Datasheet
SCBA015A
7–36
Fast GTLP Backplanes With the GTLPH1655
Measurements on GTLPH1655 Test Board
A GTLPH1655 test board has been constructed to examine the characteristics of the
SN74GTLPH1655 in a practical application. The principle of this board is shown in Figure 28.
I = 40 cm
I = 20 cm
Backplane Bus, Z
O
= 25 Ω, τ = 16.25 ns/m
MidpointSN74GTLPH1655 SN74GTLPH1655
Figure 28. Principle of Construction of GTLP Bus on GTLPH1655 Test Board
This bus consists of a straight connecting line, 40 cm long, between two SN74GTLPH1655
devices.
In practice, a backplane wiring system provides the option for multiple plug-in modules. The bus
impedance is reduced as a result of the additional input capacitances of the modules that are
connected to it (see Figure 2). This effect can be approximated by connecting capacitors
between the bus line and ground at intervals of 2 cm.
Both sides of the bus are provided with termination resistors that are connected to V
TT
. The
termination voltage is set at 1.5 V. A 1-V reference voltage (V
REF
) was chosen. In this way,
GTLP signals are transmitted on the bus.
The termination resistors for this setup were chosen to match the line impedance, which, for a
fully loaded backplane, results in Z
O
= 25 Ω. This case provides optimum line termination, with a
reflection factor very close to ρ = 0.
For the measurements, the slew rate of the GTLP output stage was varied by means of the
edge-rate control input ERC. The measurements were carried out under two different bus
conditions: a fully loaded bus (with distributed capacitors) and the bus unloaded (without
capacitors). The clock frequencies used were 10 MHz, 50 MHz, and 160 MHz, the last being
close to the maximum value of 175 MHz specified in the data sheet.
For the case of a fully loaded bus, the result is a line impedance of about 25 Ω, and a delay time
on the line of about 7 ns.
With the bus unloaded, i.e., operated without capacitors connected to it, the line impedance is
about 30 Ω, and the delay time on the line reaches a value below 3 ns.