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From: email@example.com (Bill Sloman)
Subject: Re: Cheap low frequency impedance bridge?
Date: 13 Oct 2002 14:39:23 -0700
References: <3DA6E7D6.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 13 Oct 2002 21:39:23 GMT
email@example.com (Winfield Hill firstname.lastname@example.org) wrote in message news:<email@example.com>...
> firstname.lastname@example.org (Bill Sloman) wrote
> > The Analog Devices AD9854 DDS chip that produces tightly coupled
> > in-phase and quadrature outputs
> > http://products.analog.com/products/info.asp?product=AD9854
> > is exactly what I'd love to be able to use - much neater (though
> > probably no more effective) than the scheme Win Hill set around
> > two AD DDS chips.
> 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
> Multiplier chip MPY-1 acts as a phase-sensitive error detector,
> balancing the bridge for the capacitance phase of the unbalance
> signal. MPY-2 with Cr acts as a voltage-controlled capacitor
> to servo-balance the bridge, with the multiplier's DC drive
> voltage indicating the value of Cx. The clever use of a trans-
> capacitance error amplifier adds variable frequency capability
> (a small delay adjustment can be added to cancel the effect of
> this amplifier's high-value DC feedback resistor).
> The resistor Rs is optional, if you want it can serve to balance
> some of the Cx series resistance, so as to avoid saturating the
> error amplifier for higher loop gains and faster bridge balance.
> This circuit can be highly sensitive and stable, even to the
> sub-ppm level in some versions I've made.
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.
The fact that the AD9854 is programmable in phase and amplitude
suggests other bridge-balancing schemes that might be extended up to
maybe 50MHz or so - way beyond the 10kHz of a mulitplying D/A
converter, and probably better than the the 10MHz limit of the AD734
(though the AD834 is pretty good and much faster).
Fun stuff, if you can find an application to pay for the development.
Bill Sloman, Nijmegen
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