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Analog input pins for ADC14L040

Jose Sales
Prodigy 70 points
Other Parts Discussed in Thread: ADC14L040, LMH6554

 Hello all,

I am currently designing an interface board for the ADC14L040. As stated in the datasheet, the chip supports a Vcm range between 0.5V and 2.0V, and the analog input should never be higher than 2.6V or the reference voltage.

My question may sound elementary, as I already have a differential pair to be used as input, what should I do to guarantee this Vcm range? Should I use a kind of biasing circuitry, such as a voltage divider before connecting to the ADC, for example?

Thank you for your attention,

Best regards,

Jose Sales

  • 0
    TI__Expert 4340 points

    Hi Jose,

    Ensuring the proper input common-mode bias depends on how you are driving the ADC. If the application does not require frequencies near DC, then AC coupling caps may be used to isolate the ADC input bias from the driving circuitry and placing resistors at the ADC input pins terminated to the VRM voltage reference is sufficient. If you are considering a transformer for single-ended to differential signal conversion, then you can consider injecting the VRM bias into the transformer secondary center tap (see pg.22 of the datasheet).

    For DC coupled applications, many ADC driving amplifiers have a configurable output common-mode that is set by routing the VRM reference to the output common-mode control pin (check out the VCM pin on the LMH6554). If such control is not available and the driver common-mode is fixed, you can consider shifting the driver voltage supplies, or creating a more elaborate interface between the driver and ADC to set the common-mode as you suggested. Using the ADC's VRM pin as a common-mode reference is the preferred method because it will track with the ADC over voltage/temp.

    You may find this app note useful: http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=snoa565a&fileType=pdf

    Regards, Josh