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How to select the correct Analog Switch (TTL<->CMOS)

Discussion in 'Electronic Design' started by Lathe_Biosas, Apr 7, 2005.

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  1. Lathe_Biosas

    Lathe_Biosas Guest

    Hi

    Would the following Analog Switch (AD7510)

    http://www.analog.com/en/prod/0,,768_833_AD7510,00.html

    help to switch between two signal lines that are connected as follows?


    LINE 1: CMOS Chip A -------
    \
    SWITCHING AD7510 ---- CMOS Chip C
    /
    LINE 2: TTL Chip B -------


    An aditional TTL Chip D would control the switching of AD7510

    Unfortunatelly I don't know that much about currents and I don't want
    to burn my circuit. If the AD7510 is the correct one, what voltage
    should I apply to V_ss and V_dd?

    While selecting the Analag Switch I get options with different ranges
    of impedances, i.e. 50 to 100 Ohm, what is the meaning of that?

    Any help or information would be kindly appreciated
    Best Regards
     
  2. Pooh Bear

    Pooh Bear Guest

    Why do you think you need an *analogue* switch if you're only switching
    logic signals ?


    Graham
     
  3. Lathe_Biosas

    Lathe_Biosas Guest

    Hi Graham

    Thanks for answering
    I have heard that an Analog Switch would help do that
    I had a reed relay but it seems the current from the TTLs is not enough
    to drive it.
    I even don't know exactly how "analogue" switches function
    Sorry for the ignorance, if you could recomend a book or information
    about then would be great

    Regards
     
  4. Pooh Bear

    Pooh Bear Guest

    A standard TTL gate is unlikely to be able to drive even a reed relay.
    Are your logic levels all 5V ?

    You can do what you want simply using logic gates ( much cheaper ).


    Graham
     
  5. Lathe_Biosas

    Lathe_Biosas Guest

    Hi Jim,

    Thank you very much for the answer



    The chip B is a 74F
    Is it possible to use a 74F as selector?

    Best Regards
     
  6. Jim Thompson

    Jim Thompson Guest

    ONLY if the CMOS chip (A) is capable of driving a 74F input, which I
    doubt.

    ...Jim Thompson
     
  7. Lathe_Biosas

    Lathe_Biosas Guest

    I think I explained bad and refered wrong to the Analogue Switch
    modell, I offer an apologize

    What I would like to have is following connection:

    CMOS Chip A -------(S1) AD7512 (OUT 1) -------- CMOS Chip C

    and been able to switch to the following connection

    TTL 74F Chip B ---------(S2) AD7512 (OUT 1) -------- CMOS Chip C

    Will the AD7512 be the correct one to have those connections?

    Is a 74F capable of driving a AD7512 through the (A1) terminal?

    Regards
     
  8. Jim Thompson

    Jim Thompson Guest

    Looks like it would be OK.

    The way I read the data sheet, A1...A4 are logic inputs, specified as:

    Logic Low maximum level = +0.8V

    Logic High minimum level = +2.4V

    Which is the TTL specification.

    ...Jim Thompson
     
  9. Pooh Bear

    Pooh Bear Guest

    Which is why an HC/HCT device is a good choice - maybe even AC if line 2 has
    really fast signals ?


    Graham
     
  10. Lathe_Biosas

    Lathe_Biosas Guest

    Perfect. Thank you.

    Now one more question:

    On the following connection:

    TTL 74F Chip B ---------(S2) AD7512 (OUT 1) -------- CMOS Chip C

    wouldn't be a problem if the 74F terminal has 4.8 Volts and the CMOS
    Chip can receive only 3.3? Will the AD7512 manage this voltage
    difference?
     

  11. Yeah that is a potential problem unless the CMOS Chip C has adequately
    overvoltage tolerant inputs. In practice the 74F device won't output logic
    highs of 4.8V, but whatever it does output may still be too high to remain
    in specifications and avoid activating the CMOS Chip C ESD protection
    structure. The AD7512 is an analog switch, so it is designed not to
    interfere with voltage levels. It behaves something more like a 75 ohm
    resistor would.

