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How to make input impedance of EVAL-AD7982-PMDZ to be high?

JYC
on Mar 1, 2021

I am trying to use EVAL-AD7982-PMDZ to digitize voltage signals from a capacitive amplifier.  I want the input impedances of the input signal ports to be high (~1 MOhm).  However, in EVAL-AD7982-PMDZ, there are bypass capacitors (C2, C3, C5) and a bypass resistor (R4) between the input signal port and the ground, which makes the input impedance to be a few hundred Ohms.   I am wondering what is the purpose of these bypass resistors and bypass capacitors and how I can make the input impedance to be high.   

Thank you.

 

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    •  Analog Employees 
    on Mar 1, 2021 1:48 PM

    Thanks for your query,

    The purpose of the R1/R3 and R2/R4 resistor dividers on the amplifier inputs is to ensure the factory default EVAL-AD7982-PMDZ boards have a common mode input voltage of VREF/2 on the AD7982 inputs.

    This is because the AD7982 has a common mode input voltage requirement/specification of VREF/2 (nominally). In other words, the average voltage on the IN+ and IN- pins needs to be VREF/2 for the AD7982 to function properly.

    If that common mode voltage can be achieved in another way, then these resistor dividers aren't necessary and can be removed. But be sure to keep the common mode voltage requirement in mind when designing the capacitive amplifiers you mentioned.

    The bypass capacitors are also optional, they are just there to reduce the noise from the amplifier generating the VCM signal (A1).

    Thanks,

    Tyler

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