13 Dollars for an electronic switch ?? (was: 4066's. Eat em up, Y um)

[Haible Juergen]

	>I gotta mention. These DG201 and similar families are really
nice to use.
	>When you pick 'em up to put 'em in the board they just feel
right. Like the
	>difference between the suspension on a race car and a cadilac.
We're talking
	>smooth as silk. You just wanna caress these guys. And @ 13
bucks a piece
	>they'd wanna feel like that. They're a bit weird to get use to
in a way
	>because the controls are upside down. But man they drive well.

Just another idea, before you spend 13 Dollars for an electronic
switch:

Why not use a cheap OTA with 100% feedback instead?
Take a good old CA3080 and switch the bias current on and off.
You think 3080's are noisy? Only if you want linear amplification
of its differential input voltage. But if you connect it like an
ordinary
opamp as a unity gain buffer, and then switch Iabc on and off,
SNR and distortion will be very different (better).

Try this for a start:

10k from your signal input to pin 3 of 3080.
10k from pin 6 to pin 2 of 3080.
1.2k from pin 2 to pin 3 of 3080.
(or replace the 1.2k by two 1n4148's in parallel,
reverse polarity)
You may also need a small capacitor at the output
for compensation.

I have used similar configurations from time to time, but
I don't remember the details, and I haven't made any
measurements.
Depending on the circuit environment, you might need 
input and output buffers (opamps), but you may as well
succeed without buffers, or with one buffer shared by all
OTAs in a multiplexer configuration.

I really don't know where the limits of this topology are.
But if you consider how well a 3080 works in S&H
circuits of steep filters, I'd expect not too much crosstalk.

Just try it and experiment, if you like.

If you want to take it a step further, build one OTA from
discrete transistors, and duplicate only the differential 
input pair for several switches in mux configuration.

JH.


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