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參數(shù)資料
| 型號： | AD73311LARUZ-RL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 16/36頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PROCESSOR FRONT END LP 20SSOP |
| 標(biāo)準(zhǔn)包裝： | 1,000 |
| 位數(shù)： | 16 |
| 通道數(shù)： | 2 |
| 功率（瓦特）： | 50mW |
| 電壓 - 電源，模擬： | 3V |
| 電壓 - 電源，數(shù)字： | 3V |
| 封裝/外殼： | 20-SSOP（0.209"，5.30mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 20-SSOP |
| 包裝： | 帶卷 (TR) |
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AD73311
–23–
REV. B
In order to produce a direct sampling rate of 8 kHz, it is
necessary to reduce the external master clock to 8.192 MHz and
to set the master clock divider to a ratio of 4, which results in a
sample rate of 8 kHz. In this case, the response of the Sinc
3
decimation filter may affect the response in the voice BW as its
first null occurs at 8 kHz. In Figure 17, Trace A shows how the
SNR remains approximately constant up to FS/2 regardless of
the sample rate FS; Trace B shows that the SNR achievable in
the voice BW is proportional to the sampling rate FS. These two
traces intersect at an FS of 8 kHz which is the point where FS/2
equals the voice BW.
SAMPLING FREQUENCY (DMCLK/256) – kHz
816
24
32
40
48
56
64
80
70
S/(N+D)
–
dB
65
55
75
60
SNR IN VOICEBAND
(300Hz–3400Hz)
SNR IN BAND UP TO FS/2
TRACE B
TRACE A
Figure 17. SNR vs. Sampling Frequency
If the input signal is externally band-limited, it is possible to
achieve the 8 kHz sampling rate directly from the 64 kHz
sample rate by decimating the sampled data in the DSP. This
technique will alias the band between 8 kHz and 64 kHz into
the 8 kHz band, therefore it is necessary to have good quality
external band-limiting on the input signal. Figure 18 shows the
spectral response of using this decimation technique for sample
rate reduction.
FREQUENCY – kHz
0
–120
0
4.0
0.5
dB
1.0
1.5
2.0
2.5
3.0
3.5
–20
–60
–80
–100
–40
S/N+D = 59.545
Figure 18. FFT (ADC 8 kHz Subsampled)
Encoder Group Delay
The AD73311 implementation offers a very low level of group
delay, which is given by the following relationship:
Group Delay (Decimator) = Order
× ((M–1)/2) × Tdec
where:
Order is the order of the decimator (= 3),
M is the decimation factor (= 32) and
Tdec is the decimation sample interval (= 1/2.048e6)
=> Group Delay (Decimator) = 3
× (32–1)/2 × (1/2.048e6)
= 22.7
s
If final filtering is implemented in the DSP, the final filter’s
group delay must be taken into account when calculating overall
group delay.
Decoder Section
The decoder section updates (samples) at the same rate as the
encoder section, MCLK/256, which gives a 64 kHz rate from an
external MCLK of 16.384 MHz. Figure 19 shows the spectral
response of the decoder section sampling at 64 kHz. Again, its
sigma-delta modulator shapes the noise so it is reduced in the
voice bandwidth dc–4 kHz. For improved voiceband SNR, the
user can implement an initial anti-imaging filter, preceded by
8 kHz to 64 kHz interpolation, in the DSP.
FREQUENCY – kHz
0
–140
032
5
dB
10
15
20
25
30
–20
–60
–80
–100
–120
–40
S/N+D = 58.454842
Figure 19. FFT (DAC 64 kHz Sampling)
As is the case with the encoder section, voiceband SNR is re-
duced if the DMCLK rate is reduced as shown by the example
of 16 kHz sampling in Figure 20. This is due to the noise-shap-
ing of the sigma-delta modulator being compressed into a
smaller bandwidth, which increases the noise in the voice BW.
FREQUENCY – kHz
0
–140
08
12
3
4
5
67
–20
–60
–80
–100
–120
–40
dB
S/N+D = 57.441659
Figure 20. FFT (DAC 16 kHz Sampling)
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