# [SI-LIST] Re: AC Termination

• From: "Peterson, James F (FL51)" <james.f.peterson@xxxxxxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Mon, 3 Dec 2001 08:07:55 -0500

```Jim,

These are good questions. I think we have confusing sources of information.
I'm going to go out on a limb - I believe Johnson's position has a problem.
Black Magic states that the C in an AC termination scheme needs to be large
enough so that the RC time constant is sufficient to only allow the signal
to charge thru 1/2 of Vcc before switching again. This is to reduce drive
strength needs. He asks "if the driver can source the full amount of
deltaV/R then why don't we just"..remove the C and use the resistor?

I believe the problem in this position is there is a difference in the ac
drive strength and the rated dc drive strength of the driver. The ac drive
strength can typically tolerate driving a 50 ohm load and the dc drive
strength typically can not. So the C only has to block the dc portion.

Hall, Hall, McCall, states that you should size the c so that the RC time is
equal to one or two rise times. The premise of this approach is that the
edges of your signal see the resistor and the "dc" sections of your signal
see hi-z. This is qualified with the advice to simulate the termination. I
agree with this advice - ac terminations should always be verified using a
good representation of your topology and drivers.

Best regards,
Jim Peterson

-----Original Message-----
From: Jim Hanson [mailto:hansonjb@xxxxxxxxxxx]
Sent: Sunday, December 02, 2001 5:44 PM
Subject: [SI-LIST] AC Termination

In Johnson's test "High-Speed Digital Design", page 237, it states that if
the duty cycle is 1/2, then across the capacitor the voltage will be 1/2 of
delta V.

In "High-Speed Digital System Design" by Hall, Hall, Mc Call, it states on
page 35 that if an edge is impressed across the capacitor, it will charge
up
to Vs.

Johnson states that the time constant should be very large while HHMc
states
that tau should equal 1 or 2 rise times.

Also, what about the Xc of the capacitor at the clk frequency?  If Xc is
small compared to R, then won't it react as a low impedance to gnd?  And
how
is the value of C chosen to accomplish the following:

(a) provide the proper time constant and, also
(b) not have the Xc react as a low impedance to gnd?

Robert Hanson
Americom Seminars, Inc.

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