Single-Ended Input

With this setting, the ADC measures the value of one input signal. The difference between this setting and differential measurement is that the negative input is always connected internally to a defined level depending on if signed or unsigned mode is being used. For signed mode the negative input is tied to GND while in unsigned mode it is connected to VREF/2 – ΔV.

Figure 1. Single-ended Input in Signed Mode

ΔV is a fixed and internally generated voltage of approximately 0.05* VREF. This offset needs to be measured by connecting the positive input to ground (GND). The offset will typically correspond to a value of about 200 when measured.

The advantage of ΔV is that it will be possible to measure a negative offset in the ADC block because ΔV will be larger than any offset. ΔV will allow the XMEGA ADC to be used in applications where it is essential to know and compensate for offset errors. The disadvantage is that some of the upper range is lost since any measurement above VREF – ΔV will saturate to the top value.

In addition to connecting the negative input the ADC will in unsigned single ended mode automatically add 2048 to the result. This gives a possible output range from 0 to 4096 as opposed to -2048 to 2047 for signed mode.

Figure 2. Single-ended Input in Unsigned Mode
Note: Since the ADC is differential, unsigned mode is achieved by dividing the reference by two (internally), resulting in an input range from VREF to zero for the positive single ended input. The offset enables the ADC to measure zero cross detection in unsigned mode, and to calibrate any positive offset where the internal ground in the device is higher than the external ground.