🚀 go-pugleaf

In article <51764@donald.WichitaKS.NCR.COM> kthompso@donald.WichitaKS.NCR.COM (Ken Thompson) writes:
>bwood@diva.Berkeley.EDU (Blake Philip Wood) writes:
>
>)I need to turn a 13.56 MHz sinusoid with sub-volt magnitude into a 
>)a square wave oscillating between 0 and 5 V....
>)
>)Any suggestions?
>
>Drive an op amp to saturation and limit the voltage with a zener diode.

Possibly.  But, before you do, read the manufacturer's literature about
the limitations of using op-amps as if they were comparators.

Briefly: many op-amps (even high-speed ones) take a long time
(relatively speaking) to come out of saturation, once you've driven them
there.  Op-amps used as saturated comparators are _slow_, and cannot be
used in this application at frequencies anywhere near their
gain*bandwidth limit.  You're going to need a _very_ fast op-amp to
handle a 13 MHz signal if you're driving it into saturation.

Comparators, on the other hand, are designed to produce a nice clean
TTL-level (typically) output, without going into saturation.  They can
switch, reliably, with response times down to a few nanoseconds.

There are quite a few comparators with fast enough response times to be
able to handle a 13.56 MHz signal... the CMP08 and the AD or Maxim 9686
look as if they'd do the job.  Check out the "Comparators" section of IC
Master for a whole big list.

It'd probably be a good idea to configure the comparator with a small
amount of positive feedback (e.g. 10 mV or so) to provide some
hysteresis.

-- 
Dave Platt                                                VOICE: (415) 813-8917
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