In many circuits you must sense current to monitor operating conditions or detect problems such as over-current conditions in a motor or a drive circuit. You can monitor either the “high side” or the “low side” of the circuit. A high-side monitor measures current flow at the power supply’s positive output, while a low-side monitor measures current that flows to ground. For DC-powered devices you pass the current through low-resistance shunt and measure the voltage across it. Then Ohm’s law lets you calculate the current: I = E/R.Conceptually, low-side measurements seem easier because you can reference the measuring circuits to ground. But a shunt resistance will slightly “raise” the ground potential on the circuit side of the shunt, which can affect other measurements and voltages seen by other circuits that reference their signals to the real ground, 0V. And as the current varies through the shunt, so will the ground potential seen by the circuit, as shown below:
In this circuit (above), you measure the voltage across the shunt resistor to calculate current, but a short circuit or sneak path bypasses the shunt. Also, circuit “sees” its ground reference, of “floating ground,” slightly above the actual zero-volt ground. That condition can add noise to signals.Also, if a component shorts to zero-volt ground, current can bypass the shunt and remain unmeasured. (Hopefully a power-to-ground short would blow a fuse or trip a circuit breaker.) Or if a designer uses a separate ground for some portion of the circuit, you won’t measure its current.So, a high-side shunt can make life easier, at least at first. As you look at such a shunt circuit, though, you realize that it must cope with high common-mode signals. That means you might measure 5.00 volts on the power supply side of the shunt and, say, 4.85 volts on the other side. You need to measure the 0.15-volt difference in the presence of the 4.85 volts common to both sides of the shunt. That can be a tough task.Some time ago, Maxim Integrated Products produced the application note: “High-Side Current-Sense Measurement: Circuits and Principles,” APP 746 that you can find at www.maxim-ic.com/appnotes.cfm/appnote_number/746. Maxim provides a PDF version of the app note, too at the same place.This helpful information includes many useful circuits. Even if you don’t use the Maxim current-sense or differential amplifier ICs shown in the examples, you’ll still get good ideas about how to measure high-side current. (Maxim no longer supplies the MAX471 device shown in the app note, but you can substitute a device in the MAX4071 family.) I welcome comments on how you make current measurements. –Jon Titus
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