A classic 5V op amp seems totally inappropriate for this kind of measurement. But with just a few external components, we are going to see that low voltage amplifiers are absolutely appropriate for sensing a current accurately without any of the common mode voltage limitations.
Schematic & description
The main goal of this application is to measure the current of an industrial motor control, powered with 150V, as illustrated by Figure 1, by means of a shunt resistor. In order to get a precise measurement for low current, a 5V precision op amp is used.
Figure 1. Typical application
Won’t 150V input burn up the op amp? Not if the V1 voltage is used to generate the positive power supply (Vcc_H) for the first op amp, OP_A.
If we use a Zener diode (BZT52C4V7S) with a 4.7V breakdown voltage, the negative power supply (Vcc_L) of the OP_A is generated. In this way, the OP_A is powered with 4.7V, from Vcc_L=145.3V to Vcc_H=150V.
The resistance Rz is used to bias the Zener diode (~5 mA) and provide a return path for the bias current of the op amp (~40 µA).
The voltage, Vsense, is the result of the current flowing through Rsense, and it is amplified as set by the R1, R2, R3 and R4 resistances.
The P-MOSFET (BSP2220) sources an accurate output current proportional to the current flowing into Rsense, and with the R4 resistance, it generates a voltage Vo with respect to ground, which is proportional to the high side current. The voltage output of the first stage can be given by equation 1:
The second op amp, OP_B, is necessary to buffer Vo voltage. A R5 resistance may be added in order to protect the intrinsic ESD diode of the OP_B in case of a high current that might flow in the input pins at start up.