At 03:52 AM 6/3/2005, Michael Rigby-Jones wrote:
>Just checking the maximum recommended input impedance of the A/D convert=
er=20
>on the 16F88.
>
>The datasheet states in section 12.1 (page 117)
>The maximum recommended impedance for analog sources is 2.5 k=D9.
>
>On the same page in the "notes section it states:
>The maximum recommended impedance for analog sources is 10 k=D9. This is=
=20
>required to meet the pin
>leakage specification.
>
>Parameter A30 in table 18-13 states
>A30 ZAIN Recommended impedance of analog voltage source  2.5 k=D9 See (N=
ote 4)
>
>Note 4 states
>The maximum allowed impedance for analog voltage source is 10 k=D9. This=
=20
>requires higher acquisition times.
>
>So anyone know which is it? Is 10K actually acceptable (with longer=20
>acquisition) or is it a typo or something left over from one of the=20
>earlier device where 10k was definitely ok?

The last time I brought this up to Microchip Tech Support, I was told tha=
t=20
the figures are 10K for 8-bit A/D inputs and 2K5 for 10-bit A/D=20
inputs.  This is because the input leakage current stayed the same even=20
though they improved the resolution.

Says pretty awful things about their input stages <sad frown>.

Staying with 10K for 8-bit A/D inputs and 2K5 for 10-bit A/D inputs keeps=
=20
leakage error to less than 1/2 LSb.

dwayne

--=20
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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