How do I read a differential signal using the Arduino Due ADC?

Question

I'm having some difficulty configuring the Arduino Due ADC to read a differential signal. I have connected the positive terminal to pin A1 (CH6), and the negative terminal to pin A0 (CH7). The common mode voltage is 3.3V / 2.

The initialization code is

pmc_enable_periph_clk(ID_ADC); // Enable the peripheral clock.

// Initialize the ADC.
adc_init(ADC, // Pointer to an ADC.
    sysclk_get_cpu_hz(), // Master clock frequency.
    2 * ADC_FREQ_MIN, // The ADC frequency.
    8); // The startup time.

// Enable individual settings for the input channels.
adc_enable_anch(ADC);

adc_configure_timing(ADC, // Pointer to an ADC.
    2, // Tracking time.
    ADC_SETTLING_TIME_3, // Settling time.
    1); // Data transfer time.

// Configure the conversion resolution.
adc_set_resolution(ADC, // Pointer to an ADC.
    ADC_MR_LOWRES_BITS_12); // Use 12-bit resolution.

// Enable the input channels.
adc_enable_channel(ADC, ADC_CHANNEL_6); // Pin A1.
adc_enable_channel(ADC, ADC_CHANNEL_7); // Pin A0.

// Configure channels as differential input.
adc_enable_channel_differential_input(ADC, ADC_CHANNEL_6); // Pin A1. CH6+.
adc_enable_channel_differential_input(ADC, ADC_CHANNEL_7); // Pin A0. CH6-.


// Configure how the ADC conversion process is triggered.
adc_configure_trigger(ADC, // Pointer to an ADC.
    ADC_TRIG_SW, // Use software triggering for conversion.
    1); // Free-running mode (no trigger is required for conversion).

// Start the ADC.
adc_start(ADC);

I am trying to read the value using

int16_t i16Value;
i16Value = (int16_t) adc_get_channel_value(ADC, ADC_CHANNEL_6);

But this gives me the common mode voltage plus half the differential voltage. If I manually calculate the value as

int16_t i16Value;
i16Value = (int16_t) ((adc_get_channel_value(ADC, ADC_CHANNEL_6)
    - adc_get_channel_value(ADC, ADC_CHANNEL_7));

then I get a (noisy), but correct value.

Please can anyone either tell me what I am doing incorrectly, or point me in the direction of some example code.

Thanks.

Answer 1

You can read the value using

i16Value = adc_get_latest_value(ADC);`

Note:-An external decoupling capacitance is required for noise filtering.(pg.1317 from datasheet)