ESP32 WROVER IDF使用i2s_adc读取电压时，打印数据都是0，如何才能读到正确的电压值？

目前我使用ESP32 WROVER IDF 参考官方例程i2s_adc_dac读取电压时，打印数据都是0。代码如下：
void init_i2s()
{
        i2s_config_t i2s_config;
        i2s_config.mode = I2S_MODE_MASTER | I2S_MODE_RX | I2S_MODE_ADC_BUILT_IN;
        i2s_config.sample_rate = I2S_SAMPLE_RATE;
        i2s_config.dma_buf_len = I2S_BUFFER;
        i2s_config.bits_per_sample = I2S_BITS_PER_SAMPLE_16BIT;
        //i2s_config.channel_format = I2S_CHANNEL_FMT_ONLY_LEFT;
        i2s_config.channel_format = I2S_CHANNEL_FMT_RIGHT_LEFT;
        i2s_config.use_apll = false;
        i2s_config.communication_format = I2S_COMM_FORMAT_PCM;
        i2s_config.intr_alloc_flags = 0;
        i2s_config.dma_buf_count = 5;

        // install and start i2s driver
        ESP_ERROR_CHECK( adc_gpio_init(ADC_UNIT_1, ADC_CHANNEL_0) );
        ESP_ERROR_CHECK( i2s_driver_install(I2S_NUM_0, &i2s_config,  0, NULL) );
        ESP_ERROR_CHECK( i2s_set_adc_mode(ADC_UNIT_1, ADC1_CHANNEL_0) );
        ESP_ERROR_CHECK( i2s_adc_enable(I2S_NUM_0) );
}

void sample_i2s()
{
        ESP_LOGI(TAG, "Task called.");
        //uint16_t i2s_buff[I2S_BUFFER];
        int i2s_read_len = I2S_BUFFER;
        char* i2s_buff = (char*) calloc(i2s_read_len, sizeof(char));
        size_t bytes_read = 0;
        //ESP_ERROR_CHECK( i2s_read(I2S_NUM_0, (void*) i2s_buff, i2s_read_len, &bytes_read, portMAX_DELAY) );
        int error=i2s_read(I2S_NUM_0, (void*) i2s_buff, i2s_read_len, &bytes_read, portMAX_DELAY);
        printf("error=%d rn",error);  //error=0
        for(int i=0;i