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

3.1 Single-Stage Difference Amplifier

Based on conventional difference amplifiers, this topology achieves input and output functionalities in a single stage. Compared with a conventional difference amplifiers, the most significant distinction is the capability to withstand common-mode input voltage that is significantly higher than device supply voltage while maintaining high gain. For example, the working input common-mode voltage could be 28 V while the device supply is only 3.3 V. Shown in Figure 3-1 is the simplified diagram of such a device. In this diagram, Ri and Rf represent the ideal values of the input and feedback resistors respectively.

The resistor network must be closely matched to achieve good Common Mode Rejection Ratio (CMRR) and Gain Error (GE) performance. The resistor network is often trimmed for matching. Although sometimes it is possible to meet relaxed design goals by optimizing circuit design and layout without trim.

When driving the reference pin of a single-stage difference amplifier, ideally the voltage source should have zero impedance to maintain the balance of the resistor network.
Figure 3-1 Single-stage Difference Amplifier

The difference amplifier transfer function is the familiar equation:

Equation 1. V o u t _ i d e a l = R f R i V d i f f + V r e f _ x