Instrumentation Amplifier There are 2 types of instrumentation amplifiers 2 op-amp (e.g. INA122) 3 op-amp (e.g. INA333)

Instrumentation Amplifier Notice both have gain equations so you can vary the gain Notice the input impedance is that of the non-inverting terminal of a non-inverting amplifier High-Z Nodes Difference Amp High-Z Nodes Variable Gain

Instrumentation Amplifier Recall the 0V-0A input properties of an ideal amplifier Let’s analyze the INA333

Instrumentation Amplifier From the 0V criteria we can determine that the voltage across Rg=Vin1-Vin2 V=0V + VRg=Vin1-Vin2 - V=0V

Instrumentation Amplifier We can now solve for VRg in terms of Vcm and Vd. Notice the common-mode voltage cancels!

Instrumentation Amplifier From the 0A criteria we know that the current through the R3 resistors must be the same as the current through Rg. 0A I I 0A

Instrumentation Amplifier Current through Rg From Ohm’s law we therefore know that Substituting for Vin1-Vin2 and re-arranging we get Differential Gain

Instrumentation Amplifier-INA333 From previous slide:

Instrumentation Amplifier Input stage also known as: Diff-in, diff-out stage Gain stage The difference amplifier stage is also known as: Output stage Diff-in, single-ended output stage From our difference amplifier discussion we know the differential gain of the output stage is 1. Therefore, Same as PDS