Instrumentation
Amplifier
Dr. Mohamed Ibrahem Waly
Assistant professor biomedical engineering and systems
Biomedical Technology department
m.waly@mu.edu.sa
Instrumentation Amplifier
Instrumentation amplifiers are commonly used in environments with high commonmode noise such as in data acquisition systems where remote sensing of input
variables is required.
An instrumentation amplifier is a differential voltage-gain device that amplifies the
difference between the voltages existing at its two input terminals. The main
purpose of an instrumentation amplifier is to amplify small signals that are riding on
large common-mode voltages. The key characteristics are high input impedance,
high common-mode rejection, low output offset, and low output impedance. A basic
instrumentation amplifier is made up of three operational amplifiers and several
resistors. The voltage gain is set with an external resistor.
A basic instrumentation amplifier is shown in Figure 14-1. Op-amps A 1 and A2 are
noninverting configurations that provide high input impedance and voltage gain. Opamp A3 is used as a unity-gain differential amplifier.
Instrumentation Amplifier
A basic instrumentation amplifier is shown in Figure. Op-amps A1 and A2 are
noninverting configurations that provide high input impedance and voltage gain. Opamp A3 is used as a unity-gain differential amplifier.
Instrumentation Amplifier
The gain-setting resistor, RG, is connected externally as shown in Figure. Op-amp A1
receives the differential input signal Vin1 on its noninverting (+) input and amplifies
this signal with a voltage gain of
Op-amp A1 also has Vin2 as an input signal to its inverting (-) input through op-amp
A2 and the path formed by R2 and RG. The input signal Vill2 is amplified by op-amp
Al with a voltage gain of
Instrumentation Amplifier
The gain-setting resistor, RG, is connected externally as shown in Figure. Op-amp A1
receives the differential input signal Vin1 on its noninverting (+) input and amplifies
this signal with a voltage gain of
Op-amp A1 also has Vin2 as an input signal to its inverting (-) input through op-amp
A2 and the path formed by R2 and RG. The input signal Vill2 is amplified by op-amp
Al with a voltage gain of
Instrumentation Amplifier
Also, the common-mode voltage, Vcm on the noninverting
input is amplified by the small common-mode gain of opamp A1. (Acm is typically less than 1.) The total output
voltage of op-amp A1 is
Instrumentation Amplifier

Op-amp A3 has Vout1 on one of its inputs and Vout2 on
the other. Therefore, the differential input voltage to
op-amp A3 is V out2 - V out1
Instrumentation Amplifier
Notice that, since the common-mode voltages (Vcm) are
equal, they cancel each other. Factoring out the
differential gain gives the following expression for the
differential input to op-amp A3:
Instrumentation Amplifier
Instrumentation Amplifier
Applications:
The instrumentation amplifier is normally used to measure small differential signal
voltages that are superimposed on a common-mode voltage often much larger than
the signal voltage.
Applications include situations where a quantity is sensed by a remote device, such as
a temperature- or pressure-sensitive transducer, and the resulting small electrical
signal is sent over a long line subject to electrical noise that produces common-mode
voltages in the line. The instrumentation amplifier at the end of the line must
amplify the small signal from the remote sensor and reject the large common-mode
voltage.
Instrumentation Amplifier
Isolation Amplifiers
An isolation amplifier provides dc isolation between input and output.
It is used for the protection of human life or sensitive equipment in those applications
where hazardous power-line leakage or high-voltage transients are possible. The
principal areas of application are in medical instrumentation, power plant
instrumentation, industrial processing, and automated testing.
An isolation amplifier is a device that consists of two electrically isolated stages. The
input stage and the output stage are separated from each other by an isolation
barrier so that a signal must be processed in order to be coupled across the isolation
barrier. Some isolation amplifiers use optical coupling or transformer coupling to
provide isolation between the stages. However, most modern isolation amplifiers use
capacitive coupling for isolation.
Isolation Amplifiers
Each stage has separate supply voltages and grounds so that there are no common
electrical paths between them. A simplified block diagram for a typical isolation
amplifier is shown in Figure. Notice two different ground symbols are used to
reinforce the concept of stage separation.
Isolation Amplifiers
The input stage consists of an amplifier, an oscillator, and a modulator.
Modulation is the process of allowing a signal containing information to modify a
characteristic of another signal, such as amplitude, frequency, or pulse width, so that
the information in the first signal is also contained in the second. In this case, the
modulator uses a high-frequency square wave oscillator to modify the original signal.
A small-value capacitor in the isolation barrier is used to couple the lower-frequency
modulated signal or dc voltage from the input to the output. Without modulation,
prohibitively high-value capacitors would be necessary with a resulting degradation in
the isolation between the stages. The output stage consists of a demodulator that
extracts the original input signal from the modulated signal so that the original signal
from the input stage is back to its original form.