High-side versus Low-side Current Sensing
Both of these approaches have their pros and cons. High-side current sensing has the benefit of being able to detect current related faults, such as short-circuits or an open circuit that could affect the load. Also, with high-side current sensing, the load can be referenced directly to ground, as we’ll explore later. The main disadvantage of high-side current sensing is that the common mode voltage is relatively high (based on the supply voltage), hence a high common mode amplifier is required.
As opposed to high-side current sensing, on the low-side the common mode is referenced to ground, hence a cheaper, more readily available low voltage amplifier can be used. As eluded to earlier, one disadvantage of low-side current sensing is that the shunt resistor is placed between the load and ground, which can cause ground loop issues since the load may not be at the exact same ground potential as the rest of the circuitry.
As mentioned earlier, for low-side current monitoring, essentially any single-supply, low voltage amplifier can work, as the common mode voltage is ground referenced. Selecting the best amplifier for this situation depends on price versus required performance, as the amplifiers offset voltage, offset drift, common mode and power supply rejection and transient response may all be critical considerations.
For high-side current sensing, the amplifier must be able to support the higher common mode range, as well as handle any voltage transients that may occur on the power line. A system designer can again use a standard operational amplifier configured as a difference amplifier, as shown in Figure 2. However, there are some limitations to this approach. First, the input resistance is relatively low, determined by the external resistors, not to mention that the input currents are not matched, which will limit the common mode rejection. Speaking of which, the common mode rejection will also be limited by how well matched the resistors are, which leads to subpar performance.
Due to these limitations, amplifier manufacturers have created specialty amplifiers for high-side current sensing, such as Microchip’s MCP6C02 high-side current sense amplifier. The MCP6C02 is a single amplifier designed specifically to sense the current through a shunt resistor and convert this measurement to a proportional output voltage. The input pins can support voltages up to 65V specified, 68V operational. The output voltage range will be determined by a separate supply pin, which spans 2.0V to 5.5V to make it easy to interface to a lower voltage ADC or MCU. The MCP6C02 comes in three fixed gain options of 20, 50 and 100 V/V. An external reference pin allows a dc shift in the output voltage, which enables the MCP6C02 to monitor current flow in both directions if so desired.