Hello
I have a problem with ADC measure and accuracy.
I want to read a voltage (U_BAT+) between 2.5 – 4.5V with an accuracy of < +/- 50mV
I have two scenarios.
- No load
- With 1A load
Schematic
ADC Configuration
Souce code snippets:
// ADC configuration
static nrf_drv_adc_channel_t ADC_I_discharge = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_I_discharge_PIN); // Channel instance. Default configuration used
static nrf_drv_adc_channel_t ADC_U_HE = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_U_HE_PIN); // Channel instance. Default configuration used
static nrf_drv_adc_channel_t ADC_U_BAT = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_U_BAT_PIN); // Channel instance. Default configuration used
static nrf_drv_adc_channel_t ADC_I_BAT = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_I_BAT_PIN); // Channel instance. Default configuration used
static nrf_drv_adc_channel_t ADC_U_charger = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_U_charger_PIN); // Channel instance. Default configuration used
//***************************************************************************
//* ADC Init
//***************************************************************************
void Init_ADC(void)
{
nrf_drv_adc_config_t adc_config = NRF_DRV_ADC_DEFAULT_CONFIG;
err_code = nrf_drv_adc_init(&adc_config, NULL);
APP_ERROR_CHECK(err_code);
}
//***************************************************************************
//* Get Battery Voltage
//***************************************************************************
void get_battery_voltage(uint8_t mode)
{
uint32_t messung;
uint8_t cycles = 5;
messung = 0;
write_SW(SW_U_bat_mess,ON);
for(uint8_t i = 0; i<cycles; i++)
{
nrf_delay_ms(1);
DEBUG_LED_ON;
nrf_drv_adc_sample_convert(&ADC_U_BAT, &adc_buffer[2]); // get Battery voltage
DEBUG_LED_OFF;
messung = messung + adc_buffer[2];
}
ADC_Bat_U = messung / cycles;
write_SW(SW_U_bat_mess,OFF);
}
Timing
Blue U_BAT_mess
Red Debug LED (ADC is sampling)
Measurements
No load
|
|
Measured |
calculated |
|
U_BAT+ |
4.312V |
|
|
U_BAT_mess |
0.925V |
0.917V |
|
ADC |
781 (ADC_Bat_U) |
781.5 |
|
|
Measured |
calculated |
|
U_BAT+ |
3.0V |
|
|
U_BAT_mess |
0.645V |
0.638V |
|
ADC |
546 (ADC_Bat_U) |
543.7 |
With 1A load
|
|
Measured |
calculated |
|
U_BAT+ |
3.986V |
|
|
U_BAT_mess |
0.853V |
0.847V |
|
ADC |
717 (ADC_Bat_U) |
722.4 |
|
|
Measured |
calculated |
|
U_BAT+ |
3.18V |
|
|
U_BAT_mess |
0.675V |
0.676V |
|
ADC |
572 (ADC_Bat_U) |
576.3 |
The measured ADC (output on display) does not change if I connect the probe to U_BAT_mess.
Questions
- Why is the measured and the calculated U_BAT_mess not the same ?
(The ADC impedance of 130k || to 270R should not make a difference)
- Why does the measured ADC value not match to the measured U_BAT_mess ?
(1.2V ref / 1023 * ADC is not equal to U_BAT_mess)
- Why is there a difference if I measure with and without load ?
