What is this strange symbol?

[GibsonGM]

Hi,

When looking thru some design notes from TI, I came across this parallel line symbol...anyone know if this means "is proportional to" or something?  I thought maybe, but that doesn't make much sense to me in the context...it's about "the rules" for biasing opamps for single supply.  I was under the assumption that R1 and R2 are independent of R3, and as long as R1 is over 1 to 10K everything else 'doesn't matter'....  THX.

[BlueToad]

I think JL hood uses that to denote "in parallel."

[GibsonGM]

Ahhhhhh, thanks toad     I KNEW it was something with a "P", ha ha.  Makes perfect sense now...

[MartyMart]

Doesn't that mean that R3 should equal the sum of  R1 + R2 in series  ??
So if they were close to 10k, like 9k6 and 9k8 R3 needs to be 19k4  or as close as !

just a theory ... could be wrong

[R.G.]

Doesn't that mean that R3 should equal the sum of  R1 + R2 in series  ??

Yes.

So if they were close to 10k, like 9k6 and 9k8 R3 needs to be 19k4  or as close as !

If R1 and R2 were 10K, R3 needs to be 5K.

The point behind the note is that opamp inputs pull some bias current, however small. The bias current comes through the resistance that the input sees, so that if you want best DC accuracy, the bias currents which are nominally equal should come through equal resistances. The resistance that the inverting input sees is the parallel combination of the input and feedback resistor, so you put a resistor close to equal to that in the path to the + input for best DC accuracy.

If you get close, it's better than being far off, so in the 10K/10K case, 4.7K is probably fine, and is better than 2.2K, which is better than a wire. If you don't need DC accuracy, it doesn't matter, and a wire is as good as 5.0000K.

[scaesic]

Doesn't that mean that R3 should equal the sum of  R1 + R2 in series  ??

Yes.

i'm pretty sure you meant to say "no" there. R3 should be the equivalent of R1 and R2 in parrallel, not in series, which is what you explained anyway...

[GibsonGM]

Thanks guys...I recall seeing that symbol somewhere in the distant past, and it did mean parallel now that I hear it again.  This fits the previously-mentioned "rules" i'd seen governing bias current, and is an easy rule of thumb for picking the correct R3.  Every once in a while I still come across something odd regarding this hobby...    
I have been trying to avoid "stealing" biasing setups from distortion ckts just because they work, and to use the actual 'right way' of setting things up for best operation rather than going blindly....

[R.G.]

i'm pretty sure you meant to say "no" there. R3 should be the equivalent of R1 and R2 in parrallel, not in series, which is what you explained anyway...

Uhhh... yeah, what I meant, not what I typed... 

My typing speed is a problem. I get an idea and put it in the output buffer for typing while my mind goes off to other tasks and comes back later to check on the output queue. Sometime the queue data integrity is not all it should be. 

[Alex C]

If "||" is not specified, is it assumed that the components are in series?
as in:
Rc = Ra + Rb
or is there a complementary symbol for series?

[PenPen]

If "||" is not specified, is it assumed that the components are in series?
as in:
Rc = Ra + Rb
or is there a complementary symbol for series?

That would be series. R1 + R2 is adding both resistances together, which is series for resistance.

[scaesic]

If "||" is not specified, is it assumed that the components are in series?
as in:
Rc = Ra + Rb
or is there a complementary symbol for series?

No, || is always short hand for parrallel, think about it, it's two parralel lines..., you don't need a short hand for series because + is already shortened.

i.e R1 + R2 is the total resistance of two resistors in series

R1 || R2 is the total resistance of two resitors in parrallel.

it's a maths/physics thing.

[jonathan perez]

it stands for BBQ...

in between soldering r1 and r2, youre supposed to go fire up the grill, and throw a pair of hot dogs on.......

right?

[Alex C]

No, || is always short hand for parrallel, think about it, it's two parralel lines..., you don't need a short hand for series because + is already shortened.
i.e R1 + R2 is the total resistance of two resistors in series
R1 || R2 is the total resistance of two resitors in parrallel.
it's a maths/physics thing.

I understand the parallel notation, but I try to be cautious of assuming that what makes sense and seems simplest to me is the way things are done. 
As I study science and engineering, I find that very often extra care is taken to reduce the possibility for error, miscalculation, and so on; basically eliminating the need to assume, and possibly make mistakes.

I guess what I'm asking is whether that's how it's done.  Do I need to know any other notation or will others know that I mean series if I simply say Ra + Rb?

What about other components?  Let's say caps, where values are additive in parallel.  In this case it seems to me that it would be intuitive that Ca + Cb is in a parallel configuration (because of the additive properties of parallel caps), thus rendering the || symbol redundant .  If this is not the case, how is series denoted? 

[scaesic]

No, || is always short hand for parrallel, think about it, it's two parralel lines..., you don't need a short hand for series because + is already shortened.
i.e R1 + R2 is the total resistance of two resistors in series
R1 || R2 is the total resistance of two resitors in parrallel.
it's a maths/physics thing.

I understand the parallel notation, but I try to be cautious of assuming that what makes sense and seems simplest to me is the way things are done. 
As I study science and engineering, I find that very often extra care is taken to reduce the possibility for error, miscalculation, and so on; basically eliminating the need to assume, and possibly make mistakes.

I guess what I'm asking is whether that's how it's done.  Do I need to know any other notation or will others know that I mean series if I simply say Ra + Rb?

What about other components?  Let's say caps, where values are additive in parallel.  In this case it seems to me that it would be intuitive that Ca + Cb is in a parallel configuration (because of the additive properties of parallel caps), thus rendering the || symbol redundant .  If this is not the case, how is series denoted? 

i don't think the convention is that well defined, your over-thinking it. C1+C2 could equally be regarded as C1||C2, but you're right, it'd be redundant. In the case of Resistance it just happens to be a lot easier to write R1||R2 than R1R2/(R1+R2). I don't think there would be a short hand way of writing C1 and C2 in series, it'd just become confusing.

In physics/endineering i've found that || and an upside down T are often used to denote parralell and perpendicular, when decomposing vectors.

[Gilles C]

I learned electronics over 30 years ago, and what we used then for the parallel symbol was //

And we were using pencils to write formulas in those days....

I still prefer to use // but both ways seem to be correct, // or ||

http://searchdatacenter.techtarget.com/sDefinition/0,,sid80_gci803019,00.html

It seems it was used first in 1673 and 1677

http://members.aol.com/jeff570/geometry.html

Gilles