Other Parts Discussed in Thread: DAC38J84
Hello,
I'm measuring the SNR of the DAC38j84 by applying a single tone (const=0x0000, fout=400MHz) at the input with NCO (on the evaluation board with GUI evaluation software).
Settings: Mixer Gain=0dB, Coarse DAC Gain=15, 8 x Interpolation, fdac=2.457GHz. No external LPF.
Here is what I got on the spectrum analyser:
- LF to fdac/2 (2.4576GHz/2=1.2288GHz)
I got SNR = FS - noise floor (displayed) - process gain (10log(fdac/2/RBW)) = -1.81 - (-76) - 10log(1.2288GHz/220kHz) = 36.7dB !?
However when I applied this equation to get the "theoretical" SNR for 400MHz which is calculated as: FS -(NSD + 10log(fdac/2) = -1.81- (-153dBm/Hz@400MHz + 10log(fdac/2)) = 60.3dB......
When I reduce the SPAN of the spectrum analyser to 100MHz centered @400MHz, I got:
- 350MHz to 450MHz
SNR = FS - noise floor (displayed) - process gain (10log(fdac/2/RBW)) = -1.49 - (-85) - 10log(1.2288GHz/10kHz) = 32.6dB !?
while again the theoretical gives: SNR = -1.49- (-153dBm/Hz@400MHz + 10log(fdac/2)) = 60.6dB...
I'm wondering why there is such a difference between the theoretical one and the measured one?
Thanks in advances,
BR,
XL