What Is a Current Sense Amplifier?
A current sense amplifier is a device element that detects the value of the current flowing in an electric circuit. The current can be calculated by measuring the voltage at both ends of a shunt resistor used to detect the current. Since the voltage drop across both ends of the shunt resistor is amplified by an operational amplifier, even a slight potential difference can be seen as a large voltage. In addition, since a fixed gain is configured from the built-in resistor, it is not easily affected by temperature changes or external influences, enabling highly accurate measurements.
Uses of Current Sense Amplifiers
Current sensing is essential for safe and efficient system operation in many machines. Current sense amplifiers are used to measure the currents flowing in various machines. An example is the current flowing in electric vehicles. Standard current sensing in electric vehicles with high voltages is complex but can be measured using current sense amplifiers. Operational amplifiers usually cannot accept input voltages above the supply voltage, but current sense amplifiers can support input voltages due to their sizeable common-mode voltage range.
Principles of Current Sense Amplifiers
Current sense amplifiers are excellent device elements that measure currents flowing in various machines. Here we introduce its principle and features.
Current sense amplifiers measure the value of the flowing current by estimating the potential difference between resistors through which the current flows. This resistance is called a shunt resistor and is calculated based on a simple principle based on Ohm’s law. Even if the potential difference between the two ends of the shunt resistor is slight, it can be sufficiently measured by amplifying it with an operational amplifier. The characteristics of the shunt resistor vary depending on the position where it is placed.
- Low-side amplifier – A shunt resistor is placed between the load and ground in a circuit. The most significant advantage of low-side current detection is that the standard code voltage is close to zero. This allows for simple implementation without the need for complex circuit design.
- High-side amplifier – A shunt resistor is placed between the circuit’s power supply and the load. The advantage of detecting current on the high side is that it is not affected by the ground.
How to Determine the Current Sensing Resistor
One of the most critical decisions in current sensing is the selection of a current sense resistor (shunt resistor). The resistance value and wattage should be determined to obtain the desired maximum operating voltage from the maximum current that can generally be expected. When necessary, it is also essential to consider the resistor’s power dissipation before selecting the resistor’s value.
Once the current-sense resistance and wattage are determined, consider the effect of resistor tolerance. Tolerances directly affect the accuracy of the voltage and current measurements that can be taken. Also often overlooked is the temperature drift coefficient of the resistor tolerance.
Also, care must be taken in how the printed pattern is drawn from the shunt resistor to the current sense amplifiers. Since shunt resistors are usually low resistance, depending on how the pattern is drawn, the voltage drop due to the copper resistance of the pattern may also be detected, resulting in a significant error. The pattern should be separated from the shunt resistor and routed to the current sense amplifiers. In addition, the current path and voltage detection should be routed separately to eliminate errors caused by the resistance of the current path.
How to Select a Current Sense Amplifier for Current Detection
Two main types of current sense amplifiers are high-side amplifiers and low-side amplifiers. Each has its advantages and disadvantages. High-side amplifiers measure the current between the power supply and the load, so a DC voltage higher than the power supply voltage may be applied. Low-side amplifiers, in contrast, measure the current between the load and ground, so a voltage close to ground level is applied to the input pins.
When selecting current sense amplifiers, determine whether high-side or low-side current sensing will be used. For high-side current sensing, select current sense amplifiers based on the maximum advisory mode voltage required for the application and the supply voltage used. For low-side detection, select a current sense amplifier that operates at an advisory mode voltage close to 0V.
In addition, consider the direction of current detection in the system. Is recent detection limited to one direction, or does the design need to consider bi-directionality? Considering offset and gain accuracy errors based on the shunt resistance and measured current range is also essential.