AD6634BC/PCB Analog Devices Inc, AD6634BC/PCB Datasheet - Page 25

• Manufacturer Part Number: AD6634BC/PCB
• Description: BOARD EVAL SGNL PROCESS AD6634
• Manufacturer: Analog Devices Inc

Specifications of AD6634BC/PCB

• Rohs Status: RoHS non-compliant
• Module/board Type: Evaluation Board
• For Use With/related Products: AD6634
• Lead Free Status / Rohs Status: Not Compliant

where, IN is the value of IN[13:0], EXP is the value of EXP[2:0],
and rCIC2 is the value of the 0x92 (rCIC2_QUIET[4:0], and
rCIC2_LOUD[4:0]) scale register.
rCIC2 Rejection
Table III illustrates the amount of bandwidth in percent of the
data rate into the rCIC2 stage. The data in this table may be
scaled to any other allowable sample rate up to 80 MHz in
Single Channel Mode or 40 MHz in Diversity Channel Mode.
Table III can be used as a tool to decide how to distribute the
decimation between rCIC2, CIC5, and the RCF.
Example Calculations
Goal: Implement a filter with an Input Sample Rate of 10 MHz
requiring 100 dB of Alias Rejection for a ± 7 kHz pass band.
Solution: First determine the percentage of the sample rate that
is represented by the pass band.
Find the –100 dB column on the right of Table III and look down
this column for a value greater than or equal to the pass-band
percentage of the clock rate. Then look across to the extreme
left column and find the corresponding rate change factor
(M
M
rejection is 0.071 percent, which is slightly greater than the 0.07
percent calculated. Therefore, for this example, the maximum
bound on rCIC2 rate change is 4. A higher chosen M
means less alias rejection than the 100 dB required.
An M
rejection; however, the power consumption can be minimized by
decimating as much as possible in this rCIC2 stage. Decimation in
rCIC2 lowers the data rate, and thus reduces power consumed in
subsequent stages. It should also be noted that there is more than
one way to get the decimation by 4. A decimation of 4 is the
same as an L/M ratio of 0.25. Thus any integer combination of
L/M that yields 0.25 will work (1/4, 2/8, or 4/16). However, for
the best dynamic range, the simplest ratio should be used. For
example, 1/4 gives better performance than 4/16.
REV. 0
rCIC2
rCIC2
rCIC2
/L
/L
rCIC2
/L
rCIC2
M
10
11
12
13
14
15
16
2
3
4
5
6
7
8
9
rCIC2
BW
rCIC2
of 4, the frequency having –100 dB of alias
). Referring to the table, notice that for a
FRACTION
Table III. SSB rCIC2 Alias Rejection Table (f
of less than 4 would still yield the required
/L
rCIC2
=
100
–50 dB
1.79
1.508
1.217
1.006
0.853
0.739
0.651
0.581
0.525
0.478
0.439
0.406
0.378
0.353
0.331
×
10
7
kHz
MHz
=
0 07
.
–60 dB
1.007
0.858
0.696
0.577
0.49
0.425
0.374
0.334
0.302
0.275
0.253
0.234
0.217
0.203
0.19
rCIC2
/L
rCIC2
–25–
SAMP
–70 dB
0.566
0.486
0.395
0.328
0.279
0.242
0.213
0.19
0.172
0.157
0.144
0.133
0.124
0.116
0.109
Decimation and Interpolation Registers
rCIC2 decimation values are stored in register 0x90. This is a
12-bit register and contains the decimation portion less 1. The
interpolation portion is stored in register 0x91. This 9-bit value
holds the interpolation less one.
rCIC2 Scale
Register 0x92 contains the scaling information for this section of
the circuit. The primary function is to store the scale value
computed in the sections above.
Bits 4–0 (rCIC2_LOUD[4:0]) of this register are used to contain
the scaling factor for the rCIC2 during conditions of strong
signals. These five bits represent the rCIC2 scalar calculated
above plus any external signal scaling with an attenuator.
Bits 9–5 (rCIC2_QUIET[4:0]) of this register are used to contain
the scaling factor for the rCIC2 during conditions of weak signals.
In this register, an external attenuator would not be used and is not
included. Only the value computed above is stored in these bits.
Bit 10 of this register is used to indicate the value of the external
exponent. If this bit is set LOW, then each external exponent
represents 6 dB per step as in the AD6600. If this bit is set to
HIGH, each exponent represents a 12 dB step.
Bit 11 of this register is used to invert the external exponent before
internal calculation. This bit should be set HIGH for gain ranging
ADCs that use an increasing exponent to represent an increasing signal
level. This bit should be set LOW for gain ranging ADCs that use
a decreasing exponent for representing an increasing signal level.
In applications that do not require the features of the rCIC2, it
may be by setting the L/M ratio to 1/1. This effectively bypasses
all circuitry of the rCIC2 except the scaling that is still effectual.
FIFTH ORDER CIC FILTER
The third signal processing stage, CIC5, implements a sharper
fixed-coefficient, decimating filter than rCIC2. The input rate
to this filter is f
equation below. N
mode; otherwise N
M
increased (reference fractional rate input timing described in the
Input Timing section).
= 1) Bandwidth Shown in Percentage of f
rCIC2
can be increased, N
–80 dB
0.318
0.274
0.223
0.186
0.158
0.137
0.121
0.108
0.097
0.089
0.082
0.075
0.07
0.066
0.061
SAMP2
CH
CH
. The maximum input rate is given by the
equals 2 for diversity channel real input
equals 1. In order to satisfy this equation,
CH
–90 dB
0.179
0.155
0.126
0.105
0.089
0.077
0.068
0.061
0.055
0.05
0.046
0.043
0.04
0.037
0.035
can be reduced, or f
SAMP
–100 dB
0.101
0.087
0.071
0.059
0.05
0.044
0.038
0.034
0.031
0.028
0.026
0.024
0.022
0.021
0.02
AD6634
CLK
can be