5t 01:32 PM 4/27/2012, you wrote:
>I've done some searching around, with some mixed results on the=20
>proper interface.
>
>Since the source is 0-10V but 20Mohms (according to the customer),=20
>at minimum I plan on a 100K/100K divider to range it to 0-5V, and a=20
>zener clamp for the ...just in case...since this is an industrial applicat=
ion.
>
>Now, I was thinking...and this is where I've seen it both=20
>ways....that I really want to use a voltage follower on the input,=20
>that feeds the divider?  This gives a lower impedance input to the=20
>PIC's ADC input.  But I've seen it where some have said...not necessary...
>
>So? what are the reasons for the opamp (other than stated for=20
>impedance interfacing) or just the divider network?


As Bob has said a couple times, you need to verify that the _input_=20
impedance looking into your input
has to be 20M vs. the actual _source_ impedance being 20M. One=20
requires a 20M input impedance (obviously)
the other a 2G Ohm input Z for 1% loading (or 20G for 0.1% loading).=20
BIG difference.

If it's 20M looking into your output, you could just use two 10M=20
resistors in a voltage divider and
a low Ib amplifier running off your (presumably) 5V Vdd. You will=20
need to keep your PCB clean and dry,
including care with the type of flux used, but it's not that bad (the=20
buffer amplifier sees a 5M
impedance, so 1nA leakage will cause 5mV of error or 1 bit on a=20
10-bit ADC with 5V span. No further
"protection" should be necessary, up to a few hundred volts or the=20
rating of the resistors, whichever
comes first.

Otherwise, I think that you will need at least a 10V supply for the buffer
and then a voltage divider. If you want the input voltage defined for
disconnected input, then either 20M or 2-20G to whatever voltage you want i=
t
to approach (eg. ground).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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