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ADS8866: Voltage source "VCM" connected to the non-inverter

Gerald
Genius 9580 points
Part Number: ADS8866
Other Parts Discussed in Thread: OPA836

Hi Experts,

Our customer is interested in using the A/D converter. He has a question regarding its data spec documentation. On page 29, figure 53, there is a typical application Q Circuit for Lowest Distortion and Noise Performance at 100 kSPS. He would like to know what is the voltage source "VCM" connected to the OPA836. amplifier port. What is the purpose of it?

Can you help me with some technical information about it? Thank you.

Best regards,

Gerald

  • 0
    TI__Genius 9580 points

    I have found an answer to this question but please still confirm below:

    Checking the circuit on page 29 of the datasheet as shown below:

    I have checked the datasheet of OPA836 and the Vcm above is the same as Vref as shown below:


    VREF provides a reference point around which the input and output signals swing. Output signals are 180° out-of-phase with the input signals.

  • 0 in reply to Gerald
    TI__Guru 62826 points

    Hi Gerald,

    Apologies for the delay here. I can look into this further tomorrow if needed, but my initial assumption is that VCM is determined by the input common-mode requirements of the ADC. In many SAR applications, common-mode has a tight limitation equal to VREF or VREF / 2. Therefore, the source may actually be from an external voltage reference (or a divided version of it), or it may come directly from a buffered VCM output pin on the ADC itself.

    Best regards,

    Ryan