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Filter stage in Signal Chain solution - Need & Selection


I am designing a high resolution and accuracy solution at a low frequency (<10kHz) and aiming to achieve the best possible stability. Checking out the signal chain solutions in the Precision Narrow Bandwidth webpage, there is a filter stage in the signal chain option I am planning to design.

Why do I need an RC filter in front of a precision ADC? How do I select the RC filter values?


Thanks in advance for your support.

  • An RC filter is typically required between the ADC driver and SAR ADC to help reduce the voltage step seen at an ADCs input as a result of non-linear input current associated with charge redistribution.  This RC filter also acts to limit the wide bandwidth noise contributions coming from analog front end components. New precision ADC architectures  reduce inherent non-linear input current and allow the RC filter cut-off to be set based on bandwidth of interest rather than being limited by input settling time requirements. This gives much more flexibility in the components that can be selected for both the analog front end and the RC filter.  

    C0G or NP0 type capacitors are recommended for the RC filter due to their high Q, low temperature coefficient, and stable electrical characteristics under varying voltages. A larger value of R can be used in the RC filter with a corresponding smaller value of C, reducing amplifier stability concerns without impacting distortion performance significantly. A larger value of R helps to protect the ADC inputs from overvoltage conditions. It also results in reduced dynamic power dissipation in the amplifier. For lower input frequencies (DC to <10kHz), a small value of R can be used in the RC filter with a corresponding larger value of C with appropriate ADC driver to achieve the best SINAD performance.