When to use Quad Op-amp vs 2 Dual?

[TySN_Sound]

Hello fellow builders!!

This forum has definitely helped me in the past, but I cannot find the answer I'm looking for, so now my first post!

I have seen circuits in the past that use multiple DUAL op-amps, when it looks like they could use a QUAD in there place.
There are times that this makes sense to me, like it the PCB layout could be better with the two DUALs placed in different areas of the board.
There are other times that placement doesn't seem like it's an issue.

I've read once that you should use separate op-amps for different parts of the circuit, like maybe one DUAL for the main gain staging and another DUAL for the tone circuit (If the pedal needs that many of course).

Other than parts availability and board layout, is there a reason to use two DUAL op-amps over a single QUAD?

Thanks!

-Tyler

[merlinb]

There are many more types of dual opamp than quad, so there is more opportunity for parts substitution and opamp 'rolling'.

[R.G.]

Setting parts availability aside as requested;

I have a solid preference for duals over quads based on signal flow considerations. Quads have an issue with signal flow, with only a few exceptions I know of. The outputs are either in the middle of the rows of pins, or out on the corners. Duals have the inputs on one end of the IC, the two outputs on the other end. In a board layout, if you're worried about slgnal flow direction on your boardsa - and I think you should be - then a dial has a signal flow direction, from one end to the other. A quad has either middles to outsides, or corners to middle. In both cases, it's hard to fit this in a proper signal flow on a board.

Does this matter? It complicates layout to "escape" the output signals from the cluster of parts that are necessarily around the IC, and makes work over signal flow harder,. It can be done, and may not matter at all to the signals. In critical layouts, it can matter, especially with FET input opamps. My approach is to lay out every board as if it was a critical layout, and do the very best I can. It doesn't hurt non-critical layouts to be better.

Signal flow and layout difficulty aside, one might think that you could get a more compact layout from a single quad than two duals. Again, maybe. Two duals, end to end, have exactly two more pins than one quad. There's two more power pins. My preferred "best practice" layout for power on opamps is to run the two (or three) supply traces under the opamp, in the middle under them. This works well with duals, and OK with quads. So it amounts to two more pins. and two duals give you a preferred signal flow from one end to the other.

Bottom line as I see it? Duals offer an advantage over quads for the layouts I do. Quads work, but they are more work. If saving two pins matters, or if you have an application that naturally wants an inside-out signal flow, use a quad.

[ElectricDruid]

+1 agree with RG. I often use duals over quads for layout reasons.

One thing that really bugs me about the typical quad layout (outputs on corners) is that the +V and -V are reversed compared to most other ICs. If I put a row of chips down the middle of a PCB, I can generally run a pair of power busses along the top layer underneath the ICs. this works fine while all the ICs have +V power pins on one side and -ve power pins on the other. Quads don't.

At one point I used to use quads quite a bit, and there are still some things I might use them for (I did a 4-pole lowpass filter with an V2164 and a quad op-amp, for example - it was nice to have the whole filter core on just two chips) but in general Duals are easier and make more sense.

[anotherjim]

One application where I find quads work well for layout is a pair of balanced line drivers requiring 4 amps, since 2 of the amps are plain unity gain noninverting buffers. Unfortunately, some of the best types for that job aren't available as quads! At least I don't think so. But any design that has x2 opamp buffers can use a quad as the lack of external components means they can be pushed up against the edge of the board and the other 2 amps then have plenty more room for their external components.

It is annoying the way most quads have the + and - pins on opposite sides to most everything else.

[Rob Strand]

+1.  Agree with RG as well.   Less power pins often often doesn't end-up with an easier layout.

The quads can be cheaper per opamp.

Duals give you more options subbing parts, which might handy if you want to tweak the tone or some other aspect of the design (noise, power).  They can be used for small projects and larger projects - no need to keep duals *and* quads in stock.