As a general design practice we always put resistors (albeit 0 ohm) in series
on clock-like lines. You never know when you'll need them :-)
... Rich
On Jan 15, 1:10pm, Gary Peterson wrote:
> Subject: FCT806 Clock Driver
> Fabrizio Zanella writes...
>
> Hello SI engineers,
> I am having problems with an IDT 49FCT806BT inverting clock driver, 0.5
> micron CMOS technology. I measured the rise time on this device to be
> 800ps, and this is causing excessive reflections and false clocking. Does
> anyone know of a vendor who makes a pin compatible FCT806 clock driver,
> SSOP package with slew rate control or guaranteed rising edges around 1.5ns
> or longer? I don't need subnanosecond edges in my system, the frequency is
> below 50MHz.
> Thanks in advance for any advice.
> Hope to see many of you at Design Supercon next week in Santa Clara.
>
> Regards,
>
> Fabrizio Zanella
> Design Engineer
> ................................. Possible Solution Below...................
>
> We use IDT 807s a lot and have taken to routinely adding series dampers to
> all of the outputs to slow down the edges and control over/undershoot. I'm
> assuming you have a board in-hand since you are looking for a slower, drop-in
> replacement. We have had good luck on prototypes with gently lifting the
> output pins from their pads on the board and standing an 0805 resistor on end
> in the space between the pad and the lead. Solder the resistor in place and
> you're in business. Values between 20 ohms and 52 ohms have worked well
> depending on the loading/stubs on the output. 20 ohms should be fine for
> a single load w/ no stubs in 50 ohm transmission line.
>
> ------------
> \ ------------
> |__/ \ <-- FCT806 output lead
> | \_______
> / |_____|
> ------------ | | <-- 0805 Resistor (on end)
> |_____|
> | |
> --------------------
> | | <-- Solder pad on PWB
> ==================================================================
>
> Gary P.
> ---
> Gary D. Peterson
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>
>-- End of excerpt from Gary Peterson