Switching Small Signals With 4066

Discussion in 'General Electronics Chat' started by DangerousBill, Aug 7, 2010.

  1. DangerousBill

    Thread Starter Active Member

    Jul 21, 2010
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    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
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Just make a couple of simple reverse charge pump circuits.
     
  3. DangerousBill

    Thread Starter Active Member

    Jul 21, 2010
    30
    1
    Can you be more explicit please?

    DB
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    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.
     
    Last edited: Aug 8, 2010
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Are you using the 4066s as a multiplexer? If so, CD4051/2/3 have the level translators built in.
     
  6. DangerousBill

    Thread Starter Active Member

    Jul 21, 2010
    30
    1
    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.
     
  7. DangerousBill

    Thread Starter Active Member

    Jul 21, 2010
    30
    1
    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
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Sounds like a simple enough solution to the problem, especially since you've already got a dual supply available.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    According to the Fairchild datasheets, CD4051/2/3 have the same Ron as CD4066.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you're planning on using 74HC/HCT parts, there are 74HC/HCT4051/2/3 available also.
     
    Last edited: Aug 9, 2010
  11. DangerousBill

    Thread Starter Active Member

    Jul 21, 2010
    30
    1
    Thanks for the heads up on that. Going to d/l the datasheets now.

    DB
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The 74HC/HCT parts are faster, with lower Ron. Just be aware that max Vcc-Vee is 10 or 11 volts.
     
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