AN202 View Datasheet(PDF) - Philips Electronics

Part Name

Description

Manufacturer

AN202

Testing and specifying FAST logic

Philips Electronics 

Philips Semiconductors

Testing and specifying FAST logic

Application note

AN202

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 50Ω. 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 VCC plane goes through the center of the part too, and

connects to the VCC pin in like manner as the ground pin. See

Figure 1. As the VCC is brought on board, the VCC wire is wrapped

around a 1/2 inch ferrite core, 6-8 times, then makes connection with

the VCC plane on the top side.

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BYPASS

CAPACITORS

SOLDER AND

COPPER WIRE

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 50Ω characteristic

impedance. We recommend that the user maintain a 50Ω

environment for the input signal as close as possible to the input pin

and then terminate in 50Ω. On our jig, we terminate with a 50Ω chip

resistor. The signal is brought on board through an SMB connector

to the 50Ω 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 450Ω chip resistor

soldered to it. The other side of this resistor, R1, is soldered to a

50Ω trace on the bottom side of the board that runs to an SMB

connector on the edge of the jig. This connects to the 50Ω input of

the Sampling Oscilloscope. This 450Ω resistor in series with the

50Ω input of the scope creates a 10X divided 500Ω 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,

VS1 – VS3, by use of a DIP switch on each pin. It may also be left

open and the 50Ω 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

50Ω input of the Sampling Scope. With the designed-in flexibility of

Jumper 1 and Jumper 2, and the selectable nature of VCC and

Ground pin designations, one can configure this board for any VCC

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 VCC/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 450Ω resistor and the 50Ω of the scope

comprising the 500Ω 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.

June 1987

3

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