Datasheet

Observing an Output

2-5

Setup and Equipment Required

Figure 2-4. External Termination for Single-Ended LVPECL Inputs to EVM

50 Ω

OUT

Signal Source EVM BOARD

NOTES: A. Add jumper Jmp2 and 0-Ω R5

B. Remove R1

2.3 Observing an Output

Direct connection to an oscilloscope with 50-Ω internal terminations to ground

is accomplished without R2, R3, and R4

. The outputs are available at J6 and

J7 for direct connection to oscilloscope inputs. 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 (J1, J2, and J3) 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 2-2, 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 J6 and J7. With power applied as

shown in Figure 2-2, the common-mode voltage seen by the SN65LVDS100

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 would include high-impedance

receivers, which would not require the setup steps outlined above.

As delivered R2, R3, and R4 are not installed