[sdiy] Module input isolation?

[Jim Patchell]

    What you need to add greatly depends on the chip and what you need
to do.  For bringing out an LM311 input pin, I would have a resistor in
series with the pin (1K -> 10K range I would think), and then you would
want to have back to back diodes (like 1n914's) from the + to - input,
plus clamp diodes to the two power supply rails ( two diodes,  Cathode
of D1 to V+, Anode D1 to input, Cathode D2 to input, Anode of D2 to V-,
per input).  Doing all this will probably make the thing close to bullit
proof.

    Another problem with bringing LM311 inputs to a panel, however, will
be unwanted feedback.  From the input to output of the LM311, you have a
real high gain and a wide bandwith.  So, depending on how it is hooked
up, the LM311 could posibly oscilate.  You will need to make sure that
everything is very well shielded, but in general, you want to keep the
connections to the LM311 very short (i.e. less than an inch).

    -Jim

John Speth wrote:

>
>
> Hi list!
>
> Is there any kind of isolation needed to protect silicon devices that
> serve as inputs to the outside world?
>
> Let's say I have a synth module that uses an LM311 comparator.  And
> the input to this module needs to go directly one of the LM311 input
> pins.  Does this pin need any protection circuitry?  I'm afraid I'm
> not familiar with any kind of bad stuff that might happen at that pin.
>
> I'll probably hang a 1M resistor to ground to keep it from floating
> when not connected.
>
> Thanks, John Speth