Periodic Link Loss in AD9680 Data Acquisition

Question

I am currently using an FPGA to implement data acquisition with JESD204B Subclass 1 and the AD9680 ADC. The FPGA model is Xilinx Zynq-7035. When operating in Subclass 1 mode, periodic link loss occurs during tests at different data rates.

For instance:

With an input clock of 312.5 MHz (sourced and synchronized for both FPGA and ADC), lane count L=2, and a line rate of 6.25 Gbps; 
 And with an input clock of 500 MHz and a line rate of 10 Gbps.

In both cases, the link drops after approximately 0.5 ms. Via Vivado debugging, we confirmed the GT_PLL_LOCK signal is lost. The GT_PLL_LOCK signal recovers automatically after around 113 core clock cycles.

We also conducted tests in JESD204B Subclass 0 mode. After the SYNC signal is asserted, the data link breaks shortly after acquisition starts, which rules out issues related to the SYSREF signal.

We noticed a mismatch in the trace lengths of the clock lines feeding the ADC and FPGA on the PCB. The external clock first connects to the ADC, then travels an additional 10 centimeters before reaching the FPGA&rsquo;s clock pin.

Could this link loss be caused by the inconsistent clock trace lengths in the PCB design? If yes, please explain the underlying principle. Are there any other potential root causes? Your professional advice would be greatly appreciated

JAlipio · Accepted Answer

Hi ad9680 
 Based on your setup, the configuration appears to be L, M, F = 2, 2, 2; N' = 16; Decimation = 1 (full bandwidth) &mdash; please confirm if this is correct. 
 The issue is not primarily due to the trace length mismatch between the ADC and FPGA; it is more likely driven by clock quality degradation. It is important to ensure that a clean, low-jitter clock is delivered to both devices, and that the differential clock pairs to the ADC and FPGA are well matched in terms of length, impedance, and routing to preserve signal integrity. 
 Additionally, take into account the PLL lock time (~2 ms) when evaluating link stability.)

SLA Status	Assignee	Support Status
SLA Closed. Met after 44 hours	JAlipio	Awaiting Company Response

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Periodic Link Loss in AD9680 Data Acquisition

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