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| 型號： | AN202 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | Testing and specifying FAST logic |
| 中文描述： | 快速測試，并指定邏輯 |
| 文件頁數(shù)： | 3/5頁 |
| 文件大?。?/td> | 72K |
| 代理商： | AN202 |
Philips Semiconductors
Application note
AN202
Testing and specifying FAST logic
June 1987
3
GROUNDING
One of the biggest contributors to waveform degradation is improper
grounding. In reference to the test jig, the grounding is best done
with one or more large ground planes that are directly connected to
the ground pin of the test socket. The Philips Semiconductors AC
Test Jigs, both DIP and SO styles, are constructed as a four layer
PC board with the 2 internal layers as ground planes. Ground planes
are also interdigitated between all signal lines to decrease crosstalk.
There are holes drilled in these and they are plated through to
connect with the internal 2 layers and the top and bottom layers.
See Figure 3 to see the interdigitated ground planes on the PCB
layout of the SO jig. This grounding scheme has been used with
great success in 10k and 100k ECL fixturing. The board is laid out
so that the characteristic impedance of the signal lines is 50
. This
is done by using industry standard stripline techniques. The ground
plane also passes down through the center of the part on the bottom
side of the board and the ground pin is soldered to it using copper
wire to connect the pin and the ground plane. On the top side of the
board, the V
CC
plane goes through the center of the part too, and
connects to the V
CC
pin in like manner as the ground pin. See
Figure 1. As the V
CC
is brought on board, the V
CC
wire is wrapped
around a
1
/
2
inch ferrite core, 6-8 times, then makes connection with
the V
CC
plane on the top side.
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SOLDER AND COPPER WIRE
DUT
V
CC
PIN
GROUND
PIN
SOLDER AND
COPPER WIRE
BYPASS
CAPACITORS
V
CC
PLANE
SF01268
Figure 1.
INTERCONNECTS
The next concern is getting the input signal to the part and the
output signal to the measurement system. As stated before, the
Philips Semiconductors jig is laid out for a 50
characteristic
impedance. We recommend that the user maintain a 50
environment for the input signal as close as possible to the input pin
and then terminate in 50
. On our jig, we terminate with a 50
chip
resistor. The signal is brought on board through an SMB connector
to the 50
trace on the top side of the board. The signal is
terminated by the chip resistor, R3, see Figure 2a and 2b. the signal
proceeds to the DUT pin, a distance of about 0.5 inches, through
Jumper 1 (in the Input Only position), and the rest of the trace. The
same pin on the opposite side of the board has a 450
chip resistor
soldered to it. The other side of this resistor, R1, is soldered to a
50
trace on the bottom side of the board that runs to an SMB
connector on the edge of the jig. This connects to the 50
input of
the Sampling Oscilloscope. This 450
resistor in series with the
50
input of the scope creates a 10X divided 500
probe for the
scope and provides impedance matching for the scope. See Figure
2b. This circuit also doubles as the resistive portion of the FAST AC
Output Load and thereby allows the output to be sensed in the same
fashion. When the input is not used for a signal or generator input,
the line may be switched to one of three voltage sources,
V
S
1 – V
S
3, by use of a DIP switch on each pin. It may also be left
open and the 50
pull-down resistor that is used for an input
terminator pulls the line to ground and can be used as a hard low
level. See Figure 2b. This scheme eliminates excessive cabling to
each input to provide static input levels and thereby reduces
parasitic inductances and cross-talk. It also eliminates the need for
bulky and sometimes unreliable high impedance probes by using the
50
input of the Sampling Scope. With the designed-in flexibility of
Jumper 1 and Jumper 2, and the selectable nature of V
CC
and
Ground pin designations, one can configure this board for any V
CC
and Ground pin designations, select which pins are outputs or inputs
and even provide the proper pull-up for 3-State outputs. This makes
the board entirely universal for designated V
CC
/Ground
configurations. To explain this, the output of the device is connected
to its capacitive load by Jumper 1 in the Output Only position. This
means that no pin can be both output and input at the same time,
but can be either. Jumper 2 allows an output to be connected to the
3-State pull-up resistor, R2, and have that connected to the needed
7V. See Figure 2a and 2b. The scope is connected in the same way
as the input, with the 450
resistor and the 50
of the scope
comprising the 500
needed for the FAST load. One other
consideration exists. In small part quantity testing, the elimination of
a socket is very desirable, using inserted pins that are flush with the
jig. In larger quantity testing, sockets may be needed, however. If
this is the case, some degradation in the performance will occur due
to the increased lead inductance for each pin, which is observable,
and the addition of group delay through the socket may alter or
affect the readings obtained.
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