Subject:
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Re: input impedance
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Thu, 11 Jun 1998 02:08:44 GMT
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Original-From:
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David Kott <(dakott@)antispam(alpha.delta.edu)>
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Viewed:
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2150 times
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On Wed, 10 Jun 1998, Joe Martin wrote:
> Does anyone know the impedance onto the sensor inputs on the HB and/or any
> suggestions for setting them to receive signals sent at 220K Ohm.
> Joe Martin
> joema@mpx.com.au
>
>
Hmmmmmm.. as I recall, there are 47k pullups on those inputs. These are
used to condition the signals when used as a digital input. The 'HC11
pink book suggests that any input resistance of greater then 10K will
"...degrade A/D accuracy." The ADC module draws 400nA continuously. Your
220K will drop ~80 mV. This will result in only 6(ish) bit precision.
Also, the ADC subsystem is modeled as 20pF cap. 220K driving such a cap
will increase your acquisition time significatly. You will definately
want to condition this signal with a low impedance driver. I suggest an
instrumentation amplifier (a special kind of rail to rail, high frequency
Op-Amp) to condition that signal. National Semiconductor offers many
single supply, rail to rail devices. Analog Devices has a pretty keen
selection too. I don't remember the particular part number, but NS has a
swift selection tree. The output impedance of an OpAmp is quite low.
This means you will be able to leave the 47K's on the HB ADC inputs alone.
-d
The box said "Requires Windows 95/NT or better"...
So I got Unix.
Free the Source. Free your Computer... http://www.FreeBSD.org
http://www.NetBSD.org
http://www.OpenBSD.org
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Message is in Reply To:
 | | input impedance
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| Does anyone know the impedance onto the sensor inputs on the HB and/or any suggestions for setting them to receive signals sent at 220K Ohm. Joe Martin joema@mpx.com.au (27 years ago, 10-Jun-98, to lugnet.robotics.handyboard)
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