2 Setup and Required Equipment
2.2 Observing an Output
In order to minimize the parasitic capacitance in high-speed measurements (probe capacitance), the 4x4 crosspoint switch EVM provides an offset power plane (layer 5). This power plane allows the user to offset the device so that the common-mode output is compatible with the 50 ohms to ground termination within the scope. Terminating the EVM within the scope eliminates any bandwidth limitations introduced by a probe.
Direct connection to an oscilloscope with 50-Ω internal terminations to ground is accomplished without requiring installation of resistors R5 – R16 on the EVM. The outputs are available at J9–J16 for direct connection to oscilloscope inputs. All cabling used to source and measure signals must be electrically matched in length to prevent any skew between conductors of the differential inputs.
Referring back to 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 device under test (DUT) ground. With this power scheme, the common- mode voltage seen by either the SN65LVDS125A or the SN65LVDS250 is approximately equal to that of the oscilloscope, thus preventing significant common-mode current flow. Using dual supplies and offsetting the EVM ground relative to the DUT ground are simply steps required for the test and evaluation of devices. Actual designs include high-impedance receivers, which do not require the setup steps outlined above. If the EVM outputs are to be evaluated with a high-impedance probe, direct probing on the EVM is supported via installation of a 100-Ω resistor across the solder pads (R5, R8, R11, and R14).
Note: Power Supply 2
.Power supply 2 must be able to sink current.
2.3 Typical Test Results
Figure 2-3 is a typical result obtained with the EVM setup shown in Figure 2-1. The inputs (J1–J8) were stimulated with a 223–1 PRBS signal at
1.5Gbps. The input levels for both clock and data were a differential voltage of 400 mV, with a common-mode voltage of 0 V (referenced to the ground of the pattern generator).
