9.2.1 Design Requirements

The CDCLVD1204 shown in Figure 16 is configured to select two inputs: a 156.25-MHz LVDS clock from the backplane, or a secondary 156.25-MHz LVCMOS 2.5-V oscillator. The LVDS clock is AC-coupled and biased using the integrated reference voltage generator. A resistor divider is used to set the threshold voltage correctly for the LVCMOS clock. Either signal can be then fanned out to desired devices, as shown. The configuration example is driving 4 LVDS receivers in a line card application with the following properties:

• The PHY device is capable of DC-coupling with an LVDS driver such as the CDCLVD1204. This PHY device features internal termination so no additional components are required for proper operation.
• The ASIC LVDS receiver features internal termination and operates at the same common mode voltage as the CDCLVD1204. Again, no additional components are required.
• The FPGA requires external AC-coupling, but has internal termination. 0.1-µF capacitors are placed to provide AC-coupling. Similarly, the CPU is internally terminated, and requires only external AC-coupling capacitors.

9.2.2 Detailed Design Procedure

See Input Termination for proper input terminations, dependent on single ended or differential inputs.

See LVDS Output Termination for output termination schemes depending on the receiver application.

Unused outputs can be left floating.

In this example, the PHY, ASIC, and FPGA or CPU require different schemes. Power supply filtering and bypassing is critical for low-noise applications.

See Power Supply Recommendations for recommended filtering techniques. A reference layout is provided in Low-Additive Jitter, Four LVDS Outputs Clock Buffer Evaluation Board (SCAU043).

9.2.3 Application Curves

The CDCLVD12xx's low additive noise is shown in this line card application. The low noise 156.25 MHz source with 67 fs RMS jitter drives the CDCLVD12xx, resulting in 80 fs RMS when integrated from 12 kHz to 20 MHz. The resultant additive jitter is a low 44 fs RMS for this configuration.

Reference signal is low-noise Rohde & Schwarz SMA100A

Figure 17. CDCLVD12xx Reference Phase Noise, 67 fs rms
(12 kHz to 20 MHz)

Figure 18. CDCLVD12xx Output Phase Noise, 80 fs rms
(12 kHz to 20 MHz)