AD6624AS Analog Devices Inc, AD6624AS Datasheet - Page 19

• Manufacturer Part Number: AD6624AS
• Manufacturer: Analog Devices Inc

Specifications of AD6624AS

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REV. B
where input_level is the largest fraction of full-scale possible at
the input to the AD6624 (normally 1). The rCIC2 scale factor
is always used whether or not the rCIC2 is bypassed.
Moreover, there are two scale registers (rCIC2_LOUD[4:0]
Bits 4–0 in x92), and (rCIC2_QUIET[4:0] Bits 9–5 in 0x92)
that are used in conjunction with the computed S
determines the overall rCIC2 scaling. The S
be summed with the values in each respective scale register and
ExpOff, to determine the scale value that must be placed in the
rCIC2 scale register. This number must be less than 32 or the
interpolation and decimation rates must be adjusted to validate
this equation. The ceil function denotes the next whole integer
and the floor function denotes the previous whole integer. For
example, the ceil(4.5) is 5 while the floor(4.5) is 4.
The gain and passband droop of the rCIC2 should be calculated
by the equations above, as well as the filter transfer equations
that follow. Excessive passband droop can be compensated
for in the RCF stage by peaking the passband by the inverse
of the roll-off.
where: IN is the value of IN[15:0], 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.
The table can be used as a tool to decide how to distribute the
decimation between rCIC2, CIC5, and the RCF.
M
L
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
rCIC2
CIC5
scaled input
scaled input
Table III. SSB rCIC2 Alias Rejection Table (f
Bandwidth Shown in Percentage of f
/
–50 dB –60 dB –70 dB –80 dB –90 dB –100 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
_
_
OL
S
rCIC
CIC
=
=
2
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
2
IN
IN
=
=
ceil
×
×
L
rCIC
2
2
(




M
−
−
log
mod(
mod(
2
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
rCIC
2
×
Exp rCIC
7



2
2
−
M
2
S
Exp rCIC
rCIC
+
)
rCIC
+
2
×
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
2
2 8
input level
, )
+
2 8
, )
,
floor
ExpInv
SAMP
,
ExpInv
_
rCIC2
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



M
L
rCIC2
rCIC
=
rCIC
value must
SAMP
0
=
2
1
2
which

 ×

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
= 1)
(7)



2
×
M
–19–
rCIC
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 passband.
Find the –100 dB column in Table III and look down this column
for a value greater than or equal to your passband percentage of
the clock rate. Then look across to the extreme left column and
find the corresponding rate-change factor (M
ring to the table, notice that for a M
having –100 dB of alias 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
the 100 dB required.
An M
rejection; however, the power consumption can be minimized
by decimating as much as possible in this rCIC2 stage. Decima-
tion 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 determine 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.
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 con-
tain 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 con-
tain the scaling factor for the rCIC2 during conditions of weak
signals. In this register, no external attenuator would 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, each external exponent repre-
sents 6 dB per step as in the AD6600. If this bit is set to HIGH,
each exponent represents a 12 dB step.
2
−
BW
L
rCIC2
rCIC
FRACTION
2
/L
×
rCIC2
floor
=
of less than four would still yield the required
100



rCIC2
M
L
rCIC
rCIC
×
10
/L
2
7
2
rCIC2
kHz
MHz
+
1






means less alias rejection than







=
rCIC2
0 07
.
/L
rCIC2
rCIC2
of 4, the frequency
/L
AD6624
rCIC2
). Refer-
(6)
(8)