AD9764 output

Question

Hi 
 I have designed a DAQ borad including AD9764, AD6644 and it communicates with Xilinx Spartan 3AN FPGA through FX2 expansion connector. 
 As indicated in the attachments we can see the input&output connections and the clock signal (which is RC filtered version of Clockin at 16MHz coming from the FPGA) . I am wondering why i am getting the same voltages at the IOUTA and OUTB nodes at the AD9764 output (simply 0 or 20mV at the output of AD9631 opamp ) although I apply the code "00000001111111" from the FPGA. I checked the DAC inputs and see that they are as the FPGA output. Is it about the AD9764 or the AD9631 opamp or the noise level? 
 I will appreciate any help . 
 Thank you very much 
 HIDIR

ADIApproved · Accepted Answer

Hi HIDIR - 
 I recommend you add 25 ohm resistors to ground in between IOUTA and R18 as well as between IOUTB and R19. With 2K resistor R15 you set IOUTFS to 20ma. The voltage swing at !OUTA and IOUTB will then be 0 to .5 volts. 
 Thanks, 
 Larry)

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AD9764 output