"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
|
