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Impedance of digital inputs

Discussion in 'Electronic Basics' started by Patrick, Oct 3, 2005.

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  1. Patrick

    Patrick Guest

    Would it be safe to say that ALL digital inputs (pins on
    microprocessors, microcontrollers, gates, etc.) are always high input
    impedance? i.e. When doing circuit analysis is it okay to model a
    digital input as a very large (100K or higher) resistance to ground?
    Thanks in advance...
     
  2. Essentially yes, if they are *CMOS*. They look like, say, 1p-10p
    capacitance, with a very high dc impedance. For the most part they can
    be considered open circuits for DC. A more accurate model would be to
    include diodes from the inputs to the supply rails to model leakage.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  3. Patrick

    Patrick Guest

    Sounds good. What about digital inputs of other logic families, e.g.
    TTL? If I remember correctly TTL inputs draw a few micro-amps of
    current with a logic HIGH (5V) input.
     
  4. John Fields

    John Fields Guest

     
  5. err....do people still use TTL?


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  6. John Fields

    John Fields Guest

    ---
    Do it matter?

    His question was:

    "Would it be safe to say that ALL digital inputs (pins on
    microprocessors, microcontrollers, gates, etc.) are always high
    input impedance?"

    and the answer is clearly "no".
     
  7. Pooh Bear

    Pooh Bear Guest

    They draw a hell of a lot more when LOW.


    Graham
     
  8. Rich Grise

    Rich Grise Guest

    Actually, they're _sourcing_ current when low. ;-) ;-)

    Cheers!
    Rich
     
  9. John Larkin

    John Larkin Guest

    I use the occasional 74F38, which is almost TTL.

    John
     
  10. Pooh Bear

    Pooh Bear Guest

    Yeah, yeah, I know ! Didn't want to spoil the flow of it though.

    Graham
     
  11. Well, actually, no they are not. There are sinking electrons *into* the
    input!


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  12. John Fields

    John Fields Guest

    They're sourcing _conventional_ current, Kevin.
     
  13. The way kewl thing about this is, it doesn't matter! To switch from
    conventional to electron flow, just change all the pluses to minuses,
    and vice versa. ;-)

    The thing that really weirds me out [but The Philosophizer loves it]
    is that, in all of the equations that relate electricity and
    magnetism, you can swap those two, and the answers still come out
    right!

    I'm working on a 3-D diagram of the two 7-dimensional hypertoruses
    that the Universe is derived from...

    Cheers!
    Rich
     
  14. Pooh Bear

    Pooh Bear Guest

    I thought 74F was analogue ! ;-)

    Graham
     
  15. They can't. Conventional current is imaginary...


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  16. John Fields

    John Fields Guest

    ---
    So's the square root of minus one, but that doesn't stop us from
    using it.

    Don't be an ass, Kevin. We all know that and we all use
    conventional current every day.
     
  17. PeteS

    PeteS Guest

    On the OP question, I just had an interesting experience with a Xilinx
    part. In *unprogrammed* mode, the bus hold circuitry is active in
    series with an equivalent resistor of about 25k to the I/O pin.

    I caught that just as I was connecting things up to it (because I
    wanted to leave it unprogrammed for initial testing of other parts of
    the board) and had to whip up a simple piece of verilog to force inputs
    to *be* inputs and outputs to be at the suitable level. It didn't do
    anything else, so I simply had a bunch of assignments.

    The old rule of thumb that FPGA IO pins are inputs when unprogrammed
    doesn't always hold ;)

    Cheers

    PeteS
     
  18. Jasen Betts

    Jasen Betts Guest

    One exception is the Hall effect.

    that one depends on the sign of the charge on the charge carriers and has
    been measured to have the opposite sign in P-type semiconductors than it
    does in regular metals and N-type semiconductors.
    huh?

    Bye.
    Jasen
     
  19. John Larkin

    John Larkin Guest

    Isn't everything analog(ue)?

    John
     
  20. John Fields

    John Fields Guest

     
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