Subject:
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Re: analog header
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 5 Apr 1997 21:27:19 GMT
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Original-From:
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Jeffrey Strauss Morehead <MOREHEAD@ECE.UTEXAS.antispamEDU>
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Viewed:
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1421 times
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Certainly the voltage follower will work. But I had a similar problem
adjusting the sensitvity of a photocell that I solved by paralleling a
pullup resistor of my own to get the proper impedance. You might try
putting a 1 ohm (yep, thats one NOT 1k) from your input node
to plus 5v on the board. This will guarantee your input impedance is less
than 1 ohm and will effectively nullify the 47k pullup on the board.
It may not work for your case however.
jm
On Fri, 4 Apr 1997, Chuck McManis wrote:
> Adam wrote:
> > BUT when I hook this up to the HB the
> > sensitivity is lost because I then have to deal with a 47K
> > pullup on the analog ports.
>
> This is known as an 'impedence mismatch'. Basically your NPN
> amplifier is incapable of driving the analog input low (which
> in itself seems a bit weird since you only need to sink .1 mA
> to do that which suggests that perhaps there is feedback in your
> output circuit such that the voltage level of the A/D input pin
> is interfereing with your amplifier) HOWEVER, a simple way to
> rectify this (read a hack) is to use an op-amp in a voltage
> follower mode (connect your signal to the '+' input, connect
> the output of the op-amp to the A/D input and back to itself
> on the minus input. The output of the op amp will follow the
> input from your circuit and your circuit will believe its
> connected to air (op amps have a very high input impedence)
> Use an Opamp that works well on a single supply like a 301 or
> something (_don't use a 741_)
>
> --Chuck
> --
> cmcmanis@netcom.com http://www.professionals.com/~cmcmanis
> All opinions in the non-included text above are the sole opinions of
> the author.
>
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Message is in Reply To:
 | | Re: analog header
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| (...) This is known as an 'impedence mismatch'. Basically your NPN amplifier is incapable of driving the analog input low (which in itself seems a bit weird since you only need to sink .1 mA to do that which suggests that perhaps there is feedback (...) (27 years ago, 5-Apr-97, to lugnet.robotics.handyboard)
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