1.4.3 The Advantage of Differential Inputs

A relatively new feature on some microcontrollers is the ability to sample a signal differentially. Single-ended sampling is a measurement of a signal concerning the ADC ground. Figure 1-7 shows a simple single-ended signal being acquired.

Figure 1-7. Signal Ended Sampling

However, if the single-ended sample has a large common-mode voltage with a small signal on top, it is hard to get a good measurement since the resolution of the ADC must encompass both the signals. Amplifying the entire signal with an op amp or similar will increase the common-mode voltage and the desired signal.

Figure 1-8. Example of a Signal with a Large Common-Mode Voltage

An alternative method is to acquire the signal differentially. In this configuration, the ADC measures the difference between its inputs. The common-mode voltage is significantly attenuated, which leaves only the differential element to be measured and reduces the reference voltage required to acquire the signal (see Signal Scale and Resolution) without clipping, which further improves the resolution of the measurement. See Figure 1-9.

Figure 1-9. Differential Measurement of a Wheatstone Bridge
Note: Always keep input signals within the absolute maximum ratings specified in the data sheet, or permanent damage may occur.