Signal integrity (SI) power integrity (PI) learning notes (XXIV) differential pair and differential impedance (IV)

Differential pair and differential impedance （ Four ）

1. When any transient common mode signal propagates along a differential pair , Will feel a high impedance at the end and return to the source . Even if there is a jumper between two signal lines 100Ω The resistor , Since the common mode signal has the same voltage on the two signal lines , It will not feel the resistor , Affected by the drive impedance , Any common mode signal generated will oscillate back and forth , Ringing effect occurs . If the difference pair is asymmetric , It will make some differential signals become common mode signals , However, this asymmetry is encountered again when the common mode signal oscillates back and forth , Some common mode signals may be converted back to differential signals , This will cause differential noise .

2. Terminating the common touch signal is not eliminating the common touch signal , It just prevents the common mode signal from oscillating back and forth between circuits . If the common mode signal causes electromagnetic interference , Then terminating the common mode signal does slightly reduce electromagnetic interference . But the important thing is to carry out innovative design . To eliminate the common mode signal source .
Termination common touch ： Connect a resistor between each signal line and the return path , When these two resistors are connected in parallel, the impedance value is equal to the common mode impedance .

3. Terminate differential mode and common mode impedance
（1）π Type topology ：
The equivalent resistance of the common mode signal is equal to the common mode impedance , The equivalent resistance of the differential signal is equal to the differential impedance . The equivalent resistance of the common mode signal is two resistors R2 Parallel connection of .

The equivalent resistance of the differential signal is two resistors R2 Connect in series and then connect with R1 parallel connection .

（2）T Type topology ：
The equivalent resistance of the differential signal is two resistors R1 Series connection of ,

The equivalent resistance of the common mode signal is two resistors R1 Parallel and then R2 Series connection ：

4. In the realization of π Type termination or T Type termination , The most important factor to consider is the potential DC load of the drive . The smaller the even mode impedance , The greater the current flowing from the driver . A typical differential drive cannot control the low DC resistance on the low voltage side . So it is not practical to terminate the common touch signal .
（1） Another alternative to terminating the differential and common mode signals is at T A DC isolating capacitor is added in the type a termination , Resistance and basic T The resistance value in type a structure is equal . When selecting capacitors , Ensure that the time constant sensed by the common touch signal （RC） It is much larger than the value of the period corresponding to the lowest frequency component in the signal , This ensures that the impedance of the capacitor is less than that of the resistor in the lowest frequency component of the signal .

（2） The other is for in-chip termination schemes where each signal line is connected with a separate Vtt Termination between power supplies .

5. It is very important to maintain the quality of the differential signal , Use the following guidelines when implementing ：
（1） Use controllable differential impedance ;
（2） Minimize the mutation of the difference pair ;
（3） Terminate the differential signal at the far end ;
（4） Routing symmetry .
Differential line distortion ：
（1） The asymmetry between the wires and the dislocation between the drivers will also cause the distortion of the differential signal ;
（2） The misalignment of the two differential drivers during switching will distort the differential signal ;
（3） If certain factors , Affects one line of the difference pair without affecting the other , The differential signal will be distorted .

6. In a microstrip line or stripline , Distortion due to asymmetric misalignment occurs independently , It has nothing to do with the coupling between lines . No matter the difference pair is uncoupled between the two lines , The distortion still appears .

7. Dislocation and distortion can have some adverse effects . Any asymmetry factor will transform the differential signal into a common mode signal . Any asymmetry factor will transform the differential signal into a common mode signal , This includes crosstalk , Drive misaligned , Line length deviation and asymmetric load, etc . An important purpose of keeping errors to a minimum is to minimize the conversion from differential signals to common mode signals .

8. A small misalignment may not affect the quality of the differential signal , But it can significantly affect the common touch signal . The differential signal component is terminated by a resistor at the far end , But the common mode signal component feels an open circuit at the far end , And will return to the low impedance source , This will produce a ringing . Even differential and common mode signals are terminated , Due to various asymmetries , Common touch signal will still be generated . With the increasing dislocation between the two signal lines , The amplitude of the common touch signal will increase accordingly .

Small driver misalignment can produce obvious common touch signals , This requires minimizing asymmetries , In order to minimize the generation of common mode signals , The path must be made as symmetrical as possible .

9. in fact , The return path of common mode signal of unshielded twisted pair is ground , floor , Or other adjacent conductor signal lines, the coupling between the nearest conductor surface is usually less than that between adjacent signal paths , So the common mode impedance is usually very high , About several hundred euros .
The impedance of the common mode signal on the circuit board and in the twisted pair may not match , The connection between the two return paths is the path that any current can find . At high frequencies , The return path is mainly connected by the ground of the circuit board - Bracket - The stray capacitance between the backplanes constitutes .

10. In the U.S. ,FCC The radiation field grade standards for two types of products are formulated .A Class a products are used in industry or manufacturing ,B This product is used in the home or office .B The radiation field intensity ratio of this kind of product A Smaller for class products .

The common mode current in unshielded twisted pair is easy to generate radiation , Even though there is little common touch current in unshielded twisted pair , The product will not pass the test and certification of electromagnetic interference . If the common mode current value is greater than 3uA, Can't pass FCC Of B Class authentication .