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THP210: Connecting Vcm

Gerhard Kreuzer
Expert 2580 points
Part Number: THP210

Hi,

there where some sample circuits in the data sheet, showing that Vcm is connected to analog ground, especially when a balanced supply is used.

But at the end of the data sheet the user gets the information to left Vcm open and apply a capacitor of 100nF, if no external signal is connected to Vcm.

I use a balanced supply (+-14V) and want to set Vcm to analog ground. I have two amplifiers in the chain, first did the amplification and the second stage is an active anti aliasing filter. My ADC expects Vcm at analog ground level.

With best regards

Gerhard

  • 0
    Guru 47535 points

    Morning Gerhard, 

    Most of the FDA's like the THP210 have a default midsupply bias through very high R's internally, that is shown on the equivalent internal ckt diagram where the THP210 as 5Mohm each. Normally the sense resistors for the CM loop across the outputs need to be relatively low for CM loop bandwidth reasons. - values not shown below. 

    In any case, floating the CM pin will give a DC midscale but relatively high CM noise - hence the external capacitor. To the extent that CM noise gets into the final output differential noise (through R imbalance in the feedback networks, etc) it will add to the the signal noise floor. Driving the CM pin with a low impedance, low noise, source (or grounding it if that works in the design) will improve this, 

  • 0 in reply to Michael Steffes
    TI__Mastermind 49455 points

    Hello Gerhard,

    As Michael has mentioned, in this case, since the device is powered with bipolar supplies and the ADC expects the common-mode at the analog ground level, the best option is to connect the VOCM pin to the analog GND. 

    Thank you and Regards,

    Luis