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

2 One, Versus Two Reference Pins

A CSA measures target current by deriving a small differential signal (voltage or current) that is proportional to the magnitude of the current. Signal conditioning circuity then turns this small differential signal into a stable and noise-free output for further processing down the signal chain. For shunt-based current sensors, either non-isolated or isolated, the input differential signal is created by inserting a shunt resistor in the path of the target current. Magnetic sensors work without making physical contact between the sensor IC and the target current. For example, the magnetic field generated by the load current can be sensed by a Hall sensor, which is then conditioned and amplified by a low-noise amplifier.

Figure 2-1 Simplified Block Diagram of a Bidirectional Current Sense Amplifier

Figure 2-1 shows a block diagram of a typical bidirectional CSA with a single reference pin. The input stage is responsible for extracting the differential input signal while rejecting the typically very high input common-mode voltage. The input stage can take on many forms, including but not limited to, voltage feedback, current feedback, and isolated technology. The output stage takes care of output drive capabilities to interface effectively with downstream circuitry.

The output stage is typically a classic difference amplifier. To enable bidirectional measurement capability, the output stage is equipped with a reference pin. By providing a positive reference voltage to the reference pin, the output is level shifted to a desired quiescent output voltage. Typically, when a positive differential input is applied, the output moves away from the quiescent voltage, toward the supply. Conversely, when a negative differential input is applied, the output moves away from the quiescent voltage toward ground.

Matching of the resistor network is important. One of the parameters that reflect how well the resistors match is Reference Voltage Rejection Ratio (RVRR). This parameter measures net change (relative to Vref) in output voltage for a given amount of change in reference voltage. If RVRR is listed in the data sheet and is input referred, the device gain should be used as a multiplier in calculating the corresponding change in output.

Figure 2-2 Bidirectional CSA With Two Reference Pins

Some bidirectional devices come with two reference pins which are connected internally to form a voltage divider. Figure 2-2 shows such an arrangement.

As an example, for INA240, the reference divider is made up of two equal-value resistors. Real-world differences directly influence the reference voltage. For this reason, the divider accuracy is specified in the data sheet, However, if the two reference pins are shorted together and driven with a voltage source, then the divider function is not used. The divider accuracy specification is not a concern in this situation.

A common scheme of creating a reference voltage, called splitting the supply, is shown in Figure 2-3. One of the reference pins is connected to the device power supply, while the other connected to ground. This results in a reference voltage that is half of the supply. In similar fashion, this scheme can be used to create customized references, with voltage rails at different potentials.

Figure 2-3 Reference by Splitting the Supply

The two-pin arrangement brings flexibility without incurring additional error compared with external resistor dividers. When the two reference pins are shorted together, they function exactly the same as a single pin and can be treated as such.