Cirrus Logic AN339 - Manual (Page 9)

AN339
AN339REV1
AN339REV1 9
7. JITTER MEASUREMENT TO DAC THD+N CORRELATION
The results presented in Section 5 show that the amount of baseband jitter present on the recovered master clock
of each receiver is reduced as the sample rate of the input S/PDIF signal is increased. This relationship is a direct
result of the increased phase detector update rate, which is proportional to the incoming sample rate.
Also, the results of Section 6 show that baseband jitter present on the recovered master clock has the most signifi-
cant affect on the converter’s THD+N performance when its audio input signal level and frequency are both high.
Using this information, a simple test was devised to help correlate the level of baseband jitter to the DAC’s THD+N.
To do this, a S/PDIF signal with a fixed 0 dBFS, 18 kHz audio tone was transmitted into both receivers. The sample
rate of the S/PDIF signal was then varied, and both the recovered clock baseband jitter and the THD+N of the
CS4398 were measured at each sample rate step. The results are shown in
Figure 30 and Figure 31 below.
The plots confirm the relationship between recovered clock baseband jitter and DAC THD+N. It can again be seen
that the baseband jitter and THD+N for the CS8416 when PDUR = 1 is better than that with the CS8414. For sample
rates greater than 96 kHz (up to 192 kHz), the jitter and THD+N for the CS8416 with PDUR = 0 is comparable to the
performance of the CS8414 with a sample rate of 48 kHz.
-160
+0
-150
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
r
A
20k 120k 40k 60k 80k 100k
Hz
-160
+0
-150
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
r
A
20 20k 50 100 200 500 1k 2k 5k 10k
Hz
Figure 28. FFT - -60 dBFS, 997 Hz
Fs = 192 kHz
Figure 29. FFT - No Input (w/ Dither)
Out of Band, Fs = 192 kHz
CS8414 (48 kHz)
CS8416 PDUR=0
CS8414 (48 kHz)
CS8416 PDUR=0
10
100
1000
40 60 80 100 120 140 160 180 200
Fs (kHz)
Baseband Jitter (ps)
-115
-110
-105
-100
-95
-90
-85
40 60 80 100 120 140 160 180 200
Fs (kHz)
18 kHz THD+N (dBr)
Figure 30. Baseband Jitter vs Sample Rate Figure 31. THD+N vs Sample Rate
CS8414
CS8416 PDUR=0
CS8416 PDUR=1
CS8414
CS8416 PDUR=0
CS8416 PDUR=1