How Current Return Paths Affect Signal Integrity

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How Current Return Paths Affect Signal Integrity

Bài gửi  XREDXR on Sat Oct 30, 2010 11:15 am

As the speed of electronics increase, engineers are increasingly concerned with fundamental electronic design issues to ensure the proper operation of their systems and product designs.
Donald L. Sweeney, D.L.S. Electronic Systems Inc. -- Design News, October 28, 2010
When a signal is passed through a trace, it is safe to expect the majority of the signal will be contained in the trace. But when a signal returns from its load, there are many paths it can take. No matter what the return path is, from a non-ground grid to a ground grid or even a ground plane, the return current will take the lowest impedance path. When designing with a ground plane, the signal is expected to flow under the signal trace as a mirror current (See Figure 1), but it is probably returning at lower frequencies as shown in Figure 2. Why is this?
How Current Return Paths Affect Signal Integrity"Figure 1: Return current at 1 MHz."

Affects of Mutual Inductance
When the current is flowing in opposite directions, which the intended signal and return current are generally doing, the total inductance looking into two wires is given by Lt = L1+L2 - 2M (Ott Eq. 10 -10)1. Lt is the total inductance looking into the two wires. L1 and L2 are the inductance of the intended and return paths in the wires and M is the mutual inductance between the two wires. This analogy of using two wires is an approximation and really only works when using non-grounded wires (not a trace and a ground plane), but it helps to illustrate what is happening on the circuit board. L1 and L2 are often nearly the same value if the two paths are wires and are identical. To minimize Lt, when L1 and L2 are almost the same value, the value for M should be equal to L1 or L2. If this were to happen, Lt would be very small. The values of L1 and L2 come very close to each other in a coaxial cable where the two wires are positioned one inside the other, and the value of M is
approximately L1 or L2 , making the total inductance very low.
How Current Return Paths Affect Signal Integrity"Figure 2: Return current at 1 KHz."

Considering the various circuit elements in this example, note that the dc resistance of the U path (the yellow path in Figure 1) is 1.5 Ohms and the inductance is 10.7 nH looking into the source and ground. This inductance is low because of the mutual inductance between the trace and the ground plane.

When looking at the path the U trace uses across the ground plane as its return path from load to source, this total dc path is still 1.5 Ohms because of the small amount of dc resistance added from the ground plane of approximately 1 m Ohm. There is an inductance associated with this new path of 491nH due to the significant area added between the U and the short path from load to source.

Before moving on to the affects of different frequencies on this example, the following points highlight the main aspects of this circuit:

* 1.5 Ohms in the U trace.
* 1 m Ohm across the left end on the ground plane.
* 10.7 nH looking into the source and returning via the ground plane under the trace.
* 491 nH looking into the source and returning via the left end of the ground plane.

Different Frequencies
At 1 KHz our path is basically dc around the U trace of 1.5 Ohms and across the very low dc resistance of the ground plane of 1 m Ohm. Figure 2 illustrates how most of the current is flowing in the trace and across from the load to the source.

At 1 MHz our path is basically around the U trace of 1.5 Ohms and our return will be under the trace with its low inductance of 10.7 nH, giving a return impedance of .07 Ohms. The current will now prefer this path compared to returning through the large loop, as it did at 1 KHz, since our larger loop impedance would now be just a little over 3 Ohms because of its 491 nH (See Figure 1). Here you can see most of the current is returning as intended.

Remember, the current will always flow through the least impendence path. As a result, on a circuit board with a ground plane, an unsuspecting designer might be surprised to learn the return current at lower frequencies could take a totally unexpected path (See Figure 2). Once the engineer realizes this, he or she must also recognize that these currents can affect other currents also flowing through these unintended paths.

In addition, intended signals can also be distorted and interfered with by those other intended currents. (Author's note: I have seen interference with an analog signal at 180 Hz, where the interfering signal was 4.5 times greater than the intended signal.)
How Current Return Paths Affect Signal Integrity"Figure 3: Return current from plane to plane, no control and with control."

Click here for larger image

Layer-to-Layer Transfer
Another area where the quality of the signal might be compromised is when a signal moving across a circuit board must transfer from one layer to another using a via (see left image in Figure 3), where the return current is on the bottom of the right plane and needs to be on the top of the left plane.

In this case, the engineer must be concerned with what happens to the return current if the mirror current on one layer is not common with the mirror current on the layer the signal is now flowing above or below. This must be known to know how it will transfer from one plane to another.

If you don't make this transfer happen the way you want it to, the return current will find its own way. It might be through a decoupling cap far from the via or it might transfer as displacement current using only the layer-to-layer capacitance of the planes as seen on the left image in Figure 3. When this happens, the current is spread over a large area, greatly increasing the possibility of cross contamination with other signal currents creating a loss of signal integrity. A better way would be to add decoupling capacitors (the closer the better) near the via that the signal is passing through, as shown in the right image of Figure 3. This contains the current to the vicinity of the signal via.

Donald L. Sweeney, senior EMC engineer and president of D.L.S. Electronic Systems Inc. has been teaching for more than 30 years at the University of Wisconsin as well as at independent EMC design seminars.

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