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CMOS input protection

Discussion in 'Electronic Design' started by Pimpom, Jan 10, 2013.

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  1. Pimpom

    Pimpom Guest

    I'm designing a project based mainly on CD4000 series logic
    devices. The finished product will consist of two different
    sections linked by a long (~40m) 2-core shielded cable. The
    signals are very low frequency pulses (a few Hz at most) and
    transition times are not critical.

    It's not practicable to let the two sections share a common
    power supply. Neither is it possible to ensure that the
    separate PSUs will be switched on and off at the same time
    or in a specified sequence. Therefore, one unit may already
    be sending a signal to the other before the latter's PSU is
    turned on.

    To protect the receiving unit, I'm thinking of placing 10k
    resistors in series with the CMOS inputs (in addition to
    parallel terminating resistors) plus Schottky diodes from
    the IC input pins to Vdd and ground.

    Is this OK? Is it necessary? Is the integrated protection
    good enough?
     
  2. Pimpom

    Pimpom Guest

    I have BAT85 in stock. I haven't worked out the leakage at
    worst case temps but I suspect that it might be a bit too
    high with 100k. I could use the less leaky BAT43. I have a
    limited range to choose from where I live.
    They're all B series types. The input stage already is a
    Schmitt gate. The caps might be a good idea.
    I'll keep that in mind. Thing is, this is a simple project -
    only 6 ICs in the receiver - and I want to keep it as simple
    as possible. Thanks for the suggestions.
     
  3. Joerg

    Joerg Guest

    Almost ok but not quite. Add a few hundred ohms from your external diode
    and the IC pin. This makes sure that the lion's share of unwanted
    current flows through the external diode and not the substrate paths.
    Also, then you can use regular diodes such as the BAV99. Or a BAV199 if
    you need low leakage.

    What can happen is that the signals coming in from the powered unit try
    to power up the other unit. The VCC there can then slowly creep up. Make
    sure this situation is handled properly and nothing can go kaboom
    because it resulted in undefined logic states. It can affect other units
    in return because the "half-powered" crcuit can send out undefined
    voltages on its outputs or possibly oscillate somewhere.
     
  4. Rocky

    Rocky Guest

    Remember the transmit. Maybe a bit naive, but I used to use 1k
    on the outputs and 10k on the input. Seemed pretty robust. Never
    did production in those days though.
     
  5. mike

    mike Guest

    This kind of vague thinking is the root of much design evil.
    You know what you mean, but we don't.
    The audio guy and a guy who builds picosecond laser drivers
    will have very different perceptions of "critical transition times".
    If the time is not critical, make it infinite and you don't need
    the cable.
    Numbers were invented for a reason. Use 'em.
    Must be the phase of the moon. Today, there seem to be a lot
    of very vague questions with fromthehip answers.
    It's like a blindfolded quick-draw contest with live ammo.

    You've been given some good advice, but there are a lot of
    issues to consider.

    I take a lot of flak here for insisting that people define
    exactly what they're trying to accomplish.

    A typical scenario involves tunnel vision. Some very smart
    engineer comes up with a solution that has one little problem.
    She asks about that problem and gets a solution to THAT
    problem, not realizing that it created two additional problems.
    It's often worth a few minutes to rethink the system.

    The design goals for intended operation are no more important
    than the specs for what happens when some idiot gets their hands
    on it and does unspeakable things. Unfortunately, people tend
    to skip both.

    You don't say what your "product" is, but there's a wide range
    of system requirements depending on the target demographic.
    A simple catch-all starting point relevant to this
    discussion is electrostatic discharge.

    What ESD protection do you require and how are you gonna verify
    that you meet it? A 10K resistor sounds like a lot of
    protection until you start poking a 15KV ESD generator around.
    The devil is in the details. And the current doesn't always
    go through the resistor like you'd expect if you looked at the
    schematic. If you don't believe it, watch a few lightning
    strikes up close.

    A properly deployed opto-isolator can cure a lot of common-mode
    system ills, but it's not a panacea.
    It might protect your CMOS input, but now, you have to protect
    the opto-isolator from ESD...unless you use fiber for the whole
    run. And that will address a number of other issues like EMC.

    Diodes are another potential gotcha.
    You're gonna need a minimum of two resistors.
    One to protect the input from the peak overvoltage allowed by
    the diodes and strays. And another resistor to protect the diodes.

    For 5V hobby circuits, I've taken to using 5.1V zener diodes as the
    protection element...and here's why.

    I once built a GPIB interface from a PIC processor. I unplugged the
    power to the chip and the GPIB kept right on controlling.
    I immediately started writing a paper on how I'd written code that
    tapped into zero-point-energy and ordering parts for my water-powered car.
    Then, I realized that the GPIB was powering the chip through the
    forward-biased input protection diodes on the PIC. Not any part of it
    met spec, but it worked just fine.

    Depending on the circuit, a diode clamp to VCC can let an
    input transient take out every IC on the board. That's probably
    not the protection you expected. It's easy to assume that you can
    shunt arbitrary current into VCC, but the math might surprise you.

