Switching Small Signals With 4066

Thread Starter

DangerousBill

Joined Jul 21, 2010
30
Greetings,

My planned circuit may use a 4066 analog switch to select feedback resistors in a current follower circuit. The 4066 is powered with +5V, -5V, but the control signals are TTL, referenced to 0 V. I need to translate these control signals into the L = -5V, H > -2.5V signals needed to drive the 4066 inputs.

Funny thing, I designed a ckt to do this very thing back in 1983, but those notes are long gone, as are the manuals I searched for the solution.

I''ve thought of using the big-hammer solution, a quad comparator, to convert the control signals, but there must be elegant way to do it with resistors or something.

Thanks in advance for any assistance, or for a reference to a workable solution.

For what it's worth, I found a bilateral switch that has TTL/CMOS inputs, the Analog Devices ADG1421, but it only comes in pkgs with .025" lead spacing. I'm not ready to risk a whole PCB on hand soldering this sucker, and I can't find prototyping boards for .025" spaced leads either.

Dangerous Bill
 

marshallf3

Joined Jul 26, 2010
2,358
Unfortunately I'm not feeling well enough today to do much thinking, had a couple of circuits to finish building for work this weekend and ended up blowing most of the day so I was up until 3 AM getting a good start on one of them. One of these days I'll have to descrine some of the various projects I've built that alter or improve the operation of many of the aspects of the way our business operates including the 45 year old HVAC system. That thing fascinates me, it's totally done with recirculating chilled and heated water proportionately controlled at every air handler by pneumatics. It's more complicated than it sounds but I've found it a joy to work with and a challenge to integrate a fair amount of modern electronic control circuitry in with the pneumatics.

Now to your problem. If I read it directly you're just trying to change the polarity & level of some control signals to one of my old favorite ICs.

When we think of charge pumps the first thing that pops into our head are simple buck/boost coverters that change a voltage level or that convert positive to negative. No reason one can't do both when you understand the theory behind them simply by adapting what's out there.

Read through these links and think for a bit, then we'll get back to talking about it. Simple transistor circuits could also be used but I think these are a bit more elegant to deal with.

http://datasheets.maxim-ic.com/en/ds/ICL7662-Si7661.pdf

http://www.maxim-ic.com/app-notes/index.mvp/id/782

You may notice I post a lot of Maxim links. No, I don't work for them but back during their early boom years they often incorporated some good therory writeups and application comments into their data sheets, they're also still one of the easiest and fastest "free sample" places around.
 
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Thread Starter

DangerousBill

Joined Jul 21, 2010
30
Are you using the 4066s as a multiplexer? If so, CD4051/2/3 have the level translators built in.
I am indeed looking for a multiplexer. The 4052 could be the solution. Before committing, I'm going to keep hunting for something with a lower Ron. But I think I can accommodate the high on-resistance if necessary. Thanks.
 

Thread Starter

DangerousBill

Joined Jul 21, 2010
30
So far, I've checked parts cost and availability, plus other inconvenience factors, and it looks like I can do the job with a cd4066 strung between +5V and -5V, and a quad comparator like the LM339 with the threshold set by a resistor ladder at +1.5 V to accommodate both TTL and CMOS inputs. Not elegant, but cheap, and board space isn't an issue here.

Thanks for the suggestions, and I'll keep watching this space.

Dangerous Bill
 

Ron H

Joined Apr 14, 2005
7,063
If you're planning on using 74HC/HCT parts, there are 74HC/HCT4051/2/3 available also.
 
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