9.1.3.3 SYSREF Signaling

The SYSREF input may be driven by a number of different types of signals. The supported signal types, shown in Figure 73 (in single-ended form), include periodic, gapped periodic, and one-shot signals. The rising edge of the SYSREF signal is used as a reference to align the internal frame clock and local multi-frame clock (LMFC). To ensure proper alignment of these system clocks, the SYSREF signal must be generated along with the CLKIN signal such that the SYSREF rising edge meets the setup and hold requirements relative to the CLKIN at the ADC31JB68 device inputs.

For each rising clock edge that is detected at the SYSREF input, the ADC31JB68 device compares the current alignment of the internal frame and LMFC with the SYSREF edge and determines if the internal clocks must be re-aligned. In the case that no alignment is needed, the clocks maintain their current alignment and the JESD204B data link is not broken. In the case that re-alignment is needed, the JESD204B data link is broken and the clocks are re-aligned.

Figure 73. SYSREF Signal Types (Single-Ended Representations)

In the case of a periodic SYSREF signal, the frame and LMFC alignment is established at the first rising edge of SYSREF, and every subsequent rising edge (that properly meets setup and hold requirements) is ignored because the alignment has already been established. A periodic SYSREF must have a period equal to n × K / F_S where ‘F_S’ is the sampling rate, ‘K’ is the JESD204B configuration parameter indicating the number of frames per multi-frame, and ‘n’ is an integer of one or greater.

Gapped-period signals contain bursts of pulses. The frame and LMFC alignments are established on the first rising edge of the pulse burst. Any rising edge that does not abide by this rule or does not meet the setup and hold requirements forces re-alignment of the clocks.

A one-shot signal contains a single rising edge that establishes the frame and LMFC alignment.

For all types of SYSREF signals, the minimum pulse width is 2 × T_FRAME.

TI recommends gapped-periodic or one-shot signals for most applications because the SYSREF signal is not active during normal sampling operation. Periodic signals that toggle constantly introduce spurs into the signal spectrum that degrade the dynamic range of the system.