52833 adc deviation

Question

Hi 
 We are using 52833 adc, but the actual value of the adc is not within our expected value. We want to know the adc accuracy, can you give us some advice？ Thanks 
 sdk: sdk17.1.0 
 adc: 12bit NRF_SAADC_REFERENCE_INTERNAL NRF_SAADC_GAIN1_4 
 real voltage range: 0v - 1.5v

config

nrf_drv_saadc_config_t p_config = { 
 .resolution = (nrf_saadc_resolution_t)NRF_SAADC_RESOLUTION_12BIT, 
 
 .oversample = (nrf_saadc_oversample_t)SAADC_CONFIG_OVERSAMPLE, 
 .interrupt_priority = SAADC_CONFIG_IRQ_PRIORITY, 
 .low_power_mode = SAADC_CONFIG_LP_MODE 
 };

nrf_saadc_channel_config_t channe1_config = 
 NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0); 
 channe1_config.gain = NRF_SAADC_GAIN1_3; 
 nrf_saadc_channel_config_t channe2_config = 
 NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1); 
 channe2_config.gain = NRF_SAADC_GAIN1_3;

APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 1 , &channe1_config)); 
 APP_ERROR_CHECK(nrf_drv_saadc_channel_init( 2 , &channe2_config));

start collect

APP_ERROR_CHECK(nrf_drv_saadc_buffer_convert(m_buffer_pool, SAMPLES_IN_BUFFER )); 
 nrf_drv_saadc_sample();

process adc value

static void mt_saadc_callback(nrf_drv_saadc_evt_t const *p_event) 
 { 
 if (p_event->type == NRF_DRV_SAADC_EVT_DONE) { 
 m_adc_buff[m_adc_collect_times].channel1 = m_buffer_pool[ 0 ]; 
 m_adc_buff[m_adc_collect_times].channel2 = m_buffer_pool[ 1 ]; 
 
 nrf_saadc_task_trigger(NRF_SAADC_TASK_STOP); 
 
 } 
 } 
 
 Below is my circuit diagram, We only use adc to calculate the resistance of 1MΩ to avoid power supply voltage fluctuations. But it seem no effect,the adc value changes of ain0 and ain1 are not synchronized sometimes. 
 R = 27kΩ * (ain1adc / ain0adc -ain1adc)

The attachment is the result of my test and calculation, Can you give me some advice? Thanks 
 
 adcvalue.xls

Hieu · Accepted Answer

Hi Август, 
 In your implementation, I see there are a few things that can be improved: 
 
 Add calibration . It is recommended to perform calibration at least once before using SAADC, and every time the ambient temperature changed by 10°C 
 Use a higher resolution. The nRF52833 is capable of 14-bit SAADC resolution if oversampling is used 
 Configure oversampling. In your code, it is unclear how much oversampling you are having 
 
 Another point to consider is, what is the actual exact value of the resistors you are using? 27k and 1M are just the nominal value. There could be some tolerance that you have not accounted for. 
 All that is not to say that the SAADC on a nRF52833 is perfectly accurate. It is not intended to be extremely accurate. You can usually expect some fluctuations in some of the LSBits of your ADC readings. 
 Considering such fluctuations, the value in your Excel table doesn't look too bad to me. What is the level of accuracy you are expecting? 
 Hieu

52833 adc deviation

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