Zynq-7000 PCB Design Guide
32
UG933 (v1.8) November 7, 2014
Chapter 3:
Power Distribution System
PDS Measurements
Measurements can be used to determine whether a PDS is adequate. PDS noise
measurements are a unique task, and many specialized techniques have been developed.
This section describes the noise magnitude and noise spectrum measurements.
Noise Magnitude Measurement
Noise measurement must be performed with a high-bandwidth oscilloscope (minimum
3 GHz oscilloscope and 1.5 GHz probe or direct coaxial connection) on a design running
realistic test patterns. The measurement is taken at the device’s power pins or at an unused
I/O driven High or Low (referred to as a
spyhole measurement
).
V
CCINT
and V
CCAUX
can only be measured at the PCB backside vias. V
CCO
can also be
measured this way, but more accurate results are obtained by measuring static (fixed logic
level) signals at unused I/Os in the bank of interest.
When making the noise measurement on the PCB backside, the via parasitics in the path
between the measuring point and AP SoC must be considered. Any voltage drop occurring
in this path is not accounted for in the oscilloscope measurement.
PCB backside via measurements also have a potential problem: decoupling capacitors are
often mounted directly underneath the device, meaning the capacitor lands connect
directly to the V
CC
and GND vias with surface traces. These capacitors confuse the
measurement by acting like a short circuit for the high-frequency AC current. To make sure
the measurements are not shorted by the capacitors, remove the capacitor at the
measurement site (keep all others to reflect the real system behavior).
When measuring V
CCO
noise, the measurement can be taken at an I/O pin configured as a
driver to logic 1 or logic 0. In most cases, the same I/O standard should be used for this
“spyhole” as for the other signals in the bank. Measuring a static logic 0 shows the crosstalk
(via field, PCB routing, package routing) induced on the victim. Measuring a static logic 1
shows all the same crosstalk components as well as the noise present on the V
CCO
net for
the I/O bank. By subtracting (coherently in time) the noise measured on static logic 0 from
the noise measured on static logic 1, the noise on V
CCO
at the die can be viewed. For an
accurate result, the static logic 0 and static logic 1 noise must be measured at the same I/O
location. This means storing the time-domain waveform information from both logic states
and performing the subtraction operation on the two waveforms in a post-process math
computation tool such as MATLAB or Excel.
Oscilloscope Measurement Methods
There are two basic ways of using the oscilloscope to view power system noise, each for a
different purpose. The first surveys all possible noise events, while the second is useful for
focusing on individual noise sources.
