From: Winfield Hill
Subject: Re: Inductance to Voltage Converter
Date: 29 Sep 2002 04:36:18 -0700
Organization: Rowland Institute
References: <email@example.com> <firstname.lastname@example.org>
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Tony Williams wrote...
> Winfield Hill wrote:
>> 7 to 9mH
>> sensor Lx ,...............,
>> ______ ____ ____ : shielded box :
>> ,-)_____)-o--OOOO--o-)___)--o--, :
>> | coax : | transresistance
>> o (to 20m) ,...........' | 10k amp :
>> | : / ____ +-- SJ ----->-,
>> +---> AD734 -----\/\/----OOOO--' : |
>> | MPY : / 10mH ref Lr : |
>> | | '...........................' |
>> | inverting |
>> | DC summer |
>> | | | error AD734 |
>> | | '-----<--+-- integrator --<- MPY -<--' 10V at 7mH
>> | offset | | 0V at 9mH
>> | '-------> filter --- | ---->> OUT
>> | |
>> +--------->---- integrator ---->-------'
>> | -90 deg phase Vo = 80 (Vs Lr/Lx -0.44)
>> 1kHz, 400mVac Vo = 0.32/Lx - 35.6
> SJ amps with inductive sources always makes me nervous.
> The low source-R results in a high dc-gain, maybe enough
> for input offsets to saturate the SJ's output.
Right. The "circuit" of course is just a block diagram. We
have no interest or use for the dc signals, as our concern is
only the in-phase balance at 1kHz. Therefore ac coupling can
be used throughout as desired, up to the output of the error-
detecting multiplier, where a dc signal is first generated.
> PS: I'm looking at the output of that -90 phase-shifter.
> It's lagging, so it should be possible to use that as
> the signal source for the other AD734, and replace Lr
> with a resistor.
Nice idea. As it stands the 1/RC gain of the integrator isn't
at all critical (nor are the gains of the error amplifiers or
the error-detection multiplier). But reasonably-stable resistors
and capacitor are much smaller than a stable 10mH inductor; so
routing the integrator's output to the bridge-balance multiplier
could be an attractive proposition. However, an interesting idea
is to use a 10mH inductor constructed with the same methodology
as the sensor, creating a temperature-tracking bridge.