Common Mode Voltage

Signal conditioning modules typically have two types of front-ends; either single ended (SE) or differential ended (DE) inputs. Single ended signals are referenced to the module's power supply common, which should be at the reference potential of the signal being measured. If this reference is not zero potential, then the actual input signal includes an unwanted voltage, which is amplified along with the desired differential signal producing serious errors on the output.
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Differential ended inputs are simply two SE inputs referenced to a common potential where the "information" signal is the difference between these two SE inputs. Signal conditioning modules must use differential input front-ends to amplify the difference between two SE input signals. Any non-zero potential on the reference is common to both SE inputs. Figure 1 illustrates that neglecting all source resistors; V2 is Vcm+Vsignal2 and V1 is Vcm+Vsignal1. Note, (V2-V1) is (Vsignal2-Vsignal1).?