From: Winfield Hill
Subject: Re: Cheap low frequency impedance bridge?
Date: 13 Oct 2002 15:37:18 -0700
Organization: Rowland Institute
References: <3DA6E7D6.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
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Bill Sloman wrote...
> Winfield Hill wrote
>> Here's my circuit, modified for your purposes...
>> 1 to 100uF
>> sensor ,...............,
>> ______ Cx ____ : shielded box :
>> ,-)_____)-o---||---o-)___)--o--, :
>> | coax : | trans-capacitance
>> o (to 20m) ,........' | amplifier 0.1uF
>> | |\ : Rs / Cr +- SJ ------->-,
>> +--> AD734 --| >----\/\/--||---' : |
>> | MPY-2 |/ / : / 10uF ref : |
>> | | / '........................' | error
>> | inverting 0.05 to 5Vac _|_ gain
>> | DC summer \ /
>> | | | error AD734 |
>> | | '---<---+-- integrator --<- MPY-1 -<--'
>> | offset | |
>> | '-------> filter ---- | ---->> DC OUT
>> | |\ | 10V at 100uF
>> +------->---| >-------------->--------' 0.1V at 1uF
>> | |/ 5V
>> 75Hz, 500mVac
> I'm a bit surprised that you've used analog multipliers rather than
> multiplying D/A converters for a 75Hz bridge, though I guess the
> capacitative feed-throughs in a multiplying D/A converter might be
> a bit too big and erratic for comfort.
AD734 multipliers are simple to use and accurate to better than 0.1%,
but a D-A is a good idea at 75Hz. Digitize the integrator output
(or use a crude V-F with a counter integrator) and use a multiplying
D/A to replace MPY-2. MPY-1 can be replaced with a low-cost circuit.
> The fact that the AD9854 is programmable in phase and amplitude
> suggests other bridge-balancing schemes ...
An expensive way to get variable amplitude, and we know what phase
you want, zero or 180 degrees! So I don't see the appeal for a DDS
here. Low-performance 2% oscillators are adequate for my bridge.
> Fun stuff, if you can find an application to pay for the development.
Bill, I'm suggesting that you pursue it! Unless you want to just
run out and buy or rent a good used HP digital bridge. HP 4274A
bridges have 5.5-digit resolution and are often available over the
net for under $2500. Or how about the slightly less accurate 4276A?
HP4276A LCZ/Impedance Meter w/GPIB, 100Hz-20kHz (800 freq's),
4-terminal measurements, pushbutton open/zero compensation, etc.
In very nice shape with rackmount ears. ... $625
This is from my good friend John Bau, who tests all his stuff
before selling it. http://www.users.qwest.net/~jbau/tstequip.htm