[sdiy] Opinions on input protection

[Scott Nordlund]

If I'm not mistaken, the CMOS protection diodes are at least in part parasitic structures (existing only as a side effect of the CMOS process and not intentionally created), and current through these could cause latchup.

Maybe what you describe for the TTL case would be safer here?

I guess in general it would be a good idea to test these things before deploying them, though it had really never occurred to me before.  In some cases there are expensive or irreplaceable components to be protected, and I wouldn't want to leave that to an untested circuit.

I'm not sure how necessary it would be to protect an inverting amplifier input, as the op amp will clip (and limit the current draw) but shouldn't be harmed unless the inverting input pin exceeds the supply voltages (which would require a severe overload).  Other summing node inputs (like OTAs) should be easier to damage.  Maybe in this case the input resistor could be split, with reverse-parallel diodes somewhere "inside" to limit the maximum input current?

----------------------------------------
> From: georgehearn at btinternet.com
> To: jgrisso at det3.net; synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Opinions on input protection
> Date: Thu, 26 Mar 2009 13:22:37 +0000
> CC:
>
> For CMOS I usually just use a resistor in series with the input to limit the
> internal protection diode current. For TTL inputs I use a diode clamp to
> +5V/0V and a resistor after the clamp in series with the input for
> additional protection. For inverting opamps I use a pair of anti-parallel
> diodes from the (-) input to ground, with the input resistor (Rin) limiting
> the fault current. This is just my way of doing things, not necessarily the
> correct (or best) way to protect, others may differ but it works for me. G
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Joe Grisso
> Sent: 26 March 2009 13:13
> To: synth-Diy diy
> Subject: [sdiy] Opinions on input protection
>
> Hey all,
>
> I've been thinking a lot about how to interface general trigger
> and gate signals into interior digital logic, and wanted to get some
> opinions from the group. This specifically applies to when you're
> taking an external binary signal (0/5V, etc) and sending it into a 5V
> or 3.3V micro. There are many methods of input protection including
> suppression diodes, using a transistor as a switch, or even just
> running the input into a logic buffer and using the internal
> protection circuit of the IC to shift the 0-10V gate/trig signal down
> to something the processor can digest without committing seppuku. I've
> seen each of these methods used, either alone or together in some
> larger configuration. What does everyone prefer here?
>
> Best,
>
> --
> Joe Grisso
> Detachment 3 Engineering
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