At 18:29 04/07/97 -0700, Jeff Dickson wrote:
>_Every_ other solution does not depend on converting the current into a
>voltage by sending it through a resistor.  There are ways "around it." One
>method is to measure the electric field around the wire; this is the
>approach in noninvasive procedures.  Other way is to dump the current into
>a capacitor and integrate.  This too eliminates the resistor, and is great
>for applications that require small current measurements as it avoids large
>resistance values.  Burr-brown makes a couple of switched trans-impedance
>amplifiers for this purpose.

You're of course right in a general sense, but the original message was
talking about a 20mA loop, which is designed to feed into a resistor;
there's usually no need for a noninvasive measurement, and the currents
usually are not that small as to making it difficult to transform them into
a voltage easy to measure.

And with the cap charging method, just as with the electric field around
the wire (like a hall effect sensor), you finally get a voltage output --
which then has to be fed into an ADC, which usually uses some resistors
(amd maybe even opamps) in the analog signal path. I just didn't see the
point why trying to avoid a resistor in the signal path leading to the ADC
or trying to avoid opamps to generate the 20mA output current (always
taking into account that we're talking about a 20mA current loop designed
for load resistances in the range of probably up to around 1k).

BTW, how do you think that one adjusts the amplification of a trans-imp
amp? Most solutions I've seen use resistors, and so you just move the point
I was talking about (using resistors to transform current into voltage)
>from one place (at the input) to another (maybe in the feedback loop).
Which sometimes is of course _very_ appropriate (that's why they make them
:-), but again remembering the 20mA current loop...

And regarding precision, reliability and price I still have to see a better
solution than a resistor (for feeding an ADC) for measuring a 20mA loop.

-------------------------------------
Gerhard Fiedler <gerhard@pobox.com>
S‹o Paulo - Brazil