nRF52832 analog input high-Z

RE: nRF52832 analog input high-Z

Michael — Tue, 16 May 2017 13:58:49 GMT

Thanks Jørn. The 500 MOhms of the ADC is due to an integral input amplifier which provides a buffer thus showing a constant input impedance. If the input impedance of the analog inputs is in fact 10 MOhms, it might be something I can live with. Thanks, Michael.

RE: nRF52832 analog input high-Z

Turbo J — Tue, 16 May 2017 13:57:09 GMT

is the input impedance of the analog inputs 1 Megaohms no matter what?

I would assume that the input is 1 MOhm no matter what. Remember that the NRF52x has no dedicated analog pins, so there might be some resitual current from the GPIO part even when it is disabled.

And there is still an analog multiplexer present that switches inputs to the analog peripherials (SAADC, COMP, LPCOMP).

Those 0-R resistors are tiny in a 01005 package.

RE: nRF52832 analog input high-Z

Jørn — Tue, 16 May 2017 13:50:15 GMT

Hello Michael

The 1 MOhm resistor seen in the schematic here is a parasitic resistance equivalent caused by current leakage in the ESD structure and will be there regardless. It is not specified as 1MOhm, but as at least 1MOhm. I have been informed by a member of our design team the actual resistance is >10MOhm, and not >1MOhm as stated in the product specification. It is not a fixed resistance and it will vary as a function of temperature. The moment you sample the TACQ switch will close, and the capacitor will provide a low resistance path and draw current. It is important the acquisition time is set long enough to allow the capacitor to settle. Please see this section

I would think the 500 MOhms of your ADC is when the ADC is not sampling, and during sampling it would present a low impedance which would then be the dominant impedance in the parallel coupling.

Best regards

Jørn Frøysa