I need to measure a bi-directional current. 0-5mA. I see alot of special current sense amplifiers online.... what are these about? uni- directional high sense and low sense.... would I just use an instrumentation amplifier and a shunt resistor and forget about it? what are these one directional current sense amplifiers for? is it just better resolution?
thanks!
The instrumentation amp would have to have a common-mode range that covers the shunt voltage, and very good common-mode rejection. The current sense amps are cheap and take care of all that.
If you don't mind dropping a fairly large voltage across your shunt, it gets easier. At higher currents, that becomes a problem.
John
You can detect current with a sense resistor, or a current meter, or a current transformer and a sense resistor... or with more exotic tools (Hall sensors come to mind, and bolometers for AC).
When high voltage isolation is needed, the current transformer is a good choice. Use that if it's the current in your spark plug wires that you want to know. If the current is delivered to be grounded, the inverting node of an op amp, with sense resistor from output to (-), and (+) to ground, does a good job of converting I to V.
If the current measurement is from a DC supply rail, a variant on this kind of thing is a scaled current mirror (which has VERY GOOD common mode rejection, regardless of resistor accuracies). That's what your 'unidirectional high sense' gizmo does,, it's just a very good topology for a DC amplifier with common mode range including the (+) rail.
And for other cases (voltage compliance in the range an instrument amp can handle), the instrument amp might be a good choice. Alas, its common mode rejection is very much an issue when dealing with small sense voltages against a common mode background- you have to pay extra every time the CM rejection requirement gets bumped up. The current from an unregulated charger to a car battery is likely to have volts of ripple to be rejected, and if your shunt only drops millivolts, there's not much joy in doing this with an instrument amp built with 2% resistors.
Scroll down a bit and look at that full wave precision rectifier..
With that circuit you only need to supply a shunt R on the front end.
Jamie
Have a look on this one:
75uA 4,5-76V MAX4080/MAX4081 (bidir)Glenn
i-
The old part they fabbed at VTC was better. This one is OK, but I'd be careful if you used feedback around it.
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