    Since this is digital logic you really shouldn't be using analog switches
    anyway. Analog switches are likely more expensive and are less than ideal
    (IE, slow to switch) than using an all digital approach. By the sounds of
    it you need the functionality of a multiplexer. By the sounds of your
    voltage conversion needs, an NC7SZ157 would be perfectly suited.

    http://www.fairchildsemi.com/ds/NC/NC7SZ157.pdf

    Use one of these babies and drive it with a Vcc of 3.3V. The device has
    overvoltage tolerant inputs, so it should handle the 74F logic levels
    nicely. Additionally with a Vcc of 3.3V and switching threshold
    requirements of 0.3Vcc (Vin low max) and 0.7Vcc (Vin high min), the required
    levels are <0.99V and >2.31V. Since the 74F outputs max low voltages of
    0.5V and min high voltage of 2.5V, this should work very nicely with
    everything within specification.

    That said, why are you using 74F logic in the first place. Avoid it and all
    other TTL logic families (many of which are now officially obsolete anyway)
    like the plague.
     
  12. Fred Bloggs

    Fred Bloggs Guest

    Heheh- was wondering when anyone was going to notice that- analog
    switching of digital signals is an oxymoron operation if there ever was one.
     
  13. Pooh Bear

    Pooh Bear Guest

    Try my response - the first reply in this thread.

    Uhuh.


    Graham
     
  14. Lathe_Biosas

    Lathe_Biosas Guest

    Yeah that is a potential problem unless the CMOS Chip C has
    adequately
    Hi Fritz

    The 74F outputs 3.3 Volts, apologize again for the mistaken
    specification.

    Thank you very much for the great help

    I will wire the NC7SZ157 as follows:
    CMOS Chip A ----- Terminal 1 Terminal 6 ---- 74F Selector Chip D
    GND ------------- Terminal 2 Terminal 5 ---- +5 Volts
    TTL 74F Chip B -- Terminal 3 Terminal 4 ---- CMOS Chip C

    Would this function as follows?

    Terminal 6 HIGH --> CMOS Chip A would be connected to CMOS Chip C
    Terminal 6 LOW --> TTL 74F Chip B would be connected to CMOS Chip C

    Best Regards and again thank you very much
     
  15. Pooh Bear

    Pooh Bear Guest

    Look !

    Why don't you just learn some stuff about logic chips ?

    Don't expect easy answers. You need to understand the technology yourself !



    Graham
     

  16. Greetings Lathe Biosas.


    Okay looks good except for Terminal 5. If you expect the input threshold to
    be within specification, and only want the NC7SZ157 to ouput a maximum
    voltage of 3.3V, then Terminal 5 should be hooked to a 3.3V supply
    (presumably the same power supply for CMOS Chip C).


    Yeah that is how it would function. Realize however that "connected"
    doesn't mean they are bidirectionally connected. I assumed you weren't
    using anything peculiar like two way communications used in some
    applications such as I2C busses. I assumed CMOS Chip A and TTL 74F Chip B
    are both producing logical outputs while the NC7SZ157 is feeding the logic
    input of CMOS Chip C. Assuming these assumptions are correct, and assuming
    no other relevant details have been left out, then this should work.
     
  17. !


    I think he is learning some stuff about logic chips. If this newsgroup
    isn't for sharing information, learning things, and helping each other out,
    I don't know what it is for.

    It is good to have confirmation that your understanding of something new is
    correct before you build a board around that understanding. The NC7SZ157 is
    a small SMT part so will presumably need a board before it's function can be
    personally confirmed.
     
  18. Pooh Bear

    Pooh Bear Guest

    Yeah - agreed - but this is kiddy stuff !

    Uh ?

    Graham
     
  19. Pooh Bear

    Pooh Bear Guest

    And that's meant to be understood by a beginner ?


    Graham
     
  20. I read in sci.electronics.design that Pooh Bear
    <>) about 'How to select the correct Analog
    Of course not. On s.e.d., most of the explanations (except mine) are
    solely intended to impress the other contributors. Someone asks a
    question about DC Ohm's Law and gets a lecture on complex numbers.
     
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