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).
- 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!