A PRACTICAL REVIEW OF Common Mode and Instrumentation Amplifiers:Instrumentation amplifiers (in-amps)
amplify the difference between two signals.
These differential signals typically
emanate from sensors such as resistive
bridges or thermocouples. Figure 1 shows a
typical in-amp application where the differential
voltage from a resistive bridge is amplified
by the AD620, a low-power, low-cost,
integrated in-amp. In thermocouple and
bridge applications, the differential voltage
is generally fairly small (a few millivolts to
tens of millivolts). However, the two voltages
from the bridge are equal to about 2.5 V
when each is referred to ground. This voltage,
which is common to both inputs, is
called the common mode voltage of the differential
signal. This voltage contains no
useful information about the measurement.
So ideally, the in-amp should amplify only
the difference between the signals at its two
inputs. Any common mode component
should be ignored by the in-amp. Indeed,
removing the common mode component is
often the sole reason for using an in-amp. In
practice, common mode signals will never
be completely rejected by the in-amp; some
remnant of the signal always appears at the
output.
The specification of common mode rejection
ratio (CMRR) is a measure of the extent
to which common mode signals are rejected
by an amplifier. CMRR is defined by: