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> >Is this a good solution for low power apps, like battery
> >powered where the PIC will sleep until some time, wake up
> >to check timers and input pins, then sleep again?
>
> That's a little tougher. You can get micropower op-amps, and scale the
> resistance values up so little current flows.
>
> It's the old limbo question, "How low do you need to go?"

You can supply power to the op-amp only when you need it. This
micro-power circuit uses a miniscule average amount of current
with a 300us pulse every full WDT period of 2.4s. Twiddle with
component and timing values to suit your app. The TLC271 has
a low power mode (10uA) anyway. The reduced bandwidth in this
mode is still far higher than you'd need for a comparator. You just
need to decide how often you need to sample, ie what delay can
you have between an event and detection, and what pulse length
turns the op-amp on for long enough to take a reliable sample.
This technique also works with a stand-alone sampler using a
CMOS oscillator with a high Space:Mark ratio to supply power-
up pulses

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