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THS4509 + ADS 4129

QIU LEI
Prodigy 10 points
Other Parts Discussed in Thread: ADS4129, THS4509, THS4508, ADS41B29

  Hello， My question is about the common mode voltage match between THS 4509 and ADS 4129. The VCM ADS4129 can supply is just 0.95V.However, the required vcm range of THS 4509 is form 1.1 to 3.9. Is it a problem if THS 4509 is used for buffer of ADS4509, if so, could you please supply another proper FDA buffer? Thanks.

  • 0
    TI__Genius 17310 points

    Hi Qiu,

    One option is to use the THS4508, which allows a CM voltage range of -0.3 V to 2.3 V. This is probably the best option since it will allow the ADC to set the optimal common mode input voltage. I believe the specs of the THS4508 are pretty much the same as the THS4509, but it may have a little bit higher noise spec.

    The second option is to use the THS4509, supply an external reference voltage, and use a baising network (series resistor from the amp output and pull down resistor at ADC) to drop down to the CM input level of the ADC. This will attenuate the signal coming out of the amplifier and could add noise. The accuracy of the input common mode voltage will now be dependent on the accuracy of the external reference and the tolerance of the resistors. You could gain up the VCM output of the ADC using a precision op amp and then attenuate it with the network. This may give better accuracy since the ADC VCM output is the source for the reference.

    The third option is to use the ADS41B29 which has a buffered input and a higher common mode voltage of 1.7 V.

    Regards,
    Matt Guibord

  • 0 in reply to Matt Guibord
    TI__Genius 17310 points

    Qiu,

    I clarified with the amplifier team and there are errors in my previous post. The table shown on the front page of the datasheet is actually referring to the input common mode voltage, not the voltage that can be supplied to the common mode input pin. The important spec is that the voltage of each output pin can swing within 1V of each supply rail. So if you use a 5V supply and tie the negative supply to gnd, then 0.95 V falls outside of this range. One thing that can be done is actually using a positive and negative supply, where the positive supply is 4V and the negative supply is -1 V. This keeps a 5 V total supply range and offsets the output, so now the output voltage range is 0 V to 3 V. For a 2 Vpp differential swing, or 1 Vpp per output pin, the output common mode range is then 0.5 V to 2.5 V. This would meet the requirement. Further, a -1.5 V negative supply and 3.5 V positive supply will actually center the output even better.

    A biasing network could still be used to keep the amplifier output pins at a higher common mode voltage and then drop them down to the ADC input common mode voltage or the ADS41B29 could be used since a 1.7 V common mode would fall within the acceptable range of the THS4509 output voltage requirement.

    Regards,
    Matt Guibord