Advanced Power Monitoring
High-Side Versus Low-Side Current Sensing
As energy efficiency and safety have become major concerns for designers of today’s electronics, the need to closely monitor currents has increased. This article will focus specifically on the use of a shunt resistor to monitor current.
For shunt-based current monitoring, you first need to determine where to monitor the current in your system. There are two basic options: high-side and low-side current monitoring. For high-side current monitoring, the shunt resistor is placed between the power supply and the load, as shown in Figure 1. For low-side current monitoring, the shunt resistor is placed between the load and ground, as shown in Figure 2.
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, so a high common-mode amplifier is required.
As opposed to high-side current sensing, with low-side current sensing, the common mode is referenced to ground. This allows you to use a cheaper, more readily available single-supply, low-voltage amplifier. Selecting the best amplifier for this situation depends on price versus required performance, as the amplifier’s offset voltage, offset drift, common mode and power supply rejection and transient response may all be critical considerations. 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.
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. You can use a standard operational amplifier configured as a difference amplifier, as shown in Figure 3. However, there are some limitations to this approach. First, the input resistance is relatively low and determined by the external resistors. Also, the input currents aren’t matched, which will limit the common-mode rejection. This will also be limited by how well matched the resistors are, which can lead to subpar performance.
Due to these limitations, amplifier manufacturers have created specialty devices for high-side current sensing, ranging from relatively simple voltage or current output amplifiers to more advanced solutions, such as Microchip’s PAC1921. The PAC1921 is a dedicated power monitoring device that provides a configurable analog output that can present power, current or voltage. All information is also provided on the 2-wire/I2C compatible interface. Visit the PAC1921 product page on Microchip’s website to learn more.