SBOA582 November   2023 OPA2387 , OPA387 , OPA4387 , RES11A , RES11A-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction to Differential Signaling
  5. 2Common-Mode Rejection Ratio in Difference Amplifier Circuits
  6. 3Improving CMRR with Precision Matched Resistor Divider Pairs, RES11A-Q1
  7. 4Derive Differential and Common-Mode Gain, Difference Amplifier
  8. 5Derive CMRR for Discrete Resistor Tolerance
  9. 6Derive CMRR for Matched Ratio Tolerance
  10. 7Summary
  11. 8References

1 Introduction to Differential Signaling

A differential voltage signal is defined as the difference in voltage between two signal-carrying traces. Often these signals are balanced, meaning that the two signal voltages are equal in magnitude but of opposite polarity, as shown in Figure 1-1. The differential voltage (VDiff) can be resolved by subtracting the positive signal voltage from the negative signal voltage as shown in Equation 1.

Equation 1. V D i f f = V S i g+ -   V S i g-

This subtraction function is one of the main benefits of differential signaling, as the subtraction function removes unwanted common-mode voltages from the signal chain. A common-mode voltage (VCM) is a voltage that is present on both signal traces, that is equal in both magnitude and polarity. In differential circuits, VCM is defined as the average of the two signal voltages, Equation 2.

Equation 2. V C M = V S i g+ +   V S i g- 2

These unwanted common-mode voltages can be in the form of noise, or as a DC bias from the previous signal stage. Unlike the signal voltages which are of opposite polarity, these common-mode voltages are rejected by the subtraction function defined in Equation 1. The following figures illustrate this principle for both AC and DC common-mode voltages.

GUID-20231012-SS0I-G1D0-RCV6-T87BTBWSPBF5-low.svgFigure 1-1 AC Differential Signal with DC Common-Mode Voltage
GUID-20231012-SS0I-9ZDB-CNV0-3HLRDZDZN2HP-low.svgFigure 1-2 DC Differential Signal with AC Common-Mode Voltage

In practical electrical circuits the common-mode voltage is not perfectly removed from the differential signal, however it can be significantly attenuated. The magnitude of the common-mode attenuation is determined by various non-idealities in the circuit, as discussed in the following sections.