Data Acquisition

Figure 3.22Common mode voltagesThe common mode voltage V cm can be calculated from the following:( V A + V B )V cm =2WhereV A = Voltage at the non-inverting terminal of the measurement system withrespect to the instrumentation amplifier ground.V B = Voltage at the inverting terminal of the measurement system withrespect to the instrumentation amplifier ground.An example of a common mode voltage is the output from a bridge circuit, in which thesmall differential signal is superimposed over a much larger common mode voltageintroduced by the excitation of the bridge circuit.Common mode rejection ratio (CMRR)Ideally, a differential amplifier would completely reject any common mode voltagespresent on its input signal lines and only amplify the potential difference between them.Practically, however, these devices do not totally reject common mode voltages. Thecommon mode rejection ratio (CMRR) measures the ability of a differential inputamplifier to reject signals that are common to both signal inputs.The CMRR is defined as the ratio between the common mode signal present at the inputto the amplifier and the signal produced by this voltage at the output of the amplifier, asdefined by the following equation:VCMRR = 20 log 10 (cm)V outThis ratio, normally expressed in dB, can be used to calculate the output voltage error,which would occur due to a common mode voltage appearing at the input. The higher theCMRR, the better the rejection of common mode signals, and the more accurate theoutput due to the differential signal being measured. Typically, a CMRR of 60 dB–80 dBcould be expected for a well-designed system.Common mode input voltage limitsPractically, measurement systems also have another limitation, and this is that there is amaximum and minimum common mode input voltage allowable on each input, with