Understanding SAADC

RE: Understanding SAADC

rdpoor — Wed, 25 Apr 2018 21:02:49 GMT

To echo MartinBL's comment, the input impedance of your multimeter could be skewing the results -- to measure a voltage across a 10M resistor requires a meter with very low leakage current.

To test that theory: does the ADC reading change when you connect the voltmeter?

RE: Understanding SAADC

MartinBL — Tue, 29 Aug 2017 13:12:45 GMT

I'm not sure if I understand. It sounds like your measurements and ADC results are almost spot on (3.61V vs 3.598V)?

Have you taken into account the accuracy of your resistors and the dropout voltage of your voltage regulator? Since you are using VDD as a reference voltage your ADC readings will be useless when/if the battery voltage gets so low that the regulator cannot produce 3.3V anymore.

RE: Understanding SAADC

kian79 — Tue, 29 Aug 2017 00:31:10 GMT

Hi all,

I still cannot figure out the problem. Anyone can help?

RE: Understanding SAADC

kian79 — Sat, 26 Aug 2017 00:39:04 GMT

Hi Martin, I don't know the input impedance of my meter.

Now, instead of measuring the voltage across the 10MOhm resistor, I measured the voltage across the battery instead.

And these are my observations: Battery Voltage/ADC Value

3.95/592
3.79/568
3.61/541

There seems to be a consistent pattern. At 3.95V battery voltage, the theoretical voltage at the 10MOhm resistor should be 3.95*(10/14) = 2.821V

At 3.79V battery voltage, the theoretical voltage at the 10MOhm resistor should be 3.79*(10/14) = 2.707V

The difference in voltage is 2.832-2.707 = 0.114V and the difference in ADC result is 592-568 = 24. Therefore the ADC resolution is about 0.114/24 = 0.00475

Working backwards, with a ADC result of 541, this gives a ADC measured voltage of 5410.00475 =2.57V. Take this result and multiple by 2.57(14/10) = 3.598V (battery voltage). Which is quite close to what I measured.

RE: Understanding SAADC

MartinBL — Fri, 25 Aug 2017 13:13:46 GMT

You are measuring the voltage across the 10 MOhm resistor with your volt meter right? What is the input impedance of your meter? Could it be related to this?

RE: Understanding SAADC

kian79 — Fri, 25 Aug 2017 11:58:14 GMT

Hi Martin,

I measured with a voltmeter. My VDD is 3.28V and the voltage I am feeding into AN1 now is 2.22V (after charging the battery). I am getting a converstion result of 592.

Something still seems wrong.

RE: Understanding SAADC

MartinBL — Fri, 25 Aug 2017 10:44:39 GMT

Hi,

In your code you are using the nRF52's VDD divided by 4 as reference voltage (NRF_SAADC_REFERENCE_VDD4) and a gain of 1/6. If we rearange the equation presentet here we can see that Vin is:

Vin = ADCresult * Reference / (Resolution * Gain)

In your case that should result in:

Vin = 550 * (VDD/4) / (1024 * (1/6)) = 2.66.

So something is wrong. Are you sure that VDD is 3.3V?