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AD9915 output attenuation when driving FPGA.  Explanation and solutions?

Question asked by Lionelwallace Employee on Oct 6, 2014
Latest reply on Oct 16, 2014 by DSB

Customer Question:


We are using a AD9915/PCBZ in combination with a Xilinx ZC702 eval board.  We are using a Xilinx FMC XM105 debug board to access the clock and I/O pins on the FPGA.  The sync clock output on J100 of the DDS board is connected to J9 on the Xilinx debug board.  This input connects to a multi-region clock capable pin on the FPGA that is setup for LVCMOS33 operation.  When measuring the J100 output on the scope, the 3.4V, 125 MHz sine wave shown in dds_clock_output.png is obtained.  When J100 is connected to J9, the sync clock signal is attenuated as shown in dds_125mhz_sync_clk_at_j9.png.  An MMCM in the FPGA does not lock to the clock, and the IBUFG used with the J9 input pin shows active high constantly. Do you know if a clock distribution board is needed when using the AD9915/PCBZ board with a FPGA?