"Bob Monsen" <rcmonsen@[EMAIL PROTECTED]
> wrote in message
news:NCubk.6440$LG4.3212@[EMAIL PROTECTED]
> "Bob Eld" <nsmontassoc@[EMAIL PROTECTED]
> wrote in message
> news:0zabk.16122$Ri.4973@[EMAIL PROTECTED]
> >
> > "Eeyore" <rabbitsfriendsandrelations@[EMAIL PROTECTED]
> wrote in message
> > news:486C4867.F6DAF534@[EMAIL PROTECTED]
> >>
> >>
> >> Bob Eld wrote:
> >>
> >> > "Eeyore" wrote
> >> > > Bob Eld wrote:
> >> > >
> >> > > > Insure that the max peak of the audio signal is within
> >> > > > the ADC range.
> >> > >
> >> > > Diode clamp it !
> >> > >
> >> > > Graham
> >> >
> >> > Yeah you can clamp it but then you loose that and higher values as
> > peaks.
> >> > The whole idea is to convert the peaks to numbers not to clip them
off.
> >>
> >> You can actively diode clamp. Just don't want to hurt the poor ADC
'just
> > in
> >> case'. I doubt that for flickering lights, losing anything below say
> >> 0x240
> > will
> >> hurt any
> >>
> >>
> >> > There's really no need to clamp it. The processor input will have
> > internal
> >> > "diodes" that will keep the voltage from exceeding the rail
voltage
> > plus a
> >> > little.
> >>
> >> A discussion we've had many a time before here !
> >>
> >>
> >> > Likewise the input voltage won't go below ground.
> >>
> >> It WON'T ? In what ideal world is this ?
> >>
> >>
> >> > A series resistance should be added to limit the input current and
a
> > shunt
> >> > resistance
> >> > added to keep the DC level from ****fting up due to rectification by
> > these
> >> > clamps.
> >>
> >> Uh ?
> >>
> >> Graham
> >
> > Well, if there is an internal diode from the input to the negative
rail,
> > it
> > will keep the voltage on the input from going below ground, ignoring
the
> > diode drop, of course.
> >
> > Now, if you capacitively couple to the input, this diode will rectify
the
> > negative going ****tion of the audio signal and charge the capacitor,
> > positive towards the input. This will offset the zero base line in the
> > positive direction and screw up the peak detection. A judicious
resistance
> > to ground with an appropriate series resistance will limit the current
and
> > drain off this offsetting charge. It ain't rocket surgery!
> >
> >
>
>
> One thing to remember is that cmos inputs are usually prone to latch-up
> disease. So, if the output goes higher than the positive voltage input
of
> the IC, you can end up with a triggered SCR between Vcc and ground. Not
> good.
>
> So, having a way to prevent this is a good thing, and generally consists
of
> a diode from the input to the positive rail. For digital inputs, I
believe
> most microprocessors use an internal diode. However, I'm not sure about
> analog inputs, and would not want to risk it, particularly when a cap
from
> an external input can easily cause this situation, and the solution is
so
> simple.
>
> See this link for more information:
>
> http://ww1.microchip.com/downloads/en/AppNotes/00763b.pdf
>
> Regards,
> Bob Monsen
The PIC stuff has internal protection diodes on the inputs. As far as I
know
all of their processors are so protected on the IO ****ts whether
configured
analog or digital. For a typical A/D input, see the PIC16F676 Data Sheet
page 47. There should be no trouble driving a few milliamps into these
diodes clamping off a signal without any external diodes.
I've never yet seen one latch up doing this. That's probably why the
diodes
are there.
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