Prescaling on ADC battery voltage measurement

RE: Prescaling on ADC battery voltage measurement

Eliot Stock — Thu, 27 Feb 2014 16:39:41 GMT

Bingo! Thanks very much.

I do indeed have a cap in there btw, just neglected to mention it.

RE: Prescaling on ADC battery voltage measurement

Håkon Alseth — Thu, 27 Feb 2014 16:23:09 GMT

Hi Eliot,

You setup the ADC to be 8 bit, meaning that a value of 255 = 1.2V, although you're treating the ADC result as it's a 10 bit number.

If you're getting 234 from the RESULT register, then this would be 234/255*1.2V = 1,1V on the ADC pin. Note: if you're using resistors in the Mohm-area, it's recommended to use a capacitor as well (due to ADC input impedance during conversion)

Best regards Håkon

RE: Prescaling on ADC battery voltage measurement

Eliot Stock — Thu, 27 Feb 2014 16:09:10 GMT

Here are my constants:

#define BATTERY_ADC_FACTOR (0.279f)
#define BATTERY_REFERENCE_VOLTAGE (1.2f)
#define BATTERY_ADC_MAX (1024)

And here's my arithmetic, with the values I'm seeing:

	// This gets us a fractional number from 0 to 1, a proportion of the maximum.
	float adc_result_proportion = ((float)NRF_ADC->RESULT/(float)BATTERY_ADC_MAX);
	// eg. 234 / 1024 = 0.229

	// This gets us the actual voltage on the ADC pin.
	float pin_voltage = adc_result_proportion * BATTERY_REFERENCE_VOLTAGE;
	// eg. 0.229 * 1.2V = 0.274V <-- Too low considering I measure 4.2V across the battery directly.

	// This gets us the corresponding voltage across the battery terminals.
	m_battery_voltage = pin_voltage / BATTERY_ADC_FACTOR;
	// eg. 0.274V / 0.279 = 0.983

Let me know if you need my resistor values, but they're not too far off those posted in the other thread. Same orders of magnitude, anyway.

RE: Prescaling on ADC battery voltage measurement

Eliot Stock — Thu, 27 Feb 2014 16:05:33 GMT

Oh, I see. I don't understand why I'm seeing the NRF_ADC->RESULT value I am in that case. Will post more detail below.

RE: Prescaling on ADC battery voltage measurement

Håkon Alseth — Thu, 27 Feb 2014 15:47:02 GMT

Hi Eliot,

He's using a factor of 4 due to the external resistors (1M and 250K). Could you elaborate on the error in your measurement?

Note that the ADC input resistance will change while sampling! (you'll see the numbers in the thread you linked to.)

BR Håkon