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ADS127L01EVM: VOCM Source for THS4551

Part Number: ADS127L01EVM
Other Parts Discussed in Thread: THS4551, ADS127L01

Hello. I am trying to understand how the ADS127L01EVM has implemented the THS4551, and in the user manual the schematic shows VOCM as an input (Figure 32). I do not see anywhere on the schematic that VOCM is generated. VOCM measures ~1.5 V at TP18, so it is about half of AVDD_3V as expected. How is VOCM generated on the ADS127L01EVM? Is there a voltage divider or is there another regulator that is not shown on the schematic? Thanks!


  • Hello Tim,

    Welcome to the TI E2E community.

    Good question regarding the VOCM signal.  This pin is normally an input to the THS4551 and used to set the common mode output voltage, but if the VOCM pin is not externally driven there is an the internal voltage divider that will set it to 1/2 of the supply voltage. 

    Since the ADS127L01 also works best with the common mode voltage set to 1/2 supply, the default value of the THS4551 is used (nothing external is connected to the VOCM pin other than a filter capacitor).  The single analog supply for both the ADS127L01 and the THS4551 is 3.0V, which is why you were able to measure 1.5V on this pin.

    Keith Nicholas
    Precision ADC Applications

  • Excellent. Thank you very much for this very helpful answer. Tim