SBOA551 June   2022 INA240

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2One, Versus Two Reference Pins
  5. 3Bidirectional Current Sense Amplifier Topologies
    1. 3.1 Single-Stage Difference Amplifier
    2. 3.2 Difference Amplifier Input Followed by Noninverting Output Buffer
    3. 3.3 Voltage Feedback Multi-Stage Difference Amplifier
    4. 3.4 Single-Stage Current Feedback
    5. 3.5 Current Feedback Multi-Stage Difference Amplifier
    6. 3.6 Isolated Bidirectional Current Sensors
  6. 4Options for Driving Reference Pins and Input Referred Reference Error
  7. 5Resistor Divider as Reference
    1. 5.1 Resistor Divider and Equivalent Circuit
    2. 5.2 Reference Source Impedance Error in Difference Amplifier
    3. 5.3 Reference Source Impedance Error in Voltage Feedback Multi-Stage CSA
    4. 5.4 Reference Source Impedance Error in Current Feedback Multi-Stage CSA
    5. 5.5 Reference Source Impedance Error in Difference Amplifier with Output Buffer
  8. 6Examples
    1. 6.1 Calculating Reference Source Impedance Error in Difference Amplifier
    2. 6.2 Calculating Reference Source Impedance Error in Voltage Feedback Multi-Stage CSA
    3. 6.3 Calculating Reference Source Impedance Error in Current Feedback Multi-Stage CSA
  9. 7Summary

3.4 Single-Stage Current Feedback

Figure 3-4 shows the block diagram of the INA170. Bidirectional current measurement is accomplished by output offsetting through amplifier A2. The offset voltage level is defined by an external resistor and voltage reference. This setup permits measurement of a bidirectional shunt current while using a single supply. Applying a positive reference voltage to reference (pin 3) causes a current to flow through ROS, forcing output current to be offset from zero. The transfer function is written as Equation 4.

Equation 4. Vout_ideal=ISRSRLRG+VREFRLROS

Driving the reference pin is straightforward. A simple voltage divider will suffice due to the high input impedance of amplifier A2. A filtering cap close to the reference pin may help limit noise contribution. Resistor sizing and VREFVref voltage should fit the operating range of amplifier A2.

Figure 3-4 Single-Stage Current Feedback