Designers making in-circuit current measurements can avoid ground-reference problems by putting the sense resistor on the supply side of the load. However, high-side measurement components must deal with high common-mode voltages.
To make high-side differential measurements across a sense resistor, the HV7801 and HV7802 high-side current monitors from Supertex offer an 8- to 450-V input voltage range. They also provide 700-ns rise and fall times.
In the HV7802, IOUT equals the drop across the sense resistor divided by RA, which makes it easy to digitize the current as the drop across RB with a simple instrumentation amplifier and analog-to-digital converter (ADC). The HV7802 also has a user-programmable voltage or transconductance gain (see the figure). The RP protection resistor can be added if the drop across the sense resistor could exceed +5 or –0.6 V.
The HV7801 eliminates the variable-gain feature made possible by selecting values for RA and RB in the HV7802. It provides a fixed gain of five between the difference between the IN and LOAD pins and the OUT pin.
In 1000-unit quantities, the HV7801 costs $0.56, while the HV7802 is $0.62.