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AD9642 ADC VCM pin output impedance

Question asked by on May 30, 2017
Latest reply on Jun 8, 2017 by

Dear Members


I would like to drive AD9642 ADC with the fully-differential amplifier AD8139. To generate the correct offset I intend to use the VCM pin of the AD9642 which is a 0.9V output pin. I connect the VCM not only to the Vocm of the AD8139 (3.5M input resistance) but to the negative input of the differential amplifier through 1k resistors. My questions are given below:

  1. Can the VCM pin deliver enough current to work with the ~1k input impedance or I have to envisage a buffer amplifier? What is the output impedance of the VCM output?
  2. AD9642 datasheet states on page 17: “The output common-mode voltage of the ADA4930-1 is easily set with the VCM pin of the AD9642 (see Figure 47), and the driver can be configured in a Sallen-Key filter topology to provide band-limiting of the input signal.” I managed to acquaint myself with the MFB filter topology used in the case of a fully-differential amplifier and failed to find Application notes showing how to implement the Sallen-Key topology. Would you provide an appropriate AppNote or a schematic, please?

Thank you very much for your time and help.


Yours faithfully