What are the advantages of using an instrumentation amplifier over an ordinary differential amplifier, in measuring low signals and voltages?
What is the instrumentation amplifier?
An instrumentation (or instrumentation) amplifier is a type of differential amplifier that has been equipped with input buffers, which eliminates the need to equalize the input impedance and makes the amplifier particularly suitable for use in measurement equipment and proof.
Additional features include very low DC offset, low drift, low noise level, very good aperture gain, very high common mode rejection ratio and very high input impedances. Instrumentation amplifiers are used where there is great accuracy and stability is required.
Why do we use instrumenatation amplifier more than differential amplifier?
Measuring low voltages or signals quite frequently requires extremely high gains. If you try to provide a very high gain using an ordinary differential amplifier, the output voltage is affected due to the mismatch of the resistor (mismatch of R2 and R4 and mismatch of R1 and R3) which produces significant common mode voltages at the exit.
While an instrumentation amplifier provides most of the gain through a single resistance RG of its first stage, it does not require a resistance match. This leaves your differential amplifier to take the difference of two input signals and to reject any common mode signal present, which is very critical when amplifying small signals.
In addition, the difference amplifier has a low input resistance (for differential input) of 2R1 (when R1 = R3) especially when it tries to provide a greater differential gain. This low resistance problem is solved in an instrumentation amplifier by the use of two additional operational amplifiers in the voltage follower configuration (one for each input) to provide high impedance for the input signals. Instrumental amps are designed to offer low noise, high stability, high common mode rejection dc precision and gain accuracy maintained within a noisy environment, and where large common-mode signals (usually at the ac power line frequency) are present. They usually offer input buffers too.
An instrumentation amplifier has a lower noise and a common mode rejection ratio than a standard operational amplifier. The CMRR is important because you usually need to measure a small differential voltage through a pair of inputs that can oscillate violently around the ground. You do not want this swing to be seen as a signal.