INA vs difference amp vs differential amp - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: November 15, 2020, 05:21:28 pm
content: Hi,in practicality what it the difference between instrumentation amplifier and difference amplifier and differential amplifiers?Where would you use each of them? Why would you use any one of them rather than another?Some online resources seem to use difference amplifiers and differential amplifiers interchangeably but manufacturer websites classify them separately.Thank you

author: fcb
author_rank: Super Contributor
timestamp: November 15, 2020, 06:05:51 pm
content: Generally an instrumentation amplifier is configured with +IN and -IN as very high-impedance, and usually sets the gain with one resistor (Rg).

author: Siwastaja
author_rank: Super Contributor
timestamp: November 15, 2020, 06:16:19 pm
content: For a long time, I have been under the impression that "differential amplifier" means one with differential OUTPUT, that's easily recognized by having two output pins anti-phase to each other! But Wikipedia is disagreeing with me, and claiming a bog standard opamp (with differentialinput) is a differential amplifier, so who knows?So don't get too fixated over the names; instead, try to reduce confusion by talking about "differential output amplifiers" explicitly, if you need that, and don't use the term "differential amplifier"at all.It's the same issue when some people talk about "rail-to-rail opamps". What do they mean? Input can be rail-to-rail, or output can be rail-to-rail, or both? There is no other way of fixing this than being explicit and clear.The difference between instrumentation amplifier and difference amplifier is the high input impedance of the instrumentation amp. You can think about it as first buffering the two voltage levels with unity gain opamps, then taking the difference of the buffered results. If you ignored the buffers, you would get a difference amplifier.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2020, 06:47:47 pm
content: Another important feature of most instrumentation amplifier circuits is an "output ground" sense terminal that allows the output voltage to be referenced against the voltage there (within ratings of the output and "ground" with respect to the power supply terminals).

author: gf
author_rank: Super Contributor
timestamp: November 15, 2020, 07:21:18 pm
content: Quote from: Siwastaja on November 15, 2020, 06:16:19 pmMy impression is that these ones are usually called "fullydifferential"(see for instancehttps://www.ti.com/lit/an/sloa054e/sloa054e.pdf?ts=1605418004333)

author: pwlps
author_rank: Frequent Contributor
timestamp: November 15, 2020, 07:25:47 pm
content: I would say the main distinctive and desired feature of instrumentation amplifiers is their extremely high CMRR.The Wikipedia INA page explains how it is obtained, some other differences between INA and opamp/DA and their uses are discussed e.g. here:https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-chI.pdfQuote from: Siwastaja on November 15, 2020, 06:16:19 pmYou forgot the Rgain resistance network after the buffers, increasing the CMRR. If you ignore the buffers you loose it

author: Marco
author_rank: Super Contributor
timestamp: November 15, 2020, 07:44:56 pm
content: A lot of the auto-zero instrumentation amplifiers don't have much to do with the traditional instrumentation amplifier circuits.

author: ebclr
author_rank: Super Contributor
timestamp: November 15, 2020, 08:54:43 pm
content: https://www.mouser.com/pdfDocs/analogengineer%E2%80%99scircuitcookbookopamps.pdf

author: ricko_uk
author_rank: Super Contributor
timestamp: November 16, 2020, 07:06:13 pm
content: Thank you all!!