Conversion Mode

The conversion block can be put in the unsigned or signed conversion mode. Changing between unsigned/signed and single-ended/differential modes will corrupt data already in the pipeline.

Signed mode can be used as input mode for both differential and single-ended inputs, while unsigned mode is only available for the single-ended or internal input.

In unsigned mode the conversion range is from ground to the reference voltage. To be able to have zero-cross detection a ΔV is subtracted. The ΔV is approximately 0.05* V_REF, so the ground level will be approximately 0.05 of the total value range (0.05 * 4095 with 12-bit resolution). This will also limit the maximum input voltage of 0.05* V_REF, so the maximum input voltage is V_REF - ΔV. This is illustrated in the figure below, "Unsigned and Signed Conversion Mode".

In signed mode the range is from negative to positive reference voltage, but the input voltage must be within GND and V_REF. Figure "Unsigned and Signed Conversion Mode" shows the difference in conversion ranges.

Figure 1. Unsigned and Signed Conversion Mode

The figure above shows that the unsigned mode gives higher resolution on positive values than signed mode, but cannot convert negative values. The signed mode can convert negative values, but at the cost of lower resolution overall.

Note: Negative values are not negative inputs on the I/O pins, but higher voltage level on the negative input in respect to the positive input. Even though the resulting value can be negative, voltages below GND or above VCC should under no circumstances be applied to any input port.

When the ADC uses differential input, signed mode must be used. When using single-ended input both signed and unsigned mode can be used.

Note: Conversion mode is configured for the whole ADC, not individually for each channel, which means that the ADC must be put in the signed mode even if only one of the channels uses differential inputs.

The conversion mode is configured using the Conversion Mode bit (CONVMODE) in Control Register B (CTRLB).