Texas Instruments SLAU039, User Manual

Page: 38 / 66

DAC Results
4-4
4.2 DAC Results
The DAC was characterized using a sine-wave table lookup program running
on a TMS320C5x DSK+ coupled to an AD50-EVM.
Figure 4–3. AD50-EVM DAC Distortion Measurement at 8 ksps
–60
–80
–100
–120
0 1 2 3 4 5 6
Full Scale – dB
–30
–10
f – Frequency – kHz
0
7 11 12 13
–20
–40
–50
–70
–90
–110
8 9 10 14 15 19 20 16 17 18
AD50 DAC: 8ksa/s Generating a 1 kHz Sine Wave
at–3 dB ref Full Scale
FFT: 40ksa/s, 2048 Frequency Bins, 16 Averages
Figure 4–3 shows an FFT plot of the output from the AD50-EVM. This was
measured using a Texas Instruments AD55-EVM acquisition system running
at 40 ksps coupled to a DSP development system in a PC.
The signal to noise plus distortion ratio (SINAD) measured in the pass-band
(excluding dc) was 80.5 dB. The SNR was 83.6 dB and the SDR was 83.4 dB.
These measurements have not been adjusted for the noise and distortion of
the acquisition system.
The rise in the noise floor above 4 kHz is due to the digital noise-shaping filter
in the AD50 DAC, which decreases pass-band noise at the expense of in-
creased high frequency noise. This is partially attenuated by the output filter
on the AD50-EVM but continues to rise to a plateau at approximately –100 dB.
This filter is optimized for use at 20 ksps and therefore the out of band noise
is worse than at higher sampling frequencies.