AD9879 Analog Devices, AD9879 Datasheet - Page 15

• Manufacturer Part Number: AD9879
• Description: Mixed Signal Front End Set Top Box, Cable Modem (MxFE®)
• Manufacturer: Analog Devices

Specifications of AD9879

• Resolution (bits): 12bit
• # Chan: 5
• Sample Rate: 29MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: QFP

Σ-∆ OUTPUTS
The AD9879 contains an on-chip Σ-Δ output that provides a
digital logic bit stream with an average duty cycle that varies
between 0% and (4095/4096)%, depending on the programmed
code, as shown in Figure 7.
This bit stream can be low-pass filtered to generate a
programmable dc voltage of
where:
V
V
H
L
= 0.4 V
= V
V
DC
DRVDD
= (Σ-Δ Code/4096)(V
0xFFF
− 0.6 V
0x000
0x001
0x002
0x800
8
8
t
MCLK
t
MCLK
Figure 7. Σ-∆ Output Signals
4096 × 8
H
4096 × 8
) + V
L
t
MCLK
t
MCLK
SIGMA-DELTA
AD9879
Σ-∆
Figure 9. Σ-∆ Active Filter with Gain and Offset
V
SD
R
C
V
R1
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OFFSET
(8)
R
C
R
V
TYPICAL:
OP250
OUT
= (V
In set-top box and cable modem applications, the output can be
used to control external variable gain amplifiers or RF tuners. A
simple single-pole RC low-pass filter provides sufficient
filtering (see Figure 8).
In more demanding applications where additional gain, level
shift, or drive capability is required, a first or second order
active filter might be considered for each Σ-Δ output (see Figure 9).
V
MCLK
OUT
AD9879
SD
+ V
R = 50kΩ
C = 0.1µF
f
–3dB
OFFSET
= 1/(2πRC) = 318Hz
÷8
) (1 + R/R1)/2
CONTROL
WORD
Figure 8. Σ-∆ RC Filter
TYPICAL:
12
DAC
Σ-∆
R = 50kΩ
C = 0.1µF
f
–3dB
= 1/(2πRC) = 318Hz
R
C
AD9879
DC (V
L
TO V
H
)