High performance current sensing design

Most analog integrated circuits (comparators, op amps, instrumentation amplifiers, references, filters, etc.) are designed to process voltage signals. When it comes to devices that process current signals, designers have far fewer choices and face far more challenges. This is unfortunate because there are great advantages to directly monitoring and measuring current. Motor torque, solenoid force, LED density, solar cell light levels, and battery charge are best monitored by observing the current flow. All that is needed is a circuit that can accurately measure current and convert it to a voltage, which can then be amplified, conditioned, and measured using readily available off-the-shelf voltage components (amplifiers, comparators, ADCs, etc.).

Figure 1: Conceptual diagram of current sensing circuit

　　Although resistors can convert current into voltage, resistors alone do not provide a complete solution. The most common solution is to use a sense resistor directly in series with the current and an amplifier to isolate and condition the voltage across the resistor (VSENSE). Figure 2: Practical Current Sensing Circuit 　　Combining Amplifiers and Sense Resistors At first glance, placing a resistor in series with ground seems like the most straightforward current sensing method. This method, called low-side current sensing (Figure 3A), requires that there is no ground path that could shunt current around the sense resistor or introduce current into adjacent circuits. If the chassis forms the system ground, inserting such a sense resistor may not be practical. Also, since ground is not a perfect conductor, the ground voltage may be different at different locations in the system, so a differential amplifier must be used to make accurate measurements (Figure 3B). Figure 3A: Low-Side Current Sensing Topology Figure 3B: Low-Side Current Sensing Circuit There are more serious problems when performing low-side current sensing. The resistance in the ground path means that the load "ground" will change as the current changes. This can cause common-mode errors in the system and cause problems when interfacing with other systems that require the same ground level. Because measurement resolution improves with increasing VSENSE amplitude, the designer must trade “ground noise” for increased resolution. A modest 100mV full-scale VSENSE translates to 100mV of injected ground noise. Ground level variation issues can be avoided by placing a current sense resistor between the supply and the load. This approach is called high-side current sensing. The differential voltage across the sense resistor can still be used to measure current directly, but now there is a nonzero common-mode voltage across the resistor. The technical challenge with this configuration is that the small differential sense voltage must be resolved from the supply common-mode voltage (Figure 4). Figure 4: High-Side Current Sensing For low-voltage systems, an instrumentation amplifier or other rail-to-rail differential amplifier may be sufficient for monitoring the high-side sense resistor. The amplifier output must be translated to ground without adding significant error. For high supply voltages, circuitry may be required to translate VSENSE to within the common-mode range of the amplifier inputs, or to float the amplifier to the supply voltage. In addition to increasing board space and cost, these approaches assume that the common-mode voltage will remain within a narrow, specific range. For most current sensing applications, it is useful to anticipate large common-mode variations. For example, if a current sensing circuit can operate when the supply voltage drops, it can indicate if there is a problem at the supply or load; too much current indicates current limiting and load failure, and too little current indicates a supply failure. On the other hand, current sensing circuits may face common-mode voltages that exceed the supply voltage. Many current devices, such as motors and solenoids, are inherently inductive, and rapidly changing currents through these devices can cause inductive flyback, resulting in large voltage swings across the sense resistor. These examples illustrate exactly when amplifiers are most useful. 　　Simple Solutions To overcome these current sensing challenges, high-side current sense amplifiers were created. These special amplifiers are designed to extract the small differential voltage generated by the current through a small sense resistor from the high common-mode voltage. The sense voltage is then amplified and converted to a ground-referenced signal. Figure 5 shows the basic topology of a high-side current sense amplifier. In this figure, the amplifier applies a voltage equal to VSENSE to RIN. The current through RIN then flows through ROUT, providing a ground-referenced output voltage. Given this basic function, it is obvious that a high-side current sense amplifier should have high input impedance, high gain and high gain accuracy, as well as a wide common-mode range and good common-mode rejection. Perhaps less obvious is the importance of the amplifier's accuracy. Figure 5: Basic High-Side Current Sense Amplifier         Focus on Resistors Ideally, current and voltage sensing circuits should have no effect on the load to which they are connected. This means that the voltage sensing device should have nearly infinite input impedance, which ensures that no appreciable current is drawn from the load. Conversely, the current sensing device should have nearly zero input impedance, which ensures that no significant load voltage is dropped. High-side current sensing circuits (amplifier + resistor) are constrained by both of these requirements. The amplifier used to sense the voltage across RSENSE must have high input impedance. The resistor used to sense the load current must be very small. To fully understand this, let's look at what happens when a large sense resistor is used. Because the series resistance is increased, the voltage seen by the load is reduced. The added series resistance is a source of wasted energy, and a large sense resistor can result in excessive heat dissipation, which can cause reliability issues in the long run. Is there any reason to use a large sense resistor? The main benefit is an increase in the overall output voltage (EQ1). This can be useful when the amplifier has a fixed gain or limited gain configurability. 　　[EQ1] There is a limit to the sense resistor value. The amplifier input range and maximum expected current will determine the maximum practical sense resistor value (EQ2). RSENSE_MAX = (VSENSE_MAX / ISENSE_MAX) [EQ2]

