[time-nuts] TAPR TICC boxed (input protection)
bruce.griffiths at xtra.co.nz
Tue Apr 4 17:13:34 EDT 2017
A protection diode needs to also have a fast turn on with little or no overshoot of the forward voltage.
Reverse recovery time can be an issue if one is relying on the clamp for protection against a periodic overload such as when an input is overdriven by a sinewave input and one wishes to make useful measurements whilst this occurs.
The internal protection diodes of HCMOS devices can severely degrade the device propagation delay jitter when they conduct.
> On 05 April 2017 at 06:05 David <davidwhess at gmail.com> wrote:
> Low current measurements take a lot of time on the automatic test
> equipment and time in this case is measured in seconds. The same
> applies to low frequency noise.
> For an example, take a look at the National (now TI) LMC6001 and
> Unlike National, TI does not care about input bias current in their
> selection guides so you will have to look that up in the datasheets:
> The difference in the parts is that the LMC6001 is tested for an Ib of
> 25fA and below and this is reflected in the price which is $5.76
> instead of the $0.83 of the LMC6081.
> Right about the time that the LMC6001 was released, Robert Pease wrote
> some articles talking about the bias current testing and the
> The same thing applies to all of those small signal transistors with
> 25, 50, and 100nA leakage specifications. Those numbers are simply
> good enough for typical applications and what the tester can handle in
> the time allotted and have nothing to do with the actual transistor
> So collector-base junctions make good low leakage high voltage diodes
> although they are slow which does not normally matter for an input
> protection circuit and may even be preferable. Emitter-base junctions
> make good low leakage fast diodes but with low breakdown voltages.
> The cheapest guaranteed low leakage diode is probably some variety of
> 4117/4118/4119 n-channel JFET.
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