At 07:48 PM 6/27/2003 -0700, Joel wrote:
>I am going to use one of the A/D pins on a 16F73 to
>read a voltage that is output from another chip. Do I
>want to put a resistor in between these chips?

Maybe.

>And if so how do I determine the value?

Depends.

Ok. Longer answers now.

The value depends on why you are putting it there.  The "if" depends on
your circuit.  If the "chip" supplying the signal to be read by your A/D
can supply either MORE voltage than your pic's positive supply or LESS than
your pic's negative supply (usually ground), then yes, you need something
to limit the current.  And something like diodes to clamp the voltage from
going outside the range the pic wants to see.  It's up for discussion on
this list what to limit the current to.

An example. Let's say you want to be conservative and limit the current to
1mA.  (Some would say this isn't conservative enough!) And let's say you
are measuring a signal that could go to 15 volts while your pic is running
off of +5 and gnd.  Worst case is +15V minus +5V = 10 volts.  10 volts
divided by 1mA = 10 kOhms. Use that size or larger.

Let's suppose that the source of the signal is bounded by the same supply
as the pic.  Now you don't need a current limiting resistor.  But you might
want one along with a capacitor if you need to low-pass filter the signal
to remove noise or other frequency components above the range of interest.

One other case also comes to mind.  If the signal source is coming from
"off board" like from maybe something that gets plugged in and out, then
you will want to watch out for static electricity being coupled directly
into the pic.  If that's the case, put as big a resistor as you can stand.
How big is that?  Almost, but not quite, so large that it begins to produce
a corner frequency due to stray capacitance that cuts into your band of
frequencies of interest.

Also, be sure to RTFM in the area of A/D input impedance. I don't remember
the particulars but I seem to remember that you need to be aware of what
the pic wants, impedance wise, for it's signal source.  Yup. Always Read
The Fine Manual.

>Thanks

You bet.

Tom M.

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