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. One (among several) limits to 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 75V.
The circuit of Figure 1 lets you double that limit, using two current-sense amplifiers stacked one upon the other, and a single-transistor voltage-distribution scheme. The resulting voltage gain—from the 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)—is the product of the amplifier gains (each AV = 5) for a total of 25.
Figure 1. This circuit doubles the supply voltage at which current can be sensed.
Figures 2 and 3 illustrate circuit performance over the range of differential input voltage, and the range of supply voltage. More information about the MAX4080F and a complete data sheet can be found at www.maximintegrated.com.
Figure 2. In Figure 1, the output vs. differential input voltage is linear.
Figure 3. Output for the Figure 1 circuit is relatively unaffected by changes in the supply voltage.