SBOA536 December   2021 INA240

 

  1.   Trademarks
  2. 1Introduction
  3. 2Grounding in DC Circuits
  4. 3Grounding in Isolated Current Sensing Applications
  5. 4Working Principle of Non-isolated Current Sense Amplifiers
    1. 4.1 Single or Multi-stage Difference Amplifier
    2. 4.2 Current Feedback
    3. 4.3 Switched Capacitor
    4. 4.4 Input Stage and Input Bias Current
  6. 5Grounding in Non-isolated Current-Sensing Applications
  7. 6Level Shifting for High-Voltage Current-Sensing Applications
  8. 7Grounding in Motor Current-Sensing Applications
    1. 7.1 Common-Mode Voltage of Motor Current Sense Amplifiers
    2. 7.2 Directionality of Motor Current-Sense Amplifiers
    3. 7.3 PCB Design for High-Performance Motor Drive
  9. 8Summary
  10. 9References

3 Grounding in Isolated Current Sensing Applications

Three technologies stand out as integrated, isolated current sensors. They are shunt-based isolation amplifiers or modulators, fluxgate sensors, and in-package Hall-effect sensors.

Shunt-based isolation current sensing employs isolated amplifiers or modulators. Figure 3-1 illustrates an example. The AMC1300 input is similar to a non-isolated current-sense amplifier in that the small differential voltage, which rides on top of a large common-mode voltage, is extracted and amplified. The output is separated from the input circuitry by an isolation barrier that is highly resistant to magnetic interference. The input and output operate in different power domains, each with its own power supply and ground. From the point of view of downstream measuring circuit, the load current is completely isolated, and no return path is needed for the input bias current.

The isolation between the high and low sides are evident in the physical layout. The two sides are separated by an area void of any conductive material. There is no common ground connection between the two sides.

Figure 3-1 Isolated Amplifier AMC1300 Block Diagram and PCB Layout

Magnetic sensors work without making physical contact between the sensor IC and the current it is measuring, thanks to their inherent isolation through magnetic fields. A galvanically-isolated barrier is possible that can withstand very high common-mode voltages.

Figure 3-2 Hall-Effect Current Sensor TMCS1100 Block Diagram and PCB Layout

An example of in-package Hall sensor is the TMCS1100 family, shown in Figure 3-2. Within the device, the high-voltage side load current passes through the low-ohmic lead frame path. No external components, isolated supplies, or control signals are required on the high-voltage side. At the low-voltage side, the magnetic field generated by the input current is sensed by a Hall sensor and amplified by a precision signal chain.

Also shown in Figure 3-2 is a recommended layout of the TMCS1100. The layout is optimized for thermal performance while at the same time minimizes stray magnetic field interference. A large creepage area is visible between the high side and low side. There is no common connection between the two sides and they are physically isolated.