Datasheet

Observing an EVM U2 Output
1-5
Introduction
1.6 Observing an EVM U2 Output
The outputs are available at J8 and J7 for direct connection to oscilloscope in-
puts. Matched length cables must be used when connecting the EVM to a
scope to avoid inducing skew between the noninverting (+) and inverting (−)
outputs.
The three power jacks (P2, P3, and P4) 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. When connecting the EVM outputs di-
rectly to oscilloscope inputs, setting of the oscilloscope common-mode offset
voltage is required, as the oscilloscope presents low common-mode load im-
pedance to the device.
Returning to Figure 1−2, 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 J8 and J7. With power applied, the
common-mode voltage seen by the SN65LVDS20, SN65LVDS17, or
SN65LVDS19 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 test and evaluation of devices. Actual designs would include
high-impedance receivers, which would not require the setup steps outlined
above.
LVPECL drivers need a 50-Ω termination to V
TT
. A modification of Figure 1−1
and the above instructions are used when evaluating a SN65LVP20,
SN65LVP17, or SN65LVP19 with a direct connection to a 50-Ω oscilloscope.
With Power Supply 1 in Figure 1−2 set to 3.3 V, Power Supply 2 should be set
to 1.3 V (2 V below V
CC
) to provide the correct termination voltage.
Note:
Power Supply 2 must be able to sink current.