void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t channel_config;
    nrf_drv_saadc_config_t saadc_config; // added

    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    channel_config.mode = NRF_SAADC_MODE_DIFFERENTIAL;
    channel_config.acq_time = NRF_SAADC_ACQTIME_10US;
    channel_config.reference = NRF_SAADC_REFERENCE_INTERNAL;
    channel_config.gain = NRF_SAADC_GAIN1_4;
    channel_config.resistor_p = NRF_SAADC_RESISTOR_PULLDOWN;
    channel_config.mode = NRF_SAADC_MODE_SINGLE_ENDED;
    channel_config.burst = NRF_SAADC_BURST_ENABLED;

    channel_config.pin_p = NRF_SAADC_INPUT_AIN4;
    err_code = nrf_drv_saadc_channel_init(0, &channel_config);
    APP_ERROR_CHECK(err_code);

    channel_config.pin_p = NRF_SAADC_INPUT_AIN5;
    err_code = nrf_drv_saadc_channel_init(1, &channel_config);
    APP_ERROR_CHECK(err_code);

    channel_config.pin_p = NRF_SAADC_INPUT_AIN6;
    err_code = nrf_drv_saadc_channel_init(2, &channel_config);
    APP_ERROR_CHECK(err_code); 
 
     channel_config.pin_p = NRF_SAADC_INPUT_AIN7;
    err_code = nrf_drv_saadc_channel_init(3, &channel_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}