AD5781 NSD

Hi Valentin_87,

In order to measure the DAC NSD, you have to measure the NSD of the output and the reference. Then use this equation:

DAC NSD = SQRT( VOUT_NSD^2 - VREF_NESD^2)

Cheers,

Ivan

So, how i must correctly measure VOUT_NSD? I must measure output rms noise and divide it by √BW? What value BW I must use? How BW determined?

I recommend using a spectral analyzer with the y-axis set as spectral density.

I don't have Spectrum analyzer how can I measure VOUT_NSD?

Hi Valentin_87,

It's a bit tricky and should consider a lot of things, you can give this a try to approximate the NSD:

1) for AD5781, you can build an active bandpass filter using a very low noise amplifier with cut off frequencies of 900-1100 Hz.

2) Measure the rms noise coming out of the BPF you designed using an oscilloscope. Make sure that the termination is set to 50 Ohms to minimize the NSD due to the terminating resistor. But you should also consider the maximum power rating of that resistor. So I advise to set a very low output voltage.

3) compute for the NSD = Vrmsnoise / SQRT (200)  -> 200 Hz is the BW of the BPF.

Cheers,

Ivan