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nrf52832 PPI SAADC Oversample,sample rate up to 250hz

I am using SAADC and PPI to sample 2 channels data with oversample,when I set  timer tick 5ms,the sample rate is 200hz and the system works well,now I want to improve the sample rete,So I change the timer tick to 2ms,the sample rate is 500hz.but I tested that the SAADC output data rate is up to 250hz, the timers frequency is 16M.

void ECG8233_saadc_sampling_event_init(void)
{
ret_code_t err_code;

err_code = nrf_drv_ppi_init();
APP_ERROR_CHECK(err_code);

nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;
// timer_cfg.frequency = 
timer_cfg.bit_width =NRF_TIMER_BIT_WIDTH_32; 

err_code = nrf_drv_timer_init(&m_hardware_timer, &timer_cfg, timer_handler);//m_hardware_timer即TIMER1
APP_ERROR_CHECK(err_code);


uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_hardware_timer, 2); 

nrf_drv_timer_extended_compare(&m_hardware_timer,
NRF_TIMER_CC_CHANNEL0,
ticks,
NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK,
false);

nrf_drv_timer_enable(&m_hardware_timer);

uint32_t timer_compare_event_addr = nrf_drv_timer_compare_event_address_get(&m_hardware_timer,
NRF_TIMER_CC_CHANNEL0);
uint32_t saadc_sample_task_addr = nrf_drv_saadc_sample_task_get();


err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel);
APP_ERROR_CHECK(err_code);

err_code = nrf_drv_ppi_channel_assign(m_ppi_channel,
timer_compare_event_addr,
saadc_sample_task_addr);
APP_ERROR_CHECK(err_code);
}

can anyone help me to solve this problem ,is there any configuration wrong?

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