I am using the AD9914 eval board in Direct Mode. We have been experience intermittent noise on the output. That is, over a range of output frequencies, we find normal operation (noise floor >40 dB below signal peak) except occasionally the white noise background jumps up to 10-20 dB below the signal peak. My application is very sensitive to this and we need to eliminate it.
I've also been working with the eval boards in Parallel Programming Mode and there I had several issues solved by pulling floating pins to ground with jumpers/terminators. So, it seems like this noise might be caused by charge buildup on a floating pin that then randomly flips an input. For instance, for Parallel Programming Mode, I removed the five resistors R105-R109, but have not for Direct Mode. Perhaps these resistors are coupling noise (from floating pins) into the bottom five bits of the Frequency Tuning Word that comes from the MPI header. That, it seems, shouldn't cause a white noise background, but maybe does if the noise on those bits somehow messes with the DDS chip (not just causing corresponding noise on the signal output).
Does anyone have advise for how to remove this intermittent noise?
The current jumper configuration:
Enable CLK input jumpered
P203, P204, P205 are disabled (octal buffers disabled)
EXTPDCTL-BUF jumper to gnd
OSK-BUF jumper to gnd
RESET-BUF jumper to gnd
DRHOLD-BUF jumper to gnd
P102-DRCTL-BUF jumper to gnd
PS0-BUF jumper to gnd
PS1-BUF jumper to gnd
PS2-BUF jumper to gnd
SYNC-IN (sma) 50ohm terminated