OpAmp Diagram - Page 1
Forum section: electronics / projects

author: Randy222
author_rank: Frequent Contributor
timestamp: January 26, 2024, 06:57:20 pm
content: Was browsing some opamp pdf's on a popular chip site, came across this diagram.If the OA rails should be +2.5 and -2.5, why does the neg rail show + side of voltage attached?Should it not be like the batt's I added?

author: ataradov
author_rank: Super Contributor
timestamp: January 26, 2024, 07:08:10 pm
content: This diagram makes no sense. if they meant that batteries would be the source of the +/- 2.5 V, then the bottom battery is reversed.If they meant that batteries are placed in addition to some other voltages, then it makes even less sense.Given the overall vibe of the "schematic", I would say it is just a mistake in polarity.

author: RFDx
author_rank: Regular Contributor
timestamp: January 26, 2024, 08:00:16 pm
content: Quote from: Randy222 on January 26, 2024, 06:57:20 pmThe neg. rail has the plus side attached but the voltage is "-2.5V". For CAE software that is perfectly acceptable.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 26, 2024, 08:04:57 pm
content: Quote from: RFDx on January 26, 2024, 08:00:16 pmWe must have hit POST at exact same time, my reply did not post.The reference appears to be GND, whatever GND is. So a "+" sign would indicate above GND, and a "-" sign would indicate below GND. That's the std, no?

author: Bud
author_rank: Super Contributor
timestamp: January 26, 2024, 08:18:11 pm
content: Other errors aside, one battery is 1.5V , not 2.5V.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 26, 2024, 08:23:04 pm
content: I was just illustrating + - rails using two batts.For argument sake, the "AA" batts in pic are each 2.5v.

author: RFDx
author_rank: Regular Contributor
timestamp: January 26, 2024, 08:27:31 pm
content: Yes, the "-" sign in front of 2.5V says that supply rail is 2.5V below ground.The batteries represent another (lower voltage) option to power the opamp.

author: eutectique
author_rank: Frequent Contributor
timestamp: January 26, 2024, 09:17:05 pm
content: Quote from: Randy222 on January 26, 2024, 08:23:04 pmAren't they AAA batteries?

author: Randy222
author_rank: Frequent Contributor
timestamp: January 26, 2024, 09:50:27 pm
content: Quote from: eutectique on January 26, 2024, 09:17:05 pmDid I miss an "A"?Maybe they are AAA, each 2.5v.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2024, 10:15:50 pm
content: AA or AAA batteries are normally 1.4V (fully charged NiMH) or 1.6V (fresh alkaline), not 2.5V.Other than that, the notation used on the schematic is correct. It's perfectly acceptable to label voltage sources negative, to denote the negative polarity, rather than connecting the positive to 0V.The two configurations shown below are equivalent.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 27, 2024, 09:55:10 am
content: Oh no they're not.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2024, 10:29:19 am
content: Quote from: Zero999 on January 26, 2024, 10:15:50 pmX 3/10Must try harder.

author: RFDx
author_rank: Regular Contributor
timestamp: January 27, 2024, 11:29:08 am
content: Quote from: MarkT on January 27, 2024, 09:55:10 amYes they are.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 27, 2024, 12:04:02 pm
content: Quote from: RFDx on January 27, 2024, 11:29:08 amThey are not.

author: magic
author_rank: Super Contributor
timestamp: January 27, 2024, 12:06:24 pm
content: Quote from: wasedadoc on January 27, 2024, 12:04:02 pmThey are equivalent if you accept the idea of negative voltage batteries whose positive terminal is negative with respect to their negative terminal.I would prefer the abstract voltage source symbol instead of batteries here.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2024, 09:55:21 am
content: Negative voltage batteries do exist. For example lead acid cells often charge up in the reverse polarity.If you're going to be picky, a battery symbol isn't the right one for an 1.5V alkaline cell, since it's one cell.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 02:02:33 pm
content: Quote from: Zero999 on January 28, 2024, 09:55:21 amI think that would count as a misused battery, and rather feeble edge case in this context.Basically, the bottom right quadrant of your diagram is incorrect. Setting aside the multi-cell rather than single cell symbol, you can't just use a standard polarity battery symbol (complete with added '+' sign for clarity), stuff a -1.5V label next to it and have it be ok!You either need to stick the battery symbol the other way up, or have the ground symbol connected to the top and the V- symbol to the bottom. Let's get this basic stuff right, it's not picky.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2024, 05:20:02 pm
content: Quote from: Gyro on January 28, 2024, 02:02:33 pmWell I just did that and everything was ok. I tried it in a simulator and it worked. I also performed calculations using Ohm's law and it adds up perfectly.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 05:50:40 pm
content: You might be able to get it to simulate ok by specifying a negative voltage, but it doesn'treadok. Schematics need to be clearly readable.It really hadn't occurred to me that you might be relying just on simulation to show the equivalence of the two forms of representation, rather than visual correctness.I'm slightly gobsmacked.

