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Noise on AD5754R output

Question asked by strnadp on Mar 16, 2016
Latest reply on Apr 17, 2016 by strnadp

Hello, I have connected AD5754R to FPGA and I am continuously sending data through SPI to all four output channels to generate arbitrary waveforms. I am experiencing unexpected noise on the output (oscilloscope image is attached).


System details:

- SPI receiving data continuously during waveform generation, 25MHz clock

- LDAC is always enabled, output is set after last data bit is received, as specified in the documentation

- no load on the output, connected directly through coaxial cable to the oscilloscope

- internal reference is disabled, external reference voltage is supplied, identical setup to the AD5754 evaluation board, including identical voltage reference and decoupling capacitors

- supply voltage is +/-12V and 3.3V (from ADP3303)


- noise is dependent on the voltage range, smallest 0V to 5V, largest -10V to 10V

- noise appears only when SPI data are received (yellow channel at logical 1 on scilloscope image)

- noise appears even if voltage code is constant, as long as data are coming

- on the oscilloscope image the DAC receives constant voltage code every 5us which corresponds to 1.7V

- noise is present independently on encoding (2's component or offset encoding)

- no similar noise on positive or negative analog supply voltages

- no similar noise on the input reference voltage

Any help would be greatly appreciated!

Thanks a lot!