Datasheet

SCBA015A
7–25
Fast GTLP Backplanes With the GTLPH1655
Since the bypass capacitor should have the lowest possible inductance, it is recommended that
ceramic capacitors in surface-mount packages be used. The value of capacitance can be
calculated from Equation 6.
C  I
Dt
DU
I = 50 mA For bidirectional lines with a termination resistor at both ends of each line, a
maximum of one-half the output current of an SN74GTLPH1655 (I
O
= 100 mA)
can flow through one of the two termination resistors.
∆U = 10 mV In this example, the collapse of the termination voltage V
TT
must not exceed
10 mV.
∆t = 4 ns The collapse of the termination voltage V
TT
should be postponed for at least 4 ns.
C  I
D t
D U
 50 mA 
4ns
10 mV
 20 nF
If 82-nF ceramic capacitors are used, a bypass capacitor should be provided for every four
signal lines. A proposed layout for the four termination resistors and the bypass capacitor on a
circuit board is shown in Figure 14.
Reference Voltage, V
REF
The GTL or GTLP reference voltage (V
REF
) can be derived, using a simple voltage divider and a
bypass capacitor (0.01 µF to 0.1 µF), from the termination voltage. The circuit shown in
Figure 15 has the advantage that V
REF
follows voltage fluctuations of the termination voltage,
V
TT
. In this way, the maximum possible signal-to-noise ratio (SNR) always is ensured, even with
an unstable termination voltage. Since only a very small current (maximum 10 µA) flows in the
V
REF
pin of the SN74GTLPH1655, the pin can be connected to the voltage divider without
adversely affecting the GTL/GTLP reference voltage.
Ensure that the bypass capacitor is placed as close as possible to the V
REF
pin of the
SN74GTLPH1655.
V
REF
V
TT
C
R
2R
Figure 15. Suggested Connection of V
REF
Pin
(6)
(7)