MIDI Control (ASM1)

[Harry Bissell]

OK if we had it out before we can have it out again...

1) I agree that the DG series of mux are a fine and wonderful product. Good
stuff...
and also changes with applied voltage, but not as much as the CMOS...

2) By locating the gates at the summing junction, the voltage drop is
minimized... it still changes but not so you would notics... It is also possible
to put an "on" gate in series with the feedback for a first order pass at nulling
that on-resistance change...
A trick to remember with the DG series too, if you are real serious...

3) The error is from the voltage across the gates, when on... if they are off it
doesn't matter. Assume 100K input resistors and 300 ohms for the gate, the
voltage change across the gate is only 30mV which will NOT even register on the
curves for the CMOS mux, and (as you mentioned...) the DG is even better.

4) If I had to allow honest +- 10V signal levels (lets say its a patch bay for an
existing modular and no modules can be changed...) The DG are the clear
winners...

5) But the CMOS is good like I described it... anyone who still doesn't agree,
just ship me your worthless Prophet Fives (rev 3). I can live with the CMOS
switching....

:^) Harry

Batz Goodfortune wrote:

> Y-ellow Harry, Andy 'n' y'all.
>
> At 11:47 AM 10/17/99 -0700, Harry Bissell wrote:
> >Here's a good tip... if you locate the mux switches at the summing point
> >(inverting opamp input) of an inverting op-amp... with the summing resistors
> >at the other side (away from the opamp) you can minimize the voltage drop
> >across the mux and make it virtually flat...
> >(picture a 100K resistor and an analog gate of 100 ohms... one side of the
> >mux is at "virtual" ground and the other is a 1000:1 divider)
> >
> >You can in this case use the 4000 series muxes (4051???) IF you bias the
> >negative supply pin 0.7 volts below ground. (one diode drop) This is so
> >close that you can drive the control input with a normal 0-12V (or 15 etc)
> >signal.
>
> We had this out on this list perhaps 12 months ago. The problem isn't that
> there is resistance across the gates. But that the resistance isn't
> constant and changes with voltage. This, called "the transfer function"
> means that as the voltage rises, the resistance changes. Since in the case
> of B series CMOS, this resistance can change as much as 200 or 300 ohms or
> so, presents significant problems. It isn't until you study the graphs
> carefully that you notice this. Once you spot it, as did I, your first
> words are probably going to be "Ooo Yuck!"
>
> The DG series tend to offer much better transfer function. And I mean
> "MUCH!" better. The fact you can run them from +/- 15 volts is a big plus.
> But! Harris make a series of cross point switches which may well be the
> answer to you routing problems. Since you can matrix 8 or more inputs
> across 8 or  more outputs all in the one package. Some have very good
> transfer functions as well. The only draw back is that if you were thinking
> of them being used in a system like the patch block of a VCS3, you can't
> actually place a 1K resistor across the matrix. And in a great many cases,
> this is absolutely down right necessary.
>
> Hope this helps.
>
> be absolutely Icebox.
>
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