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M1BYT > DESIGN 23.03.06 22:26l 55 Lines 2421 Bytes #999 (0) @ WW
BID : AF3975M1BYT
Read: GUEST OE7FMI
Subj: Re: Logic gate IC question
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<DK0WUE<GB7FCR
Sent: 060323/1733Z @:GB7FCR.#16.GBR.EU #:34306 [Blackpool] FBB-7.03a $:AF3975M1
From: M1BYT@GB7FCR.#16.GBR.EU
To : DESIGN@WW
G0FTD Opined with considerable skill:-
> I have a question about using commonly available logic IC's.
>
> What I need to know is what voltage window can they reliably be used at ?
>
> Example:
>
> I require an input to work at a certain voltage (let's pretend it's 5v).
Standard logic gates have a deliberate designed in grey area which is quite
wide. The only guarantee correct switching outside of that grey area. Having
said that it is possible to experiment to find out the parameters for a
individual sample of a device and use it for the purpose you describe,
provididing the hysterisis is not a problem. It is possible to get around the
problem of the grey area, but the circuit is rather complex and involves
feedback from the output. I doubt it would be a repeatable design with out
individual adjustment.
>
> At 4.9v I need it to be off - but will a comon logic IC be that accurate,
> or will I have to accept a bit of a grey are, if so what sort of window
> of operation should I expect ?
That sounds as if you need to be looking at a comparator circuit, rather than
logic IC's. For these you normally use an op-amp - two inputs and one output
per device. One input acts as a voltage reference (A), the other input the
sample you are trying to measure or compare(B). When A is greater than B it
switches smartly one way at the output, when B is greater than A it switches
the other way. The reference can be provided by a potential divider from a
stable supply or from a zenor diode.
They are very easy to implement.
Which way it switches depends upon which of the two inputs (+ or -) you use
for which.
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