This two-part series will cover the easily overlooked details of current sensing. This article, Part 1, will discuss the general setup, selection, and implementation of current sense resistors. Part 2 will discuss related circuits such as key analog front ends (AFEs) and instrumentation amplifiers.
Part 1 - Current Sense Resistors
Basics of Current Measurement
Current is one of the most common parameters used to evaluate, control, and diagnose the operating effectiveness of electronic systems. Because current measurement is such a common measurement, designers tend to run into trouble if they ignore the nuances of accurate current measurement.
The most common sensing elements used to sense current are low-value precision resistors placed in the current path. This resistor, often called a shunt, creates a voltage across it proportional to the current through it. Since the shunt resistor should not significantly affect the current flow, it is usually very small, on the order of milliohms or less than a milliohm (mΩ). The voltage developed across the shunt resistor is therefore also very small and usually needs to be amplified before being converted by the ADC.
Combining these factors, a common signal chain configuration for current monitoring involves an analog front end that amplifies the voltage across the shunt resistor, an ADC that converts the amplified voltage to a digital representation, and a system controller (Figure 1)
Figure 1: The easiest way to measure current is to use a shunt resistor (far left), where the voltage developed across the resistor is proportional to the current flowing through it. To use the full ADC measurement range, the analog front end (AFE) amplifies the low voltage across the shunt resistor. (Image source: Texas Instruments)
AFEs are typically implemented using op amps or dedicated current sense amplifiers to convert the small differential voltage developed across a shunt resistor into a larger output voltage for use with the full ADC measurement range. The ADC can be a stand-alone device or an on-chip module within a microcontroller or system-on-chip (SoC) that digitizes the voltage signal and provides the resulting information to the control processor. System controllers use digitized measurements of current to optimize system performance or implement safety protocols to prevent damage to the system when current exceeds preset limits.
In the link that converts current to voltage, resistors act as sensor elements, and any of their physical properties (resistance, tolerance, power capacity, thermal coefficient, and thermoelectric potential) affect accuracy. Therefore, choosing the right shunt resistor is critical for optimizing current measurements.
The shunt resistance value and corresponding voltage developed across the shunt resistor can disturb the system. For example, a shunt resistor with too much resistance can reduce the voltage used to drive the load and cause unnecessary losses.
For example, when measuring current in a motor winding, a reduced voltage reduces the electrical energy available to the motor, affecting its energy efficiency and/or torque. In addition, large currents (tens or hundreds of amps) flowing through the shunt resistor will cause the resistor to consume a lot of power and dissipate waste heat, making the measurement less accurate and less efficient. For the above reasons, the shunt resistor should be as small as possible.
Choosing a Shunt Resistor for Measuring Current