Electronic – How closely do the input buffer op-amps need to be matched on discrete instrumentation amplifiers

An operational amplifier is an integrated circuit, and somewhat of a building block. The inputs (V+ and V-) of an op amp are very high impedance, and very little current goes into these inputs because of that. The general formula for the performance of an op amp is Vout=A*(V+ - V-), and A is a very large number. When wired up in feedback modes, the op amp can have many different configurations (even though the "open loop" gain is a very big number.

A differential amplifier, by definition, also functions with the relationship Vout=A*(V+ - V-), but A can be a much smaller number than in an op amp operating in open loop, and the currents into the inputs are not necessarily zero. The input impedance can be relatively low, and the input impedance to each input does not need to be the same. A differential amplifier can be built out of one or more operational amplifiers and some resistors, or it can be made out of more basic parts, like transistors.

An instrumentation amplifier is a special kind of differential amplifier. In general, it is a differential amplifier, but the input impedances on the two inputs are very high (meaning very small input currents), and the same for each input. There is usually a way to change the gain with one resistor. Very often, the instrumentation amplifier has a three op amp configuration (or the equivalent), with two op amps serving as an input stage, and the output stage is a simple one op amp difference amplifier with a reference point that can be used to move the baseline around. Because the resistors on the difference amplifier are usually laser trimmed inside the integrated circuit, common mode rejection is very high. There are also two op amp instrumentation amplifier configurations, but you lose some of the benefits (like setting the gain with one resistor).

Short answer : Safety current limit for CM input range input Vcb forward bias current from 0.22Ω* current pulse as the source during OFF commutation or unintended loose wire contact when running.

• Also ground shift from poor wiring ESL. where Vemf=Ldi/dt for sense wiring L

• Normally the shunt R 0.22Ω is designed for 50mV drop at max rated current to keep I^2R losses low. But as we know Motor DCR cause start currents >8x the Imax rated current, the negative voltage on the current shunt may exceed -100mV rating for CM input range on this INA.

• If it exceeds -600mV then the input Vbe of the PNP base collector begins to conduct and will blow the front end. So a wise choice of R value is to limit the current with some experience.

• Negative voltages occur from back EMF when the motor is shunted to stop, then it acts as a generator and V = L dI/dt depends on both conduction current and rate of change of current (negative) switching when motor is turned Off also resulting in a negative current spike.