gnupic: ADC using an RC circuit instead of a ADC port??
Subject:
Re: ADC using an RC circuit instead of a ADC port??
From:
John Sheahan ####@####.####
Date:
31 Aug 2002 01:29:58 -0000
Message-Id: <20020831112907.A11556@wattle.taudelta.com.au>
not sure I'd agree here
If the signal from the pressure sensor varies significantly
in the period its being measured, then the design spec.
itself is probably wrong.
If a 2ms measurement period is available and provides adequate
sampling, then an external ADC with
s/h is probably overkill. A lowpass filter on the
pressure signal should be eonugh if even needed.
I suspect the real question could be what accuracy,
temperature variability and part to part variation
can be tolerated.
Getting say 5 bit accuracy from the RC I'd suggest
is easy enough. 10 bits would start to
imply delta-sigma techniques which would be be tricky in 2ms, and
stretch the comparator too.
regards, john
On Fri, Aug 30, 2002 at 08:00:05PM -0500, RAY MCDONALD wrote:
> From: "Ray McDonald" ####@####.####
>
> To: "Martin Peach" ####@####.#### ####@####.####
> Subject: Re: ADC using an RC circuit instead of a ADC port??
>
> Be careful making measurements with a simple RC circuit. You are measuring
> a relatively quick signal with variation that will confound your A/D
> readings. The
> RC circuit is a non linear (exponential) type of integrator and I would
> expect you would like a rather quick
> instantaneous type reading which may need synchronization with the engine
> RPM and crankshaft timing.
>
> I believe that I would use an external A/D with an included sample/hold
> function which could be strobed
> when each measurement was needed. Depending on the required A/D resolution,
> you may be able to use a serial
> interface or if bandwidth is a problem then a parallel data out unit would
> be needed.
>
> Hope this helps
>
> Ray McDonald
>
>
>
>
>
> ----- Original Message -----
> From: "Brent Picasso" ####@####.####
> To: "Martin Peach" ####@####.#### ####@####.####
> Sent: Friday, August 30, 2002 5:40 PM
> Subject: Re: ADC using an RC circuit instead of a ADC port??
>
>
> > Thanks for the great replies..
> >
> > When you say "low speed input" - what's low speed? do
> > you think I could take a measurement within 2ms?
> >
> > I want to track a pressure sensor to track changes in
> > a automobile's manifold pressure.
> >
> > I want to measure it at for at least every ignition on
> > the engine... at 10,000 RPM with a 4 cylinder engine
> > that happens every 4ms or so.
> >
> > Any ideas on a 16F628 + ADC equivalent PIC I could
> > use?
> >
> >
> > Thanks,
> > Brent
> >
> > --- Martin Peach ####@####.#### wrote:
> > > Brent Picasso wrote:
> > > >
> > > > Not sure if I saw it on this mailing list before,
> > > but
> > > > I'm trying to determine if it's feasable to do
> > > > analog-digital conversion using a RC delay using
> > > > standard IO pins, and a counting routine in the
> > > > software.
> > > >
> > > > I've searched google; didn't find anything good.
> > > >
> > > > I'd like to use a 16F628 for my project as I only
> > > need
> > > > one ADC line!
> > > >
> > > > thanks in advance for any help.
> > > > Brent
> > > There's an app note about that on the microchip web
> > > site, using a
> > > 12c50X:
> > >
> > >
> > http://www.microchip.com/download/appnote/devspec/16c5x/00513d.pdf
> > >
> > > It works fine for low speed inputs like temperature
> > > or potentiometer.
> > > /\/\Martin
> > >
> > >
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