Texas Instruments SN65LVCP22, Руководство пользователя

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Observing an Output
2-4
Table 2–1. Crosspoint Function Table
SEL0 SEL1 OUT0 OUT1 Function
0 0 IN0 IN0 1:2 Splitter
0 1 IN0 IN1 Repeater
1 0 IN1 IN0 Switch
1 1 IN1 IN1 1:2 Splitter
2.3 Observing an Output
Direct connection to an oscilloscope with 50–
Ω
internal terminations to ground
is accomplished without R3–R8 (DUT1) and R11–R16 (DUT2) installed. The
outputs are available at J5–J8 (DUT1) and J13–J16 (DUT2) for direct
connection to oscilloscope inputs. Matched cable lengths must be used when
connecting the EVM to a scope to avoid inducing skew between the
noninverting (+) and inverting (–) outputs.
The three power jacks (J17, J18, J19) are used to provide power and a ground
reference for the EVM. The power connections to the EVM determine the
common-mode load to the device. As mentioned earlier, LVDS drivers have
limited common-mode driver capability. When connecting the EVM outputs
directly to oscilloscope inputs, setting of the oscilloscope common-mode
offset voltage is required, as the oscilloscope presents low common-mode
load impedance to the device.
Returning to Figure 2–1, power supply 1 is used to provide the required 3.3 V
to the EVM. Power supply 2 is used to offset the EVM ground relative to the
DUT ground. The EVM ground is connected to the oscilloscope ground
through the returns on SMA connectors J5–J8 (DUT1) and J13–J16 (DUT2).
With power applied as shown in Figure 2–1, the common-mode voltage seen
by the SN65LVCP22 is approximately equal to the reference voltage being
used inside the device, preventing significant common-mode current to flow.
Optimum device setup can be confirmed by adjusting the voltage on power
supply 2 until its current is minimized. It is important to note that use of the dual
supplies and offsetting the EVM ground relative to the DUT ground are simply
steps needed for the test and evaluation of devices. Actual designs include
high-impedance receivers, which do not require the setup steps outlined
above.
LVPECL drivers need a 50-
Ω
termination to V
TT
. A modification of Figure 2–1
and the above instructions are used when evaluating an SN65LVCP23 with a
direct connection to a 50-
Ω
oscilloscope. With power supply 1 in Figure 2–1
set to 3.3 V, power supply 2 should be set to 1.3 V (2 V below Vcc) to provide
the correct termination voltage.
If the outputs are to be evaluated with a high-impedance probe, direct probing
on the EVM board is supported via installation of a 50-
Ω
resistor across the
solder pads for R6 and R8, and another 50-
Ω
resistor across the solder pads
for R3 and R5 for DUT1. Or, LVDS outputs can be observed by installing a
100-
W
resistor at R4, R7, R12, or R15. LVPECL outputs can be observed by
installing R3, R5, R11, R13, R14, and R16 (49.9-
Ω
resistors) and setting power
supply #2 to 1.3 V. (Note that power supply #2 must be able to sink current.)