OpAmp Diagram

[Randy222]

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?

[ataradov]

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.

[RFDx]

If the OA rails should be +2.5 and -2.5, why does the neg rail show + side of voltage attached?

The neg. rail has the plus side attached but the voltage is "-2.5V". For CAE software that is perfectly acceptable.

[Randy222]

If the OA rails should be +2.5 and -2.5, why does the neg rail show + side of voltage attached?

The neg. rail has the plus side attached but the voltage is "-2.5V". For CAE software that is perfectly acceptable.

We 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?

[Bud]

Other errors aside, one battery is 1.5V , not 2.5V.

[Randy222]

I was just illustrating + - rails using two batts.
For argument sake, the "AA" batts in pic are each 2.5v.   

[RFDx]

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.

[eutectique]

For argument sake, the "AA" batts in pic are each 2.5v.   

Aren't they AAA batteries?

[Randy222]

For argument sake, the "AA" batts in pic are each 2.5v.   

Aren't they AAA batteries?

Did I miss an "A"? 

Maybe they are AAA, each 2.5v.

[Zero999]

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.

[MarkT]

Oh no they're not.

[Gyro]

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.

  X  3/10  Must try harder. 

[RFDx]

Oh no they're not.

Yes they are.

[wasedadoc]

Oh no they're not.

Yes they are.

They are not.

[magic]

Oh no they're not.

Yes they are.

They are not.

They 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.

[Zero999]

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.

[Gyro]

Negative voltage batteries do exist. For example lead acid cells often charge up in the reverse polarity.

I 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.

[Zero999]

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!

Well 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.

[Gyro]

You might be able to get it to simulate ok by specifying a negative voltage, but it doesn't read ok. 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.

[magic]

In LTspice (where this picture seems to come from) these symbols are meaningless, it's always just an arbitrary voltage source.

[Zero999]

In LTspice (where this picture seems to come from) these symbols are meaningless, it's always just an arbitrary voltage source.

You might be able to get it to simulate ok by specifying a negative voltage, but it doesn't read ok. 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.

Both 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.

[Gyro]

You might still want to post a visually accurate one for anyone without the benefit of the asc file.

[Randy222]

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.

[Zero999]

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.

No, 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.

[Randy222]

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.

No, 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.

Are you commenting on post #1, or the new diagram?

In the new
The V- pin shows "-2.5v"
The V+ pin shows "+2.5v"

That should be 5v rail-to-rail, +2.5 to -2.5