OP Amp Question  - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: May 08, 2019, 11:44:55 pm
content: I'm trying to wrap my head around the data sheet and other stuff about Op Amps but was never any good at understanding what I'm reading.https://www.ti.com/lit/ds/symlink/opa445.pdfNow as I under stand from the data sheet the Common-Mode Voltage Range or Vcm is ±5 of ±Vss (source voltage ).So that means that the maximum input voltage on the noniverting side would be the positive voltage source minus 5 V and the inverting side would be the the negative voltage source plus 5 volts ?So for example in my diagram the positive voltage source is 35V so the positive input would be 30V maximum and on the negative side at -10V would be -5V minimum inputs on both inverting and non inverting relative to a zero V common ground.Is that correct ?

author: xavier60
author_rank: Super Contributor
timestamp: May 09, 2019, 02:44:30 am
content: "Common Mode" implies that the same voltage is applied to both inputs simultaneously.The specs don't make complete sense to me. They say -35V to +35V for a supply voltage of +/-40V which is range of 70V.So I don't understand what the "80" and "95" are supposed to mean.

author: Jwillis
author_rank: Super Contributor
timestamp: May 09, 2019, 03:25:26 am
content: I'm still learning but what I'm trying to figure out is the maximum voltages to the inputs as related to the supplied voltages relative to a common ground.

author: bson
author_rank: Unknown
timestamp: May 09, 2019, 03:28:57 am
content: Neither input can stray outside the common-mode voltage range. So if the supplies are ±25V the inputs need to stay between ±20V. Both of them.

author: Jwillis
author_rank: Super Contributor
timestamp: May 09, 2019, 03:42:17 am
content: Quote from: bson on May 09, 2019, 03:28:57 amOK but on page 8 of the data sheet under power supplies it states that the positive supply can be 80V and the negative supply can be -10v or visa-versa That means to me that the total voltage across both the positive supply and negative supply can't exceed 90V and and is not less than 20V total.So neither supply needs to be equal provided that the maximum and minimum voltage ratings arn't exceeded.

author: magic
author_rank: Super Contributor
timestamp: May 09, 2019, 07:08:57 am
content: Yes, they don't guarantee proper operation with <20V or >90V total supply voltage between V- and V+.They do guarantee proper operation with 80V total supply ifbothinputs are at least 5V away from each rail.They say nothing about other supply voltages, but in practice the limit will likely be close to 5V from the rails too.It doesn't really matter if there is some "ground" somewhere between the supplies and where it is, the opamp isn't even connected to it.

author: Jwillis
author_rank: Super Contributor
timestamp: May 09, 2019, 01:17:12 pm
content: Quote from: magic on May 09, 2019, 07:08:57 amOh ok so as long as the difference between each supply voltage doesn't exceed 80 V .then if the negative rail is say - 15 V the inverting rail cant be less than -10V and the if the positive rail is say +30V the non-inverting input must be at most +25V relative to the common ground.