Historical question about op amps - Page 1
Forum section: electronics / beginners

author: Lee Leduc
author_rank: Regular Contributor
timestamp: February 19, 2018, 03:26:45 pm
content: Why do quad op amps have their power pins reversed from single and dual packages?Just curious.

author: Paul Rose
author_rank: Regular Contributor
timestamp: February 19, 2018, 03:56:27 pm
content: The single pinout goes back before the 741. It just is what it is. It was widely copied for compatibility.The double pinout is more logical, with one amp per side, following the corner pin standard for most chips of the time.The quad uses center power pins, which is nice for bypass cap layout, and has 4 nice logical areas for each amp.There were some (not so common) DIP logic gate packages that had center pins ( I think TI promoted the idea ). If I recall correctly, they had VCC on the "left", just like thedualquad op-amp.

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2018, 10:47:42 am
content: The pin-out for the quad op-amp, makes perfect sense.I'm more interested as to, why did they choose a totally different pin-out for the quad comparator?http://www.learningaboutelectronics.com/Articles/LM339-quad-voltage-comparator-circuit.phphttp://www.ti.com/lit/ds/symlink/lm2901.pdf

author: Jwillis
author_rank: Super Contributor
timestamp: February 20, 2018, 11:46:41 am
content: Just a guess but wouldn't pin outs be placed according the the chip layout to take the shortest possible route minimizing the wiring inside the plastic case.

author: danadak
author_rank: Super Contributor
timestamp: February 20, 2018, 12:00:30 pm
content: Many factors in pinout and chip layout. Stuff likegetting power out thru the leadframe, the parasiticsassociated with routing, thermoelectric effects, diehot spots, crosstalk.....A witches brew of conflicting needs.Regards, Dana.

author: mikerj
author_rank: Super Contributor
timestamp: February 20, 2018, 12:16:09 pm
content: Quote from: Hero999 on February 20, 2018, 10:47:42 amA total guess, but it does provide separation from fast edges on the outputs with and the inputs i.e. minimising crosstalk issues.

author: Circlotron
author_rank: Super Contributor
timestamp: February 20, 2018, 12:25:08 pm
content: For the quad comparator, having the output pins all together would make it easy to strap them together for a sort of wired OR output. For the dual and quad op amp, having the (-) next to the output makes board layout easy for a unity gain buffer. There may be other reasons of course.

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2018, 09:34:34 pm
content: Quote from: Circlotron on February 20, 2018, 12:25:08 pmSounds reasonable. If that's the case, do you have any idea why they didn't do the same with dual comparators, such as the LM393?http://www.learningaboutelectronics.com/Articles/LM393-comparator-circuit.php

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2018, 01:49:45 am
content: Quote from: Paul Rose on February 19, 2018, 03:56:27 pmThere was a second dual operational amplifier pinout which can be seen in theLT1013 datasheetwhich other early precision parts also used. I have no idea how it came about. I once ordered a whole tube of these for prototype production before noticing the difference.QuoteCentered ground and power pins are used to lower ground bounce by lowering parasitic inductance. TTL FAST and AS and CMOS AC logic are examples where corner power and ground pins limited performance. Fast bus oriented logic devices which have lots of simultaneously switching outputs also used centered ground and power pins for the same reason.Quote from: Hero999 on February 20, 2018, 10:47:42 amThis isolates the fast output edge from the input signals. Coupling to the inverting input is especially problematical as it will make the comparator "chatter". Note that JFET input comparators were never popular because of the same problem; high input impedance is not compatible with the fast switched output of a comparator.Quote from: Circlotron on February 20, 2018, 12:25:08 pmFor an operational amplifier, a small amount of coupling between the output and inverting input is actually desirable to cancel capacitance to ground at that point which would otherwise lower phase margin.Quote from: Hero999 on February 20, 2018, 09:34:34 pmThat is a good question. The LM393 is not really fast enough to require the extra isolation and they may have wanted to keep the LM358 pinout for easy substitution since it is commonly used as an even slower comparator as well. High speed dual comparators use a different pinout.I like Circlotron's explanation for why the equally slow LM339 didn't use the standard quad operational amplifier pinout; bridging four open collector outputs would be more difficult than two.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2018, 08:00:27 am
content: Quote from: Jwillis on February 20, 2018, 11:46:41 amBack when silicon was expensive, this was probably a big motivation.I suspect a lot of CD4000 pinouts were designed this way. What a cluster!It was later realized that silicon is cheap, pins are huge, and PCBs are even huger; there is much savings to be had by doing all the hard routing work on die.Not that this is a universal method. A lot of MCUs have shitty pinouts. Comparing an orderly ATMEGA in QFP to a messed up MSP430 in QFN, I literally waste more space on routing signals all the way around the chip, than the package size difference alone! (Given the same 2 or 4 layer design, that is. Of course, the QFN unconditionally saves space when more layers are available, especially if blind vias or HDI are used. But, those are expensive, too.)Arguably, some of that can be "fixed in software", but it's a big pain shuffling around port bits. One option costs development time, the other costs physical space...On a related note, you sometimes see chips in different SMT packages, and one has a rotated pinout, or something weird like that. Usually this happens when the die is rectangular, and just barely fits inside the package in question -- but only in one direction, so the pins have to be bonded out differently.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2018, 04:11:56 pm
content: Quote from: T3sl4co1l on February 22, 2018, 08:00:27 amOh hey! That explains the pinout of early precision dual operational amplifiers in SO8 packages. Mystery solved.Later parts must have used an SO8 friendly layout which would have worked fine in a DIP package to avoid this problem.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: February 22, 2018, 07:59:06 pm
content: I dunno, but it always seems that once someone has established a nice regular standard that everyone can work to, someone else just HAS to ruin it.