At 11:10 PM 9/3/2003, Ian Wilson wrote:
It is common to have four connections to a current sense resistor, two on each side. One on each side will be big and fat to carry the current, and the other with be a signal trace (carrying no current) that ensures the voltage drop across just the resistor is sensed - the voltage drop across power tracks, ground planes etc are ignored.

They are ignored because they are irrelevant. This kind of arrangement is used with some kind of differential amplifier (or bridge, I suppose), so the current will be equal to the differential voltage divided by the sense resistance, quite accurately, it will be as accurate as the resistance.


However, in some circuits it might be possible to ignore the ground leg drop. For example, the ground connection might be through a ground plane, and the sense amplifer circuitry is referenced to this same plane.... I'm not describing all the details and possibilities... (do I hear a sigh of relief? :-)

The only tricky stuff with this is that it requires common-mode input ranges beyond, or at least close to, the supplies in many situations - but this is no longer rocket science.

Normally the sense resistor, as I've seen it, is in the return, as in the matter at hand. So if, for example, we have high voltage to the load, we don't need to deal with high voltages in the current-measurement circuitry, we are only dealing with voltages close to ground, well within the instrumentation supply voltage. Of course, if the fabricator opens up that virtual short, we're likely to have a fairly spectacular error indication....





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