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SAR ADC reference capacitor type

Question asked by Johannes2 on Apr 28, 2015
Latest reply on May 15, 2015 by KarenNE

Hi everyone,

 

I was reading AN-931 (Understanding PulSAR ADC Support Circuitry), by Martin Murnane and Chris Augusta.

On the last page, section "Layout", the authors state:

 

It is recommended that a capacitor be placed directly at the reference pins of the ADC. This is recommended to eliminate the switch capacitor transients. A good quality capacitor is required (for example, a tantalum or a X5R type ceramic; an NPO would not be recommended). Because this is a charge storage capacitor as opposed to a bypass capacitor, values in the range of 4.7 μF to ~22 μF can be used.

 

This surprises me, as I always thought NPO capacitors are superior to X5R on a lot of parameters.

We use NPO mainly in signal paths, since they hardly introduce any harmonic distortion. I fully understand that for the reference pin of a SAR ADC this is not relevant, but still I wonder on what parameters X5R is better than NPO and why NPO is even "not recommended" here.

 

Anyone has an idea?

 

Regards,

Johannes

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