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From: Tony Williams
Subject: Re: micro ohms measurementsWhat
Date: Sun, 08 Sep 2002 11:18:44 +0100
References: <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: Sun, 8 Sep 2002 10:18:47 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article ,
N. Thornton wrote:
> Just a couple of points to add to the thread. Firstly electrode
> resistance is not an issue with 4 wire measurements, and so doesn't
> need any compensating for. That assumes your measuring electrodes are
> sensed by a reasonably high R circuit.
> Secondly a transformer in the measuring side would draw current from
> the measurement elctrodes, thus making the result dependant on
> resistances in that circuit. For that reason it sounds like not the
> way to go. The sense electrodes really need relatively high impedance
> measurement to remain resistance independant.
Yes and no. The reason that many of the classic
bridges are classic is that they are null-seeking.
When this is done the input impedance of the detector
has no effect on the linearity of the bridge, it merely
affects the final resolution. Null-seeking also means
that the linearity of the detector has little consequence.
So the trick here would be to have a transformer, but
have a scheme that always runs the transformer with zero
input. This could either be an actual balancing of the
bridge, or a faked balance with (say) an opposing voltage
in series with the primary of the transformer.
A transformer is desirable, because the step up allows
lower noise and the isolation reduces earth-loop problems.
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