Differential Op Amp Max Input - Page 1
Forum section: electronics / beginners

author: gigavolt
author_rank: Contributor
timestamp: April 06, 2023, 06:20:44 pm
content: I am confused about a datasheet I am reading. Check out page 4 of the following PDF.https://www.ti.com/lit/ds/symlink/opa1637.pdfThis is a +/- 18V rated op amp. But, if I am reading this right, the max differential voltage is only +/- 0.5V. That is, it can only accept a maximum signal of 1 V peak to peak. Is that right? It seems really low, but a lot of the high rail voltage differential op amps have similarly low specs.

author: TimFox
author_rank: Super Contributor
timestamp: April 06, 2023, 07:27:51 pm
content: There are two important maximum values for the input to an op amp.1. Differential is the maximum allowable voltagedifferencebetween the two inputs. In this case, if you exceed that voltage there are diodes connected internally between the two input pins that will conduct too much current if that voltage is exceeded, damaging the device.2. Common mode is the allowable voltagerangeon both inputs (since they can't be more than 0.5 V from each other) with respect to the power supply terminals of the device.In normal operation of an op amp, within its allowable operating range, the (external) negative feedback should ensure that the differential voltage is very small, even when the two input voltages vary within the common-mode range.

author: gigavolt
author_rank: Contributor
timestamp: April 06, 2023, 07:58:18 pm
content: Ok, thank you. Yes, I was just confused about how feedback works with a fully differential op amp. However, yeah, the op amp should keep the inputs at the same voltage.

author: Manul
author_rank: Super Contributor
timestamp: April 06, 2023, 08:19:24 pm
content: Simple example. You build a voltage follower using op amp: you connect ouput directly to inverting (IN-) input and feed signal to non-inverting input (IN+). Gain is 1 and we can say that output is identical to input, hence "follower". It also means that IN- is identical to IN+ (because IN- is connected to output). So you give IN+ 1V and output is 1V and IN- is also 1V. It is easy to see how feedback sets the IN- to equal IN+ and that differential voltage between inputs stays at 0V (given ideal op amp). Same is with other gains and configurations. So for example you can feed 5Vpp signal into op amp follower and differential voltage will stay 0V (given that op amp has enough supply voltage).Maximum differential voltage is only of concern if op amp is overdriven and negative feedback is not enough to maintain closed loop operation. That happens when ouput saturates either negative or positive (hits the limits of it's range).

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2023, 09:02:10 pm
content: Yes, a properly designed op-amp circuit has negative feedback, which causes the voltages between its inputs to be tiny.The only reason why one would directly apply a voltage between the inputs of an op-amp is to use it as a comparator. There are specialised comparator ICs designed for that, such as the LM393. It's possible to use the OPA1637 as a comparator, just make sure the current is limited to the inputs, so the reverse diodes can do their job.