How to reduce LTC2758 Output Noise

Hi Dorappp ,

Would you mind sharing your schematic? A couple of factors that affect noise at the output of the DAC are reference noise, Supply noise, amplifier noise, interference (coming from the environment or nearby circuitry), and your PCB layout as well. What you're getting is noise at the output that combines all of these noise sources. The datasheet spec is noise that is just from the DAC itself. It is measured by eliminating all other noise sources. 

Here are a few ways to reduce the overall system noise.

• Proper supply and reference decoupling using 0.1uF/0.01uF low ESR ceramic caps that are placed very close to the input pins. 
• A clear path for the output lines in your PCB away from any noise-inducing components and with a proper ground return path that avoids the return paths of noisy signals as well. 
• A low pass filter at the output of your trans-impedance amplifier, values could be determined empirically while considering the effect it would have on your output bandwidth (or output slew rate). 

Best regards,

Ian

Are you sure the noise you see on the oscilloscope comes from the DAC? With 13nVrtHz over a 20MHz BW, you'd expect 58uV RMS noise.... Typically, an oscilloscope input has a much higher noise floor than that...

You can check the oscilloscope noise floor by not connecting any signal to the input, preferrably shorting the input to GND.