author: magic
author_rank: Super Contributor
timestamp: January 28, 2024, 07:05:41 pm
content: In LTspice (where this picture seems to come from) these symbols are meaningless, it's always just an arbitrary voltage source.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2024, 07:53:09 pm
content: Quote from: magic on January 28, 2024, 07:05:41 pmQuote from: Gyro on January 28, 2024, 05:50:40 pmBoth comments are true. I chose a battery symbol, purely because that's what the original poster showed on their schematic. I wouldn't normally draw it like that, but I've seen far worse things on schematics. There are things which annoy me far more.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 08:31:14 pm
content: You might still want to post a visually accurate one for anyone without the benefit of the asc file.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 29, 2024, 02:14:53 pm
content: I think there's some confusion.1st, I added the batts for illustrative purposes only. What those batts are, their voltage, is not relevant here.2nd, the ckt in 1st post is from a TI PDF, I did not create the ckt illustration.3rd, if the TI illustration indicates "-2.5v" for the OA "V-" connection, then the batt symbol used, along with the "+" sign should indicate neg voltage on OA V- pin.Like this.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2024, 02:34:26 pm
content: Quote from: Randy222 on January 29, 2024, 02:14:53 pmNo, that won't work, because voltages are relative. The op-amp'a power supply pins will both see 2.5V. If all the inputs are nearer 0V, bad things are likely to happen.To correct it, get rid of the negative sign.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 29, 2024, 03:15:31 pm
content: Quote from: Zero999 on January 29, 2024, 02:34:26 pmAre you commenting on post #1, or the new diagram?In the newThe V- pin shows "-2.5v"The V+ pin shows "+2.5v"That should be 5v rail-to-rail, +2.5 to -2.5

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2024, 03:57:10 pm
content: Quote from: Randy222 on January 29, 2024, 03:15:31 pmI'm talking about the new diagram. Both sides of the op-amp supply are now positive.As others have mentioned, the battery symbol is not the right one to use in this situation.The original schematic was correct, other than the battery symbols, although I consider it to be a minor issue.Here's what happens if I enter the new schematic into a simulator and ask it to calculate the steady state voltages.Verses the orginal one.

author: magic
author_rank: Super Contributor
timestamp: January 29, 2024, 04:15:48 pm
content: Quote from: Zero999 on January 29, 2024, 03:57:10 pmIt's a major issue and the whole reason why this increasingly silly thread exists.There are no negative voltage batteries whose positive terminal is marked negative and whose negative terminal is marked positive. Using such notation on paper is of course possible, but it only creates confusion, as observed here.If you want to create negative voltage with batteries, flip the battery so that + goes to ground and either mark the battery simply "2.5V" (without minus) or mark the generated voltage rail (not the battery) "-2.5V".The purpose of circuit simulators is simulating circuits, not being standards on what notation is good or bad. A circuit simulator may use the dumbest notation imaginable and still simulate correctly if you understand the notation and input the schematic in a manner which will be interpreted the way you want.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 29, 2024, 04:25:21 pm
content: Quote from: magic on January 29, 2024, 04:15:48 pmI think you are saying what I was trying to say.V- really needs to be below gnd, so showing a "+" sign on the V- pin makes no sense if the intention was to have a neg voltage (-2.5) on V-There's another way the original ckt illustration could have been goofed, they could have literally flipped the batt symbol over on V- and kept "+" sign on V-, which would still be wonky.

author: zapta
author_rank: Super Contributor
timestamp: January 29, 2024, 05:27:05 pm
content: You are right, the bottom battery symbol is reversed. A schematic typo.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2024, 06:15:44 pm
content: The battery symbol didn't confuse me, because I've seen it used for a voltage source in the past. A simulator program I used when I was a child just used the battery symbol to represent a voltage source by default. The fact it's 2.5V, made it obvious it's a voltage source, rather than a real battery.This is how I would prefer to draw it.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2024, 06:31:17 pm
content: Is itreallyso important to you that you don't loose face by simply acknowledging that you put the battery symbol the wrong way round and correcting it?!The schematic symbol polarity is perfectly clear and assigning a negative value to it in an unpublished sim just isn't ok. Magic is right, it is a big issue, schematics should read correctly.The alternative is that you attach asc files with any schematic you post in case you've played games with the component parameters! It's such a trivial representation anyway that there was no reason to to guess that you had bothered to simulate it, and that it only worked in sim.