nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

RE: nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

bonnet — Mon, 26 Nov 2018 14:44:30 GMT

Thank you for the white paper on battery measurement. It is exactly what we want to do.

RE: nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

ketiljo — Mon, 26 Nov 2018 14:20:11 GMT

See this post, this applies for the nRF52 as well.

RE: nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

Ali_1 — Mon, 26 Nov 2018 14:11:15 GMT

Unfortunately, for my application, this is not enough information. Our source impedance is 600k, so 5M input impedance would be too low. Can you please provide a better figure for the input impedance. It needs to be much bigger than 5M; hopefully it is.

RE: nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

AmbystomaLabs — Wed, 22 Aug 2018 17:39:25 GMT

As Håkon stated, it is just switched capacitance input. As are most ADC's.

You just use the e^(t/RC) relationship to figure out the sample time vs. input impedance appropriate for your design. R is of course the source thevenin equivalent from whatever your source is.

RE: nRF52840: What is the input impedance of the ADC inputs ? In spec it is greater than 1M but how much.

Håkon Alseth — Wed, 22 Aug 2018 13:38:07 GMT

Hi Thierry,

The input impedance, when SAADC is IDLE, is much greater than 5 Mohm.

When a ADC conversion is on-going, you switch in a capacitor as well as the input impedance and pin-capacitance, which will lower the impedance (see figure 8 here for more details).

Kind regards,

Håkon