The reference pin should be, in a dual-supply configuration, connected to ground. It looks like a single instrumentation amplifier, but it really is three op-amps in one package with internal precisely matched laser-trimmed resistors gain is set only by R g. Since the circuit is so common and popular, it’s available in a variety of integrated packages, such as TI’s INA118: If building with discrete amplifiers, the above considerations should make for a robust design, but there’s an easier way. Conveniently these packages are also physically quite small. The variation of all resistors in the package is less critical than the variation of resistors relative to each other, and their respective temperature drift can be tolerated so long as they drift together, also very likely. Multi-resistor packages may have a tolerance of 1% or even 5%, but the difference between one resistor and the next within the package itself will be exceptionally small. An oft’ overlooked possibility is to design with multi-resistor surface mount components. This is a good start, but a little pricey. So how does one mitigate these risks? The obvious choice is to pick resistors with sub-1% precision and very low temperature coefficients. Resistors used for both R 2 and R 3 must also be precisely matched, but since there’s no gain in this stage, the bigger danger is an offset voltage creeping into the output. If the resistors used for R 1 do not match precisely, the voltage developed at the two output terminals will have a gain mismatch error, causing the common mode voltage to be affected by the input amplitude. The phrase “perfectly matched” should send prickling sensations up and down the weary designer’s spine nothing is ever perfectly matched.
![instrumentation op amp offset instrumentation op amp offset](https://i.ytimg.com/vi/hpCu3HbAiWg/maxresdefault.jpg)
![instrumentation op amp offset instrumentation op amp offset](https://www.allaboutcircuits.com/uploads/articles/labeled-offset-null-circuit.jpg)
Spreading around the gain reduces the risk of amplifiers hitting their gain bandwidth product limit, allowing the circuit to handle a little higher frequency. If the property being measured has a frequency component, there is an advantage to using R g for only some of the gain, and the ratio of R 3/ R 2 for the rest. In this case the signal being amplified is of very low frequency:į sensor R 2. Typically constructed from several amplifiers, it has proven so effective in amplifying low-frequency differential voltages, it’s now widely available in pre-packaged form. Last post we examined the Sallen-Key topology, now we get into a specialty device, namely the instrumentation amplifier.