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

5 Grounding in Non-isolated Current-Sensing Applications

The input bias current constitutes a portion of the load current being measured and is supplied by the same supply. Although this current is normally negligible comparing to load current, it is nonzero. A return path must be provided for this current for the CSA to work properly. This is the fundamental reason a non-isolated CSA cannot be used in the same fashion as an isolated CSA.

Figure 5-1 Without (Left) and With (Right) Return Path for Input Bias Current

Figure 5-1 shows an application where current in the secondary coil is measured. The primary side of the transformer is driven by an AC source; the secondary side is connected to a load. An AC recirculating current flows through both the load and the shunt resistor, and develops a small signal AC voltage across the shunt resistor. This AC shunt voltage in turn creates an AC input bias current that is equal but opposite in direction on the positive and negative input pins of the CSA. It is important to distinguish between the AC input bias current and the DC input bias current which is denoted by straight arrows in the graph. The AC bias current recirculates in the loop made up of the secondary coil and the common-mode sensing circuity inside the CSA. The DC bias current flows into the CSA and exits through the ground pin. On the left of Figure 5-1 is a setup where no return path is provided for the input bias current. Because the secondary coil is not connected to the same ground, the CSA is not able to draw DC current from the input pins, and it is effectively isolated from the secondary coil. The right side of the image is a setup where the secondary coil is connected to the same ground as the CSA, thereby completing the current return path.

If the shunt voltage is small, the AC input bias current approaches zero. Such scenario is depicted in Figure 5-2, where the shunt resistor is removed. The CSA is chosen to be bidirectional and the reference pin is set to 3.3 V; the power supply is set to 5 V. The absolute values for reference and supply are not critical, as long as they are allowed by the CSA, and they enable free movement of the output voltage under input disturbance. This experiment demonstrates the effect of return path for the DC bias current.

Figure 5-2 Experimental Setup

The secondary coil output is presented to the CSA as common-mode input voltage, as shown in Figure 5-3 (channel 2, red). The CSA output is measured by channel 1 (yellow). Because the input pins are shorted together, ideally the CSA output should be stable and equal to the 3.3-V reference voltage. Instead severe distortion up to a few hundred millivolts is observed. Such distortion will be a problem for accurate measurements.

Figure 5-3 Output Waveform With Floating (Left) and Grounded (Right) Input

It is worth noting that even though the secondary coil output voltage is several hundred volts peak to peak, the CSA sustains no damage. This is because there is no current flowing into the CSA. In this case, isolation saves the CSA from physical destruction.

To prevent the distortion, a return path must be provided for the DC input bias current. The simplest approach is to ground the end of the coil where the shunt resistor is located, shown on the left side of Figure 5-4. This effectively turns the topology into a low-side sensing, where the CSA has a defined working common-mode voltage that is zero volts and within its specified range.

Figure 5-4 Grounded Secondary Coil for Low-Side (Left) or High-Side (Right) Current Sensing

Because most CSA have asymmetrical input common-mode input range, such as –4 V to 80 V, a high-side configuration may not be feasible for the circuit in Figure 5-4. Because the common-mode voltage detected by the CSA is the full-scale output of the secondary coil, and it may exceed the common-mode input range. Damage to the CSA is possible now that the current has a complete return path.

Now examine an example where a CSA is being used in AC-coupled configuration. As shown in Figure 5-5, the CSA input is coupled to the differential input voltage through a pair of capacitors. No external DC path for the input bias current is provided.

Figure 5-5 AC-Coupled CSA

Figure 5-6 shows a few screen shots of the output. It is evident that the DC level is random and the output is unpredictable. A pair of resistors to ground should provide the DC path for the input bias current. However, due to the relatively large input bias current, resistors of significant value may cause large offset, which may render the solution unviable.

Figure 5-6 Unpredictable Output of AC-Coupled CSA