Chapter 2
2 Setup and Required Equipment
Both the SN65LVDS125A and the SN65LVDS250 LVDS driver output characteristics are compliant with the requirements of the TIA/EIA-644 standard. LVDS drivers nominally provide a 350-mV differential signal, with a 1.2-V offset from ground. These levels are attained when driving a 100-Ω differential line-termination test load. This requirement includes the effects of up to 32 standard receivers with their ground references offset up to ±1 V from that of the driver. This common-mode loading limitation of LVDS drivers affects how the driver’s characteristics are observed with this EVM, and the test setup that follows in this manual.
By using three power jacks (J17, J18, and J19) and by optionally installing termination resistors, different methods of probing can be used to evaluate the device output characteristics. The typical setup for the 4x4 crosspoint switch EVM is shown in Figure 2-1.
2.1 Applying an Input
While the use of a split power plane allows the EVM to be terminated within the oscilloscope, offsetting the EVM ground requires the inputs to the device to also be offset. Figure 2-1 shows how to offset the EVM and the inputs to the device. Setting power supply 1 to 3.3 V and power supply 2 to
1.2V causes the voltage swing of the LVDS outputs to be within the limits of -200 mV to 200 mV instead of the typical 1 V to 1.4 V. This requires the inputs of the 4x4 crosspoint switch EVM to also be offset by 1.2 V, resulting in a voltage swing of -200 mV to 200 mV. VTERM, banana jack J20, is provided for non-LVDS input terminations such as LVPECL, which requires a termination voltage. When applying an LVDS input, VTERM can be left open or connected to GND, which is the common-mode voltage when using the aforementioned –200-mV to 200-mV input swing.
2-1
