As far as the A/D is concerned a voltage follower would be fine.
Anything to provide a nice low impedance signal to the A/D.  But if you
are measuring the power line there are other questions, such as what
exactly are you trying to measure?  If you only want to measure the
fundamental, then a cap will help in getting rid of nasty spikes and
harmonics.  If the spikes and harmonics are what you are trying to
measure then the cap is bad and the follower is the way to go.

Sherpa Doug

> -----Original Message-----
> From: dr. Imre Bartfai [mailto:root@PROF.PMMF.HU]
> Sent: Tuesday, November 27, 2001 4:15 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Potentiometer on ADC
>
>
> Hi,
>
> an (maybe silly) idea: could one use instead of the capacitor
> a voltage
> follower? (I want to measure the power line and it would be a divider
> ladder with a ratio 1:150 (the Vpp is 650V = 2 * 310V = 2 * sqrt(2) *
> 230V).
>
> Thank you in advance.
>
> Regards,
> Imre
>
>
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>
> On Sun, 25 Nov 2001, Robert A. LaBudde wrote:
>
> > At 06:56 PM 11/24/01 -0500, Josh wrote:
> > >How do you figure out the value of the capacitor, and how
> long it takes
> > >to charge? I'm not an EE, so sometimes these things are
> hard for me. If
> > >I wanted to go for simplicity, would it be best to just go
> for a 10K
> > >pot?
> >
> > The size of the capacitor and the resistor are determined
> by how frequently
> > you are making measurements with the ADC, if you are going
> to rely on the
> > capacitor to do a "sample hold".
> >
> > For your simple setup, it's a lot easier to simply use a
> lower R pot so
> > that the issue doesn't arise. The minimum size of the pot
> depends only on
> > the amount of current you are willing to consume from the
> power supply.
> >
> > For a 5V supply, a 5000 ohm pot will pull 1 mA (I = V/R). A
> 1 kohm pot will
> > pull 5 mA.
> >
> > If you are not making a low-power device (e.g., running off
> a 9 V battery),
> > then a 1 kohm pot would be fine.
> >
> > Now the capacitor is present primarily to remove unwanted
> AC noise from the
> > ADC input. The pot and the capacitor make an input low-pass
> filter to the
> > ADC input. The roll-off frequency of the filter is at fc =
> 1/(2 pi R C).
> >
> > The problem with using too low an R for the pot is that you
> would need a
> > very large C for the capacitor to protect against noise at
> relatively low
> > frequencies (e.g., 60 Hz). For R = 1 kohm and fc = 30 Hz,
> you would need C
> > = 5.3 uF. For fc = 3 Hz, this would increase to C = 53 uF.
> >
> > If you were doing a lot of high-frequency port R/Ws (which
> I doubt you
> > intend in your application), you might want to bypass the
> filter capacitor
> > with a 0.22 uF tantalum to protect against very high
> frequency noise.
> >
> > For your application (a simple low accuracy pot-controlled
> test jig for an
> > ADC), a simple 1 kohm pot alone would probably present no
> problems. If you
> > still want to remove noise, add a 100 uF capacitor from the
> ADC input to
> > ground. Make sure the capacitor is not so large that fc is
> less than twice
> > the sampling frequency. I.e., if you are going to read at
> 10 sps, make fc =
> > 20 Hz or more (e.g., 4.7 uF capacitor).
> >
> > Also be aware that use of a large value capacitor means
> that you will not
> > get an accurate ADC reading until 6 RC time constants have
> elapsed at start
> > up. If R = 1 kohm and C = 100 uF, this will be 0.6 sec at
> start up, which
> > you will have to wait out.
> >
> > You can tell whether or not you need the capacitor by
> looking at the input
> > signal to the ADC with an oscilloscope while it is in its
> most frequent
> > repetitive read loop. If the input is flat to the accuracy
> you need, you're
> > home free. If not bypass with your capacitor and check that
> it works well
> > enough.
> >
> >
> > ================================================================
> > Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
> > Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
> > 824 Timberlake Drive                     Tel: 757-467-0954
> > Virginia Beach, VA 23464-3239            Fax: 757-467-2947
> >
> > "Vere scire est per causas scire"
> > ================================================================
> >
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