Extend Range of Current-Sense Amplifiers
High-side current-sense amplifiers are relatively inexpensive devices that sense current at voltages well above the level at which the current-sense signal is processed. A current-sense amplifier works by amplifying and level-shifting the voltage drop that the current causes in a current-sense resistor in series with the current path.
One of several limits found on the application of these devices is the maximum allowed voltage difference between the current-sense resistor (connected to the inputs of the current-sense amplifier) and common (ground) for the data-acquisition circuits. Even for new-generation current-sense amplifiers, this limit is about 75 V.
The circuit in Fig. 1 lets you double that limit using two current-sense amplifiers, stacked one on top of the other, and a Darlington transistor voltage-distribution scheme. The voltage-distribution circuitry adds a 200-µA burden to the current-carrying line load.
Once stacked, the amplifiers can then be connected in cascade, the output of the top one feeding the input of the one at the bottom of the stack. The resulting voltage gain is the product of the amplifier gains (each AV = 5), for a total of 25. This gain is measured from the RS+/RS- differential input of the top amplifier (where the current-sense resistor is connected) to the output of the combined circuits (labeled “output” on the bottom amplifier). Fig. 2 and Fig. 3 illustrate circuit performance over the range of the differential input voltage and through the range of the supply voltage.