Digital Signal Processing Reference
In-Depth Information
This analysis suggests that to properly manage the return currents it is not
necessary to decouple the I/O supply. In fact, to achieve the proper trace imped-
ance, enough capacitance must be placed between the power and ground planes so
that the pair has an impedance very much less than the parallel impedance of all
the traces sharing the return. For instance, across frequency the decoupling should
have an impedance well under 0.5
Ω
if ten 50
Ω
traces were routed as in Figure 6.6.
This is explored in the Problems.
6.6.3 How Does the Return Current Flow When the Power Supply Is Not the
Signaling Voltage?
In Figure 6.8 we see a 50
signal trace sandwiched between a ground plane and a
power plane. In this situation the power plane does not connect to the pins on the
ASIC powering the I/O drivers. Instead, the stripline is formed with a plane carrying
an “unrelated voltage,” such as the power supply for the ASIC's internal logic. For
instance, the internal ASIC logic supply might be 1.8V and the I/O supply is 3.3V.
Forming the stripline with the 1.8-V internal logic plane causes the capaci-
tors (and the inductors, not shown) to be created between that power plane and
ground. Once again, we will assume that the trace is perfectly centered between the
two planes so that the capacitance to each is identical.
Figure 6.8 illustrates that, as in the previous case, the launched current evenly
divides between the two return planes. However, in this case, to discharge the up-
per capacitors, the 15 mA flowing in the power plane must flow through both
power systems before it can make its way back to the driver.
In general, the power supplies are resistive, and for frequencies above a few
tens of hertz, they are also inductive. The inductance prevents them from being an
effective path for high-frequency return current. In practice, the current mostly re-
turns by way of the circuit board's interlayer and decoupling capacitors. Interlayer
capacitance is the parallel plate capacitance naturally formed between a power and
ground plane. Each of the planes forms one of the capacitor plates, and the lami-
nate is the dielectric.
Ω
15 mA
Power plane
15 mA
30 mA
I/O
supply
Unrelated
suppl
y
I
= 30 mA
i
=
15 m
A
50
signal
trace
Ω
V
i
= 1.5V
15 mA
30 mA
15 mA
15 mA
Ground plane
Figure 6.8
Return path current when stripline is formed when using a power plane not connected
to the I/O supply. Adequate decoupling capacitance is necessary to properly steer the return current.
