Prescaling on ADC battery voltage measurement

[Eliot Stock]

I'm trying to measure a scaled down battery voltage level on an ADC pin. Given my resistor values the voltage on the pin should be about 0.279 times VBATT, to fit within 1.2V. It's a lithium ion battery that shouldn't go over about 4.1V or 4.2V.

Here's how my ADC pin is set up. Note the "NoPrescaling".

	NRF_ADC->CONFIG	= (ADC_CONFIG_EXTREFSEL_None << ADC_CONFIG_EXTREFSEL_Pos) // We don't have any external reference pins.
		| (ADC_CONFIG_PSEL_AnalogInput2 << ADC_CONFIG_PSEL_Pos) // Use analog input 2.
		| (ADC_CONFIG_REFSEL_VBG << ADC_CONFIG_REFSEL_Pos) // Use internal 1.2V bandgap voltage as reference for conversion.
		| (ADC_CONFIG_INPSEL_AnalogInputNoPrescaling << ADC_CONFIG_INPSEL_Pos) // No prescaling.
		| (ADC_CONFIG_RES_8bit << ADC_CONFIG_RES_Pos); // 8 bit resolution.

There's some discussion of the arithmetic to be done on the NRF_ADC->RESULT in this thread:

devzone.nordicsemi.com/.../what-s-wrong-with-measuring-lithium-battery-voltage

My question is, why is the code in that thread using an ADC_PRE_SCALING_COMPENSATION value when the ADC is set to no prescaling?

Multiplying by 4 would get me a value pretty close to what I'm measuring with a multimeter, as it happens.

[Håkon Alseth]

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

[Eliot Stock]

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.

[Eliot Stock]

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.

[Håkon Alseth]

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

[Eliot Stock]

Bingo! Thanks very much.

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