[SI-LIST] : Slightly different signal integrity problem

Lund, Steve (SLund@tuthill.com)
Tue, 12 Jan 1999 15:38:56 -0600

Dear SI-list Members,

I have a question regarding a slightly different signal integrity problem.
Please forgive me if this is considered off-topic.

I have an application that receives optical pulses through plastic optical
fiber and directly into an infrared receiver IC for conversion back into an
electrical signal. The IC in question is an LTC1328. This chip is intended
for the standard IRDA protocols which has a certain amount of error
rejection built-in. In my application the pulses are used as received and
there is no optical "noise" due to transmission over a plastic fiber.

The problem that we have noticed is stray pulses being received as a result
of stray EM fields. We have decided to mount the receiver IC on a double
sided PC board (about 3/4" square) with ground plane on the solder side and
a metal shield on the top side. However, there is still one open issue. In
the current configuration without the shielding on a double sided board with
ground plane there appears to be noise also picked up due to disturbances on
the ground plane. The circuit can be made to trigger by touching the ground
plane with a metallic object such as a screwdriver or connecting a scope
probe ground even when connecting at the opposite corner of the current 2" x
3" PC board.

Because of this I am seeking your input as to the advisibility of a
separating the signal ground from the shield on the receiver module. The
receiver IC in an SO-8 package has a single pin for both analog and digital
ground and this same pin is also one of the photodiode inputs which may be
the path for the ground induced noise.

So I guess the question is this: Should the shield be isolated from circuit
ground and if so how? Thanks for any and all suggestions.

Steve Lund
Tuthill Transfer Systems

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