However, the differential amplifier contains an operational amplifier which is not an ideal component. Due to this the absolute voltage of the inputs will be amplified by a certain amount too. This is called the common mode amplification. Suppose that the common mode rejection ratio is 500,000 : 1. This means the differential voltage amplification will be 500,000 higher than the common mode amplification.
A differential amplifier circuit has a low input impedance, which is unfortunately not desirable for a lot of applications. Another inaccuracy is caused by both inputs of the differential amplifier not having the same input impedance. This causes different levels of amplification with various input levels, even if the differential voltage doesn't change.
In total the differential amplifier solution has three cons:
Errors caused by resistor inaccuracy cause common mode amplification.
Both input branches donít have the same loading.
Low input impedance.
All of those are solved by using an instrumentation amplifier instead. The instrumentation amplifier circuit contains a differential amplifier too, but components are added to overcome the differential amplifier disadvantages.