MIDI Control (ASM1)

[Batz Goodfortune]

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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