Lesson 2 – DUT Board Design Considerations
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on the digital ground. Then connect the ground lines to the digital ground
plane near as to the device as possible.
WGF Grounding via Coaxial Cable
Shielding the Signals
The safest way to transport signals between analog modules and the DUT
is via coaxial cables. The typical impedance is 50Ohm (except for video
signals, which usually use 75Ohm).
The shields of digital signal cables must be connected to digital ground,
and the shields of analog signal cables to analog ground.
To avoid reflections, it is essential for all high frequency analog signals to
match the impedance of the signal source. Low voltage ECL signals, too,
should only be transported by impedance matching coaxial cables.
The cable shield is normally connected to the respective analog or digital
ground. For the DPS force and sense lines, the cable shield should be
connected to guard connectors.
Notice that if you connect a coaxial cable to a high impedance input, the
cable has a capacitance (depending on its dimensions, typically 100 pF/m).
In high frequency applications, you may need to ensure that the signal
source can drive that capacitive load.
If the requirements are not that stringent, you may also consider using
twisted-pair cables or a printed circuit stripline approach (implemented by
a broad ground conductor running physically just underneath the signal
line).
It is always a good idea to mount and mark some test pins on the DUT
board where the probe of an external instrument or oscilloscope can be
attached. This will ease troubleshooting.
Filtering Supply and DC Voltage
Real world power supplies usually have a rather high impedance for high
frequency load changes. That means a sudden current change of the DUT
will cause a voltage spike (or drop) at the DUT supply pin.
Analog
Module
signal
ground
4.1G AWG
signal
ground
Digital
Channels
signal
ground
ground
Pogo Pad
50 ohm
coaxial cable
DGND
AGND
DUT
Connect ground planes
close to the DUT
Connect the shield of the
cable to the digital ground
plane close to the DUT
Digital Ground Plane
Analog Ground Plane
DUT Board
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