Current Sense Amplifiers Extend Sense Range

Summary: Using transistors as shown, you can double-click a current sense amplifier and stack two others to allow for voltage limiting of such an amplifier. The voltage signal from the bottom is amplified and then represented as the voltage across the sense resistor, time product of the two amplifiers.

High-side Current Sense Amplifiers are relatively inexpensive devices that sense current at voltages far above the level at which the current sense signal is processed. One (among several) limitations to the application of these devices is the maximum allowable voltage difference between the current sense resistor in the data acquisition circuit (connected to the input of the current sense amplifier) ​​and common (ground). Even with the new generation of current sense amplifiers, this is around 75V.

The circuit in Figure 1 lets you double that limit by using two current-sense amplifiers stacked one on top of the other, and a single-transistor voltage-divider scheme. The voltage gain produced from the differential inputs of the top amplifier (where the current-sense resistors are connected) to the combined line output (labeled the output of the bottom amplifier) ​​is the amplifier gain product of 25.

Figure 1. This circuit draws current at twice the supply voltage.

Figures 2 and 3 illustrate the circuit performance over the differential input voltage range, as well as the supply voltage range.

Figure 2. In Figure 1, the differential input voltage output is linear

Figure 3. The output of the circuit in Figure 1 is shown relative to the effects of changes in supply voltage.