[time-nuts] TAPR TICC boxed (input protection)

[Bruce Griffiths]

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.

Bruce
> 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
> LMC6081:
> 
> https://goo.gl/LCY2vR
> 
> 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:
> 
> http://www.ti.com/product/lmc6001
> http://www.ti.com/product/lmc6081
> 
> 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
> economics.
> 
> 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
> performance.
> 
> 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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