Problem getting in read separate ADC channel in nRF52832...

Question

Hello, 
 I am working on nRF52832 SDK 15.3.0 in segger studio. 
 In my project I have 2 ADC input pin. 
 ADC initialization is as shown below, 
 #define SAMPLES_IN_BUFFER 2
static nrf_saadc_value_t adc_buffer[SAMPLES_IN_BUFFER];

/*ADC init start*/
void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
 if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
 {
 ret_code_t err_code;
 uint8_t i = 0;

 err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAMPLES_IN_BUFFER);
 APP_ERROR_CHECK(err_code);

 NRF_LOG_INFO("ADC event number: %d", (int)m_adc_evt_counter);
 for (i = 0; i < SAMPLES_IN_BUFFER; i++)
 {
 adc_buffer[i] = p_event->data.done.p_buffer[i];
 NRF_LOG_INFO("%d", p_event->data.done.p_buffer[i]);
 }
 } 
}
/**/

/*SAADC initialization*/
void saadc_init(void)
{
 ret_code_t err_code;

 nrf_saadc_channel_config_t channel_config_channel_6 =
 NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN6);

 nrf_saadc_channel_config_t channel_config_channel_7 =
 NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN7);

 err_code = nrf_drv_saadc_init(NULL, saadc_callback);
 APP_ERROR_CHECK(err_code);

 err_code = nrf_drv_saadc_channel_init(6, &channel_config_channel_6);
 APP_ERROR_CHECK(err_code);

 err_code = nrf_drv_saadc_channel_init(7, &channel_config_channel_7);
 APP_ERROR_CHECK(err_code);

 err_code = nrf_drv_saadc_buffer_convert(adc_buffer, SAMPLES_IN_BUFFER);
 APP_ERROR_CHECK(err_code);
} 
 And every 0.5 second I read channel by below function: 
 err_code = nrf_drv_saadc_sample(); // Check error
 APP_ERROR_CHECK(err_code); 
 I want to read single channel, so how can I read single channel? 
 Give me suggestion to read single channel.