Datasheet

Observing an EVM U1 Output

1-3

Introduction

1.3 Observing an EVM U1 Output

The outputs are available at J1 and J2 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 1−1, 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 J1 and J2. With power applied, the

common-mode voltage seen by the SN65LVDS16 or SN65LVDS18 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

. A modification of Figure 1−1

and the above instructions are used when evaluating a SN65LVP16 or

SN65LVP18 with a direct connection to a 50-Ω oscilloscope. With Power

Supply 1 in Figure 1−1 set to 3.3 V, Power Supply 2 should be set to 1.3 V (2

V below V

) to provide the correct termination voltage.

Note:

Power Supply 2 must be able to sink current.