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AD9684 View Datasheet(PDF) - Analog Devices

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AD9684 Datasheet PDF : 64 Pages
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AD9684
Product
Overview
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Documentation
HB1 Filter
The fourth and final decimate by 2, half-band, low-pass FIR
filter (HB1) uses a 21-bit, symmetrical, fixed coefficient filter
implementation that is optimized for low power consumption.
The HB1 filter is always enabled and cannot be bypassed. Table 21
and Figure 65 show the coefficients and response of the HB1
filter.
Table 21. HB1 Filter Coefficients
HB1 Coefficient Number
Decimal Coefficient (21-Bit)
C1, C55
−24
C2, C54
0
C3, C53
102
C4, C52
0
C5, C51
−302
C6, C50
0
C7, C49
730
C8, C48
0
C9, C47
−1544
C10, C46
0
C11, C45
2964
C12, C44
0
C13, C43
−5284
C14, C42
0
C15, C41
8903
C16, C40
0
C17, C39
−14,383
C18, C38
0
C19, C37
22,640
C20, C36
0
C21, C35
−35,476
C22, C34
0
C23, C33
57,468
C24, C32
0
C25, C31
−105,442
C26, C30
0
C27, C29
331,792
C28
524,288
Design
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Discussion
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Data Sheet
0
–20
–40
–60
–80
–100
–120
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
NORMALIZED FREQUENCY (× π RAD/SAMPLE)
Figure 65. HB1 Filter Response
DDC GAIN STAGE
Each DDC contains an independently controlled gain stage.
The gain is selectable as either 0 dB or 6 dB. When mixing a real
input signal down to baseband, it is recommended to enable the
6 dB gain to recenter the dynamic range of the signal within the
full scale of the output bits.
When mixing a complex input signal down to baseband, the
mixer has already recentered the dynamic range of the signal
within the full scale of the output bits and no additional gain is
necessary. However, the optional 6 dB gain compensates for low
signal strengths. The downsample by 2 portion of the HB1 FIR
filter is bypassed when using the complex to real conversion
stage (see Figure 66).
DDC COMPLEX TO REAL CONVERSION BLOCK
Each DDC contains an independently controlled complex to
real conversion block. The complex to real conversion block
reuses the last filter (HB1 FIR) in the filtering stage, along with
an fS/4 complex mixer to upconvert the signal.
After upconverting the signal, the Q portion of the complex
mixer is no longer needed and is dropped.
Figure 66 shows a simplified block diagram of the complex to
real conversion.
Rev. 0 | Page 42 of 64
 

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