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Understanding differential pairs and differential signals: just don't think "differential mode." It will help keep differential signals separate from odd mode impedance, eliminating the confusion.

QUESTION FROM KELLY in Boston: I'm confused about differential signals. When the signal in the two channels is a 0 to 1 V signal in one and a 1 V to 0 signal in the other, everyone calls this a differential signal. Then, when it goes down a differential pair, everyone says it's propagating in the differential mode and sees the differential impedance. But, isn't it also in the odd mode? And then why is the odd mode impedance different from the differential mode impedance?

Answer: "Everyone" is not telling the truth, and the lies that we propagate in the industry are the source of much confusion about differential pairs and differential signals. Often times the words we use influence our intuition or how we think about a problem. If you really want to understand differential pairs and differential signals, you should never use the words "differential mode."

Instead, always think of differential and common signals, and even and odd modes. A differential pair is any two single ended transmission lines, with some degree of coupling. Each line in the pair can have a single ended signal on it, V1 and V2, each measured as the voltage between the signal trace and its adjacent return path.

These two signals, in principle, can be any value with any pattern. For any arbitrary pair of signals, we always define the differential component of the signal as the difference voltage between the two signals. The common signal component is the average voltage.

When you apply a 0 to 1 V signal on one line and a 1 V to 0 signal on the other line, you are not really applying a differential signal. You are applying a differential and a common signal.

In LVDS signal levels, the outputs switch from about 1.05 V to 1.35 V. We call this a differential signal, but we are really lying. There is a differential signal component in this, of 0.6 V, but there is also a strong common signal component of 1.2 V. Of course, in principle, under ideal conditions, the common signal component is constant and does not contribute to any signal integrity issues. But in practice this is not always the case.

FIGURE 1 shows the output voltages from the two channels of an LVDS driver and the differential and common components in this signal. It is the differential signal component that carries information and to which the receiver is sensitive.

[FIGURE 1 OMITTED]

So far, the discussion has been about the signals. We can always take any signal on both lines and describe it as a combination of a pure differential signal and a pure common signal component.

When it comes to describing the properties of the interconnect, we use modes. With two transmission lines, there are two modes, the odd mode and the even mode. Modes describe a particular property of the transmission line. You don't have to have a signal on the line for the pair of transmission lines to have a mode, just like you don't have to have a voltage on a capacitor for it to have capacitance.

A mode of a transmission line pair defines a number of properties of the pair of lines. It defines a voltage pattern that would propagate undistorted down the line, an impedance of each line when in that mode and a speed of the signal that propagates in that mode.

The odd mode impedance is the characteristic impedance of either line in the pair, when the pair is driven in the odd mode. Due to the way the electric fields interact, the odd mode impedance is always lower than the even mode impedance. The difference in impedance between the modes depends on coupling between the two lines. A lot of coupling and the difference between the odd and even mode impedance is large. No coupling and the difference is nearly zero.

When a pure differential signal propagates on a symmetrical differential pair; like an edge-coupled microstrip or edge-coupled stripline, the mode the signal propagates in is the odd mode of the transmission line. The impedance the differential signal sees, the differential impedance, is twice the odd mode impedance.

Throughout this discussion, at no point did we invoke differential mode or common mode. We described the signals as differential and common. Each type of signal sees differential impedance or common impedance. Each line has odd and even mode impedance. Odd mode impedance is related to differential impedance, with the differential impedance equal to twice the odd mode impedance, but they are not the same.

Forgetting the words "differential mode" will help keep differential signals separate from odd mode impedance and eliminate the confusion.

This and other topics are covered in the public classes Eric teaches. Check his web site for the schedule: BeTheSignal.com. Send questions to DoctorIsIn@BeTheSignal.com.

DR. ERIC BOGATIN is president of Bogatin Enterprises.
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Title Annotation:THE SIGNAL DOCTOR
Author:Bogatin, Eric
Publication:Printed Circuit Design & Manufacture
Date:May 1, 2007
Words:819
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