Has anyone used a FET switch, at the bottom of the voltage divider fed into=
 the ADC, controlled by the PIC?=20

Sent from my phone

> On Jul 1, 2014, at 22:36, "PICLIST" <piclist@platinum.ca> wrote:
>=20
> Thanks for the comment on the leakage current, I had not thought of that.=
 I
> think that you are correct in that I should just run at full speed on the
> internal oscillator and use the WDT to wake up when I need to do a
> conversion.
>=20
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf =
Of
> Mike
> Sent: Monday, June 30, 2014 7:06 AM
> To: piclist@mit.edu
> Subject: Re: [PIC] Using ADC in PIC12F683 with Low-Speed Internal Oscilla=
tor
> (LFINTOSC)
>=20
> PICLIST <piclist <at> platinum.ca> writes:
>=20
>=20
>> My sampling circuit will be a relatively high impedance voltage=20
>> divider off of the 12V battery, but I'm assuming that if I give the=20
>> ADC lots of time to settle (i.e. a large fraction of a second), I=20
>> should be OK with a 200K+ input impedance.
>=20
> You really need to consider the input pin leakage currents before jumping=
 to
> any conclusions.  No matter how slow you perform a conversion, or how muc=
h
> capacitance you use for decoupling, input leakage will give you the same
> error.
>=20
> Datasheet says +-0.1uA typical, +-1.0uA maximum.  With your 200k input
> impedance that would give a typical offset error of 0.02v and a maximum o=
f
> 0.2v .  And since this is a leakage current it will very likely vary with
> temperature.
>=20
> Also check the current that the A/D requires during conversion from the V=
ref
> source, you may well be using more power by running the A/D clock so slow=
ly.
>=20
> Your requirements of simply taking a sample about 1 per second could be v=
ery
> easily implemented by using the watchdog timer to wake the part from slee=
p.
> You could then simply use the internal RC oscillator running at whatever
> speed is appropriate (i.e. 125kHz up to 8MHz) to perform any processing
> before sleeping again.  The watchdog takes around ~1uA, and the rest of t=
he
> device will be in the low picoamps range if correctly configured before
> sleeping.
>=20
> Mike
>=20
> --
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>=20
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