A/D problem
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Posts: 46,084
In the world of industrial process controllers, it seems typical that 0 - 10
V outputs are rapidly replacing 4-20mA. There is an apparent difficulty in
inputting such a device into an A/D, then into a Stamp.
The 0 - 10 V controller outputs that I'm seeing are not very hefty - they
seem to call for very high impedance on the input of the device next in
line. A/D converters generally have much lower input impedance requirements,
so they can charge quickly. If the controller data specifies, say 500k or
greater impedance, and the A/D calls for 2k or less, how would you suggest
bridging the gap w/o having to build an amplifier???
Chris
V outputs are rapidly replacing 4-20mA. There is an apparent difficulty in
inputting such a device into an A/D, then into a Stamp.
The 0 - 10 V controller outputs that I'm seeing are not very hefty - they
seem to call for very high impedance on the input of the device next in
line. A/D converters generally have much lower input impedance requirements,
so they can charge quickly. If the controller data specifies, say 500k or
greater impedance, and the A/D calls for 2k or less, how would you suggest
bridging the gap w/o having to build an amplifier???
Chris
Comments
>V outputs are rapidly replacing 4-20mA. There is an apparent difficulty in
>inputting such a device into an A/D, then into a Stamp.
>The 0 - 10 V controller outputs that I'm seeing are not very hefty - they
>seem to call for very high impedance on the input of the device next in
>line. A/D converters generally have much lower input impedance requirements,
>so they can charge quickly. If the controller data specifies, say 500k or
>greater impedance, and the A/D calls for 2k or less, how would you suggest
>bridging the gap w/o having to build an amplifier???
Hi Chris,
It's easy, IF the sampling rate is low. Put a capacitor of ~0.1uf at
the input of the A/D, and put a high resistance (1% film) voltage
divider if necessary between the controller and the A/D. The
capacitor presents a low AC impedance to the input of the A/D.
Why low sampling rate? (1) The RC time constant has to be faster
than expected changes in the controller output. (2) Each time the
A/D samples that channel it kicks out a pulse of charge, equal to a
few pf of capacitance. That is nothing in comparison with the
external 0.1uf. But it amounts to a small current as the A/D sampling
rate increases, and that current has to flow through the resistance
you put at the input of the A/D. At high sample rates, the sampling
current is integrated by the external RC circuit. What it comes down
to is, keep the sampling rate much less than the external RC time
constant.
At _high sampling rates_ and for _rapidly changing input signals_,
there has to be a buffer amplifier... high input resistance for the
controller+low output resistance for the A/D=fast response.
Most controllers will include a spec for output resistance, so you
can include it in the voltage divider equation, or you can measure
the output resistance by observing the change in output voltage when
the controller output is loaded.
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com
Original Message
> From: Chris Loiacono [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=6obb5zQpdZjdQqVMAFI7WyWBRQFfo3juGHJWKUKScTDpHNeCDX-K53yQ_PECtwZiqHpG0mj5QNuDu6I]chris@m...[/url
> Sent: Wednesday, May 09, 2001 12:37 PM
> To: 'basicstamps@yahoogroups.com'
> Subject: RE: [noparse][[/noparse]basicstamps] A/D problem
>
>
> Tracy:
> Thanks.
> I had already tried the .1 cap, without success, even though
> the sampling rate is pretty low - about 15mS. In this case, a
> look at the controller output seems to have paid off. The
> manufacturer says that it should be a straight-line DC. In
> reality, it is somewhat proportional - 60mS at the control
> level, 40 at 0. I added a 100uF cap to level it out, and
> changed my divider to >500k, and all seems to work.
> I'm not sure why the A/D is OK with this, unless the cap
> across both ends of the divider is affecting the impedance
> the A/D sees. If that's so, I'll probably need to be sure I
> haven't too low an impedance for the controller output once again.
>
> From your experience, do linear output controllers not always
> have truly linear output? I'm concerned that the controller
> output is not working properly......
>
> Chris
>
> >
Original Message
> > From: Tracy Allen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=TTarrlD0toJ4HcRnG4Zn-bBW-O9ZyhIXuZS5txSTtYaMu33sAUIokqpzwYniQ1EU2VgDtNeBVZjPATewKF0]tracy@e...[/url
> > Sent: Tuesday, May 08, 2001 12:26 PM
> > To: basicstamps@yahoogroups.com
> > Subject: Re: [noparse][[/noparse]basicstamps] A/D problem
> >
> >
> > >In the world of industrial process controllers, it seems
> > typical that 0 - 10
> > >V outputs are rapidly replacing 4-20mA. There is an apparent
> > difficulty in
> > >inputting such a device into an A/D, then into a Stamp.
> > >The 0 - 10 V controller outputs that I'm seeing are not very
> > hefty - they
> > >seem to call for very high impedance on the input of the
> > device next in
> > >line. A/D converters generally have much lower input
> > impedance requirements,
> > >so they can charge quickly. If the controller data
> > specifies, say 500k or
> > >greater impedance, and the A/D calls for 2k or less, how
> > would you suggest
> > >bridging the gap w/o having to build an amplifier???
> >
> >
> >
> > Hi Chris,
> >
> > It's easy, IF the sampling rate is low. Put a capacitor of
> ~0.1uf at
> > the input of the A/D, and put a high resistance (1% film) voltage
> > divider if necessary between the controller and the A/D. The
> > capacitor presents a low AC impedance to the input of the A/D.
> >
> > Why low sampling rate? (1) The RC time constant has to be faster
> > than expected changes in the controller output. (2) Each
> time the
> > A/D samples that channel it kicks out a pulse of charge, equal to a
> > few pf of capacitance. That is nothing in comparison with the
> > external 0.1uf. But it amounts to a small current as the
> A/D sampling
> > rate increases, and that current has to flow through the resistance
> > you put at the input of the A/D. At high sample rates, the
> sampling
> > current is integrated by the external RC circuit. What it
> comes down
> > to is, keep the sampling rate much less than the external RC time
> > constant.
> >
> > At _high sampling rates_ and for _rapidly changing input signals_,
> > there has to be a buffer amplifier... high input resistance for the
> > controller+low output resistance for the A/D=fast response.
> >
> > Most controllers will include a spec for output resistance, so you
> > can include it in the voltage divider equation, or you can measure
> > the output resistance by observing the change in output
> voltage when
> > the controller output is loaded.
> >
> > -- regards,
> > Tracy Allen
> > electronically monitored ecosystems
> > mailto:tracy@e...
> > http://www.emesystems.com
> >
> >
> >
> >
> > Your use of Yahoo! Groups is subject to
> > http://docs.yahoo.com/info/terms/
> >
> >
>