The clocking subsystem of the device has two input signals, device clock (CLK+, CLK–) and SYSREF (SYSREF+, SYSREF–). Within the clocking subsystem there is a noiseless aperture delay adjustment (t_AD adjust), a clock duty cycle corrector and a SYSREF capture block. Figure 8-2 describes the clocking subsystem.

Figure 8-2 Clocking Subsystem

The device clock is used as the sampling clock for the ADC core as well as the clocking for the digital processing and serializer outputs. Use a low-noise (low jitter) device clock to maintain high signal-to-noise ratio (SNR) within the ADC. In dual-channel mode, the analog input signal for each input is sampled on the rising edge of the device clock. In single-channel mode, both the rising and falling edges of the device clock are used to capture the analog signal to reduce the maximum clock rate required by the ADC. A noiseless aperture delay adjustment (t_AD adjust) allows the user to shift the sampling instance of the ADC in fine steps in order to synchronize multiple ADC12DJ4000RFs or to fine-tune system latency. Duty cycle correction is implemented in the device to ease the requirements on the external device clock while maintaining high performance. Table 8-4 summarizes the device clock interface in dual-channel mode and single-channel mode.

SYSREF is a system timing reference used for JESD204C subclass-1 implementations of deterministic latency. SYSREF is used to achieve deterministic latency and for multi-device synchronization. SYSREF must be captured by the correct device clock edge in order to achieve repeatable latency and synchronization. The ADC12DJ4000RF includes SYSREF windowing and automatic SYSREF calibration to ease the requirements on the external clocking circuits and to simplify the synchronization process. SYSREF can be implemented as a single pulse or as a periodic clock. In periodic implementations, SYSREF must be equal to, or an integer division of, the local multiframe clock frequency in 8B/10B encoding modes or the local extended multiblock clock frequency in 64B/66B encoding modes. Equation 2 is used to calculate valid SYSREF frequencies in 8B/10B encoding modes and Equation 3 in 64B/66B encoding modes.

Equation 2.

Equation 3.

where

• R and F are set by the JMODE setting (see JESD204C Operating Modes Table)
• f_CLK is the device clock frequency (CLK±)
• K is the programmed multiframe length (see JESD204C Operating Modes Table for valid K settings)
• E is the number of multiblocks in an extended multiblock.
• n is any positive integer