Exceeding values when using the SAADC

[Raluca]

Good afternoon, 

I am trying to use a function generator to send a sinusoidal function to my nRF7002 DK and I am vizualizing the values in Putty. I am sending with a frequency of 100kHz, 1.5 Vpp. I observed that I get a lot of values of -4096, which it`s the maximum value I can get with the SAADC. Why that might be the reason for reaching that value so easily? On the positive side, my maximum is 1200 or so. 

Thank you.

[runsiv]

Hi Raluca

It sounds to me like there might be some configuration issues here. Could you please provide the following information:

1. SDK version

2. Your prj.conf

3. Your device tree

4. The ADC part of the application

Regards

Runar

[Raluca]

Hello!

1. SDK version: v2.5.99 - dev1. Answering this I realized I am in a dev branch, I will change and test it again. 

2.This is my prj file.

 

CONFIG_LOG=y

# Step 1 - Enable nrfx drivers #
CONFIG_NRFX_SAADC=y
CONFIG_NRFX_PPI=y
CONFIG_NRFX_DPPI=y
CONFIG_NRFX_TIMER2=y

CONFIG_LOG_BUFFER_SIZE=16384

3. My device tree:
 

/dts-v1/;
#include <nordic/nrf5340_cpuappns_qkaa.dtsi>
#include "nrf5340_cpuapp_common.dts"

/ {
	model = "Nordic NRF5340 DK NRF5340 Application";
	compatible = "nordic,nrf5340-dk-nrf5340-cpuapp";

	chosen {
		zephyr,sram = &sram0_ns;
		zephyr,flash = &flash0;
		zephyr,code-partition = &slot0_ns_partition;
		zephyr,entropy = &psa_rng;
	};

	psa_rng: psa-rng {
		compatible = "zephyr,psa-crypto-rng";
		status = "okay";
	};
};

4. The configuration of the ADC:
 

static void configure_saadc(void)
{
    nrfx_err_t err;

    /* STEP 5.4 - Connect ADC interrupt to nrfx interrupt handler */
    IRQ_CONNECT(DT_IRQN(DT_NODELABEL(adc)),
    DT_IRQ(DT_NODELABEL(adc), priority),
    nrfx_isr, nrfx_saadc_irq_handler, 0);

    /* STEP 5.5 - Initialize the nrfx_SAADC driver */
    err = nrfx_saadc_init(DT_IRQ(DT_NODELABEL(adc), priority));
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_init error: %08x", err);

        return;
    }   

    
    /* STEP 5.6 - Declare the struct to hold the configuration for the SAADC channel used to sample the battery voltage */
    nrfx_saadc_channel_t channel = NRFX_SAADC_DEFAULT_CHANNEL_SE(NRF_SAADC_INPUT_AIN0, 0);

    /* STEP 5.7 - Change gain config in default config and apply channel configuration */
    channel.channel_config.gain = NRF_SAADC_GAIN1_6;
    err = nrfx_saadc_channels_config(&channel, 1);
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_channels_config error: %08x", err);
        return;
    }

    /* STEP 5.8 - Configure channel 0 in advanced mode with event handler (non-blocking mode) */
    nrfx_saadc_adv_config_t saadc_adv_config = NRFX_SAADC_DEFAULT_ADV_CONFIG;
    err = nrfx_saadc_advanced_mode_set(BIT(0),
                                    NRF_SAADC_RESOLUTION_12BIT,
                                    &saadc_adv_config,
                                    saadc_event_handler);
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_advanced_mode_set error: %08x", err);
        return;
    }
                                            
    /* STEP 5.9 - Configure two buffers to make use of double-buffering feature of SAADC */
    err = nrfx_saadc_buffer_set(saadc_sample_buffer[0], SAADC_BUFFER_SIZE);
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_buffer_set error: %08x", err);
        return;
    }
    err = nrfx_saadc_buffer_set(saadc_sample_buffer[1], SAADC_BUFFER_SIZE);
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_buffer_set error: %08x", err);
        return;
    }

    /* STEP 5.10 - Trigger the SAADC. This will not start sampling, but will prepare buffer for sampling triggered through PPI */
    err = nrfx_saadc_mode_trigger();
    if (err != NRFX_SUCCESS) 
    {
        LOG_ERR("nrfx_saadc_mode_trigger error: %08x", err);
        return;
    }

}

 

Maybe it will be clearer if I attach the entire project as well.

 5857.current_code.zip

Thank you so much!

[runsiv]

Hi

Is your signal centered around 0V? if so it will be outside the GPIO range which has a minimum level of -0.3V. If this is the case, could you try to add a positive DC offset and see if the problem is exist? 

Regards

Runar