From: Spehro Pefhany
Subject: Re: Meter movements
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org> <3DD6F1BE.4F3C0ED0@earthlink.net> <email@example.com> <3DD7ADF4.2B1E@Spam.Bots>
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Date: Sun, 17 Nov 2002 13:14:53 GMT
NNTP-Posting-Date: Sun, 17 Nov 2002 08:14:53 EST
On Sun, 17 Nov 2002 06:55:48 -0800, the renowned Mike Monett
>Winfield Hill wrote:
>> Tony Williams wrote...
>> > Redrawing the diagram into a more familiar form
>> > makes it easier to see.
>> > - + R2
>> > +---[Meter]--/\/\---+
>> > | |
>> > +---|- \ |
>> > | | >-----------+
>> > Vin-----------|+ /
>> > |
>> > \
>> > /R1
>> > \
>> > \|/ I(R1)
>> > 0v--------+-----------------0v
>Is there a need for a damping resistor across the meter movement when it
>is part of the op amp feedback network? Or does the feedback absorb the
>back EMF and eliminate the need for the damping resistor?
The meter movement just sees a high impedance, up to the point where
the op-amp output hits either rail, so there is no damping. It's
being driven by a quite high quality current source. R2 can be ignored
when analyzing this, up until the point where the op-amp output
>If so, this arrangement might speed up the respone time, which would be a
>definite improvement for sensitive movements of 25 or 50uA.
If it's line powered, you'd be crazy to use such a sensitive movement,
you'd pick a nice 1mA or so movement which has less problems in that
regard. Otherwise, I suppose you could make a feedforward circuit
that was tuned to the meter response.
"it's the network..." "The Journey is the reward"
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