　　For example, if the maximum expected current through the sense resistor (ISENSE MAX) is 50mA and the high-side current sense amplifier can accept inputs up to 250mV (VSENSE MAX), then the maximum sense resistor value is 50Ω (RSENSE_MAX).

Ideally, the designer should not be forced to increase the sense resistor to compensate the amplifier. As long as the amplifier can operate with sufficient gain and gain accuracy, the designer should focus on the minimum acceptable resistor value. This can be calculated from the input offset voltage of the current sense amplifier, the minimum current that must be resolved is:
RSENSE_MIN = (VOFFSET / IRES). [EQ3]

For example, if 1mA resolution is required (IRES) and the offset voltage of the high-side current sense amplifier is 1mV (VOFFSET), then the minimum sense resistor is 1Ω (RSENSE MIN). Equation 3 highlights a key point: the minimum sense resistor is directly related to the offset voltage of the high-side current sense amplifier.

　　A Deeper Look at Modern Current Sense Amplifiers

With precision high-side current sensing techniques in mind, designers have developed new high-side current sense amplifiers that offer significant performance improvements over previous offerings. For example, the LTC6102 from Linear Technology is a new high-side current sense amplifier that uses zero-drift technology. This amplifier has an input offset voltage of only 10uV and a maximum bias drift of 50nV/oC. Compared to previous generations of current sense amplifiers, the LTC6102 can use a much smaller sense resistor (2). If the system can tolerate a large VSENSE, the LTC6102 can accept a sense voltage as high as 2V. The low offset coupled with this high maximum sense voltage results in a dynamic range of over 106dB, allowing the LTC6102 to discern microamps from amps. Sensing very small currents is possible because any gain value can be selected with external resistors. By using precision resistors, gain accuracy can be better than 99%.

The LTC6102 also does not compromise other important current sensing features. Its high impedance input limits input bias current to less than 300pA. The LTC6102 operates with input common mode voltages up to 105V. 130dB common mode rejection produces less than 32uV offset error over the entire 100V input common mode voltage range (3). For fault protection, the LTC6102 has a 1us response time, allowing the device to quickly shut down power in the event of unexpected load or supply changes. Figure 6: Linear Technology's LTC6102 makes high-side current sensing simple and straightforward. The device is configured using RSENSE and two gain resistors. By selecting RIN and ROUT, the designer can tailor power consumption, response time and input/output impedance characteristics 　　Conclusion High-side current sense amplifiers have inherent advantages when used to monitor and control current. Technological advances in areas such as battery management and motor control have led to a great demand for current sense amplifiers with higher common mode voltages, higher accuracy and higher precision. The LTC6102 pioneered this new world with an impressive set of features and excellent precision. High-side current-sense amplifiers now match the performance levels of the industry's leading precision op amps, providing designers with a simple, versatile and highly accurate device that can replace the less accurate or more complex current-sense circuits of the past.