ad73311 Analog Devices, Inc., ad73311 Datasheet - Page 23

• Manufacturer Part Number: ad73311
• Description: Low Cost, Low Power Cmos General Purpose Analog Front End
• Manufacturer: Analog Devices, Inc.

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
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 F
the sample rate F
the voice BW is proportional to the sampling rate F
traces intersect at an F
equals the voice BW.
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.
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
–100
–120
–20
–40
–60
–80
80
75
70
65
60
55
0
0
8
0.5
S
16
SAMPLING FREQUENCY (DMCLK/256) – kHz
; Trace B shows that the SNR achievable in
1.0
S
24
of 8 kHz which is the point where F
FREQUENCY – kHz
1.5
TRACE B
SNR IN BAND UP TO F
32
TRACE A
2.0
SNR IN VOICEBAND
(300Hz–3400Hz)
40
2.5
48
S/N+D = 59.545
3.0
S
S
/2
/2 regardless of
56
3.5
S
. These two
64
4.0
3
S
/2
where:
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.
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.
Order is the order of the decimator (= 3),
= 22.7 µs
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)
–100
–120
–140
–100
–120
–140
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
1
5
2
10
FREQUENCY – kHz
FREQUENCY – kHz
3
15
4
20
5
S/N+D = 58.454842
S/N+D = 57.441659
6
AD73311
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7
30
32
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