS8422.

What if i would use an additional buffer amplifier which gets the input voltage from ADS8422 COMMOUT pin and drives it to Vocm of the THS4131? Because I already need another amplifier as a pre-amplifier and liked to use a dual-amp. So in fact, i already have one amplifier in spare. Does anybody know if that would work? Furthermore, if i do this, would still a decoupling capacitor be needed on Vocm?

 

Best regards,

Ünal

  • 0
    TI__Genius 13935 points

    Hello Ünal,

    I think there are two separate questions here.  The first is what supply voltage you need for the THS4131 and the second is whether you need a buffer on the Vcm pin for the ADS8422.

    The amplifier input common mode is 4.096V, so the THS4131 will need a supply voltage that gives enough headroom for the signal swing on a 4V center.  I think that a single 8V supply would work as would a single 10V supply.

    In the ADS8422 datasheet is says the Vcm output pin can source a maximum of 10uV, so you are correct it will be necessary to use an op amp to buffer this voltage to the THS4131 Vcm input pin. 

    As shown in Figure 29 in the THS4131 datasheet there are 30k Ohm resistors on the Vcm input pin.  This is more load than the ADS8422 can support. 


    Some amplifiers that would be suitable to put between the ADS8422 Vcm out and the THS4131 Vcm in are:

    LMH771

    OPA320

    OPA336

    OPA340

    OPA344

    OPA363

    Any other op amp with similar specifications to those above would work.

    Regards,

    Loren

    Regards,

    Loren