nRF52832 SAADC circuit R_INPUT value

RE: nRF52832 SAADC circuit R_INPUT value

Jørn — Fri, 19 May 2017 10:42:15 GMT

Was the PSEL switch connected or disconnected when you did your measurements? if it was connected, could you make sure it is disconnected and redo the measurements? This would reduce any possible additional sources of error.

RE: nRF52832 SAADC circuit R_INPUT value

Farhang — Wed, 17 May 2017 01:10:20 GMT

And to me more clear i calculated back the internal resistance of the NRF52 from Fluke meter voltage measurements of the middle point of the divider. Here are the raw values of my circuit and measurements:

voltage divider resistances 1M Pull up and 68K pull down (both 1% tol)
voltage at the high end of the divider = 12.49V
voltage at the middle point (before connecting the NRF52) = 0.790
voltage at the middle point with NRF52 #1 = 0.753V! ( => Input resistance = 1.15M )
voltage at the middle point with NRF52 #2 = 0.789V ( => Input resistance = 10.26M )

RE: nRF52832 SAADC circuit R_INPUT value

Farhang — Wed, 17 May 2017 00:45:06 GMT

Thanks for further investigation Jørn. I calculated back the internal resistance seen into SAADC and it was different from NRF52 to NRF52 unit, with no hardware changes in between our boards. I looked back at my notes and I have measured it at 1.15 MOhm for one NRF52832 and another NRF52832 measured 10.26 M! (my method for measuring the resistance was 1. confirming the resistances of my voltage divider (with a meter) with the SAADC input of NRF52 NOT connected to it, 2. then I applied a DC voltage at the high end of my divider, measured the voltage at the middle point with a Fluke multimeter (re-confirming my divider and my multimeter) 3.then I connected the NRF52 to measure the middle point. And as i explained above, different units give different measurements! Hence i posted this question. Are there possibly two different revs of the chip (NRF52832) with two different resistor values?

RE: nRF52832 SAADC circuit R_INPUT value

Jørn — Tue, 16 May 2017 09:35:49 GMT

Unfortunately R_input does not have a exact resistance as it is not a "real" component. It is a parasitic resistance equivalent due to current leakage in the ESD structure, and it will vary as a function of temperature. I have spoken to one of our designers and he could inform me that the minimum value in the datasheet is inaccurate and the actual resistance lies above 10 MOhm. Which should bring your maximum deviation down to approximately 0.67%. Also remember to make sure the acquisition time is long enough for the sampling capacitor to settle.

RE: nRF52832 SAADC circuit R_INPUT value

Farhang — Mon, 15 May 2017 16:52:35 GMT

Jørn, Thanks for the reply, but i need more explanation: First of all, we're bypassing the ladder resistor with (NRF_SAADC_RESISTOR_DISABLED aka SAADC_CH_CONFIG_RESP_Bypass), so that makes things easier. Now my only question is why the exact resistance of R_Input not listed in the spec? Is it not a simple resistance similar to R_Ladder(which is listed exactly 160K without a > sign )? Having it listed as >1M doesn't make things simple to calculate the equivalent: In our circuit, we have a 1M(pull up) and 68K(pull down) resistor voltage divider at the (external to the chip) input, now if i want to calculate the equivalent resistance of the bottom side of our divider, it can be 68K||1M=63.67K or 68K||100M = 67.95K or 68K||(infinite resistance) = 68K! This means that the scale of my divider Rdown/(Rup+Rdown) can vary from 0.064 to 0.060 which is a 7% relative difference=large error

RE: nRF52832 SAADC circuit R_INPUT value

Jørn — Mon, 15 May 2017 13:20:47 GMT

Hello Farhang

If you look at the bottom of the electrical specifications here you can see a more detailed schematic of the ADC input. The 1 MOhm resistance is the input resistor you see in parallel with Cpad. When the pin is configured for the ADC, PSEL closes and Rinput is in parallel with RLadder, which is, according to the SAADC electrical specification, 160 kOhm. The total input resistance seen into the analog pin will then at the very least be 1MOhm||160kOhm=137930 ohm, and at the most be 160kOhm.

Keep the acquisition times in mind when designing your input network. Take a look here

Best regards

Jørn Frøysa