directional

Depending on the application, the power supply current may flow in one direction (unidirectional) or in two directions (bidirectional). Unidirectional design is simple because the output voltage of the amplifier does not need to distinguish direction. However, if bidirectional design is required, how can designers distinguish the direction of current? The input of the bidirectional solution is used for reference voltage or base voltage. Then reference the output of the device to the known voltage. The output voltage higher than the known reference voltage runs in one direction, while the output voltage lower than the known reference voltage runs in the opposite direction. Figure 4 illustrates the concept of current directionality.

Output quantity

The output of current sensing solutions can be in the form of current, voltage, or digital words. A dedicated CSM can have any of the above outputs, while operational amplifiers, DA, and IA have voltage outputs.

Current output CSM typically requires an external gain setting resistor (RL), as shown in the INA138/168 schematic diagram in Figure 5. This allows for flexible selection of device gain. Similarly, if the design requires driving a large capacitive load, an external operational amplifier with appropriate compensation can buffer the output of CSM. However, since the gain of the device depends on the external resistance, it is necessary to adjust the internal resistance to an absolute value, which will increase the cost.

The voltage output CSM integrates a gain setting resistor and an output buffer, as shown in the schematic diagram of INA193-8 in Figure 6. This only requires ratio level adjustment, and its cost is cheaper than absolute adjustment. The disadvantage is that the device has a predetermined gain (such as 20V/V, 50V/V, 100V/V), and designers cannot use the feedback path of buffer amplifiers to compensate for larger capacitive loads.

The digital output CSM has an integrated programmable gain amplifier (PGA) and ADC, and reports the branch voltage in digital form through a serial communication protocol (such as I2C or SPI), as shown in the INA219 block diagram. For ease of use, the full-scale differential input must be limited within a certain range. Therefore, caution must be exercised when selecting the value of the shunt resistor.

outline

In summary, this article provides a basic understanding of current sensing solutions. I have learned about common mode voltage, high-end and low-end sensing, directionality, and output requirements. The next article in this series will carefully examine four commonly used differential amplifiers in DC sensing solutions. This includes operational amplifiers, DA, IA, and CSM.