    The zener diodes can protect the circuit from power supply injection.

    You've identified power sequencing as a potential problem.
    Consider when the protection diodes put just enough voltage
    on VCC to cause a lot of dissipation in some external power switching device
    and let the smoke out.
    Then, there's the problem that the robotic welding arm kills two
    workers if you turn power on in the wrong sequence.

    We haven't even started talking about what happens when your system
    clock happens to be just the right frequency to resonate the cable
    and makes the FCC very unhappy. Unless your operator has a ham
    license and 40-meters is the electrical length. ;-)

    Write the specs...ALL the specs including the design verification
    procedures and the customer acceptance test procedures and the
    third party certification test procedures. Consider customer abuse.

    Details matter...that's where the devil lives...right next door to
    Murphy.
     
  6. legg

    legg Guest

    Something like the NC7SZ05 claims high impedance inputs and outputs
    when powered down. I'm not sure this protects external circuitry if
    the NC7SZ05 power sequencing is not ground-contact-first and
    ground-contact-last, which may be an issue you should check for with
    remotely located subassemblies.

    You can't always guarantee that power, signals and their references
    will be connected in a specific way, unless you design them to do so.

    RL
     
  7. miso

    miso Guest

    Is there a possibility this cable with generate voltage due to a piezo
    effect?

    There will probably be scenarios where the far end is powered through
    protection diodes via the signal. Open drain would solve that.

    I'd go opto.
     
  8. MrTallyman

    MrTallyman Guest


    Piezo effect? Is there physical mechanical pressure being placed on
    it?

    That is a dumb question. why would a man, company, engineer, whoever
    make a cable that fucks up? Use some common sense. All compressing it
    would do is change the interconductor capacitance *at that location*.
    Generate a voltage? Not.
     
  9. whit3rd

    whit3rd Guest

    and that's a good solution, especially if used with a CD4050-style
    CMOS gate which lacks input clamp-to-VCC diodes, because
    it prevents your signal current from (for instance) charging
    a nonrechargeable battery. This presumes, of course, that
    your two CMOS gizmos on the wire share a common ground
    which is their negative power supply terminal.

    Another item that hasn't been mentioned yet, is a pullup or
    pulldown resistor on any CMOS input that might become
    disconnected when you unplug the long cable. You cannot ever
    let CMOS inputs float.

    Yet another concern: if the circuit isn't double-insulated (isolated
    from ground) on either end, your grounding conductor in the cable
    could carry large currents in case of a fault. If that's an issue,
    the optoisolator DOES help quite a bit; otherwise, it's just a
    power and complexity bunion.
     
  10. mike

    mike Guest

    As a condescending, potty mouthed nit-picker with considerable
    experience telling people
    how stupid they are, you'd think you'd at least attempt to scour the
    nits out of your own learned pontifications!

    Q=C*V. If you change C by mechanical deformation, what happens to V?

    I'm not saying that it makes any practical impact on the current thread...
    just that you might be hungry and could use some of your own words to eat.
    Bon Appetit!
     
  11. Guest

    He meant triboelectric effect, duh.
     
  12. Guest

    The CD4000 series should not latch up if the currents 'forced' through inputs and outputs are less than something on the order of 10ma. That is DC, but there are unspecified hazards with dV/dt induced internal currents on those terminals too. Since your signaling frequency is so low, a simple RC will eliminate any possibility of latch-up due to power supply sequencing or ESD, no BATs necessary. An internal loading of Vcc wil ensure that external powering of the dead circuit does not produce a working voltage for any other part.
    Please view in a fixed-width font
    such as Courier.

    ..
    ..
    ..
    ..
    .. VDD
    .. |
    .. .----+
    .. | |
    .. | |
    .. | |\|
    .. >----[1M]---+---------| >--->
    .. | | |/|
    .. | [100K] |
    .. === | |
    .. 0.0033u | '----+
    .. | |
    .. --- ---
    .. /// ///
    ..
    ..
    ..
    ..
     
  13. Guest

    PTFE is famous for its piezoelectric properties but the effect is very low level and only important for extremely low noise and sensitive instrumentation applications. There are specialty coaxial cables made for enhanced piezoelectric effect for other practical applications:
    http://www.meas-spec.com/product/t_product.aspx?id=2476
     
  14. miso

    miso Guest

    You cover your bases and ask the questions. How do I know where that
    cable is going? It could be buried in the dirt and cars drive over it.
    There could be a scenario where the driver is not connected, hence the
    line is high impedance.

    Clearly Tallyman (or whatever the shithead calls himself) has never put
    anything into production.
     
  15. legg

    legg Guest

    Probably, so long as there's no hot plugging of assemblies going on.
    Although power may be remote, there's always a ground either showing
    up, or missing, when least expected. Battery-powered devices are the
    worst in this regard, particularly if a charging feature is attempted.
    That gives a potential positive rail pre-contact. Ouch.

    RL
     
  16. Jasen Betts

    Jasen Betts Guest

    variable capacitances generate voltage, if biased, that's how condenser
    microphones work.
     
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