AD6624AS Analog Devices Inc, AD6624AS Datasheet - Page 20

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

Specifications of AD6624AS

• Lead Free Status / RoHS Status: Not Compliant

AD6624
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 bypassed by setting the L/M ratio to 1/1. This effectively
bypasses all circuitry of the rCIC2 except the scaling, which is still
effectual.
FIFTH ORDER CASCADED INTEGRATOR COMB FILTER
The third signal processing stage, CIC5, implements a sharper,
fixed-coefficient, decimating filter than CIC2. The input rate to
this filter is f
tion 9. N
otherwise N
can be increased, N
(reference fractional rate input timing described in the Input
Timing section).
The decimation ratio, M
(all integer values). The frequency response of the filter is given
by Equation 10. The gain and passband droop of CIC5 should
be calculated by these equations. Both parameters may be com-
pensated for in the RCF stage.
The scale factor, S
between 0 and 20. It serves to control the attenuation of the
data into the CIC5 stage in 6 dB increments. For the best
dynamic range, S
(lowest attenuation) without creating an overflow condition.
This can be safely accomplished using Equation 11, where
OL
to this filter stage. This value is output from the rCIC2 stage,
then pipelined into the CIC5.
The output rate of this stage is given by Equation 12.
S
H z
H f
f
f
rCIC2
SAMP
SAMP
CIC
( )
( )
5
OL
=
2
=
5
is the largest fraction of full scale possible at the input
=
CH
2
≤
≤
2
ceil
S
CH
CIC
S
CIC
equals two for Diversity Channel Real input mode;
f
M
SAMP2
N
1
CIC
f
1
SAMP
CLK
5
+
5
5
CH
(
CIC
equals one. In order to satisfy this equation, M
+
5
log (
5
=
×
×
CIC5
5
2
. The maximum input rate is given by Equa-



(
2






1
M
CIC5
2
CH
sin
–
1
M
S
sin
CIC
CIC
should be set to the smallest value possible
z
–
can be reduced, or f



CIC
–
z
5
is a programmable unsigned integer
π
M
+



5
CIC5
–
5
5
π
CIC
1
M
5
)
5
5
f
f
CIC
×



SAMP
SAMP
, may be programmed from 2 to 32
×
5
OL
f
OL
5
×
2
2
rCIC
rCIC



f









2
2
5
) –
)
5
CLK
can be increased
CIC2
(9)
(10)
(11)
(12)
–20–
CIC5 Rejection
Table IV illustrates the amount of bandwidth in percentage of
the clock rate that can be protected with various decimation
rates and alias rejection specifications. The maximum input rate
into the CIC5 is 80 MHz when the rC1C2 decimates by one.
As in Table III, these are the 1/2 bandwidth characteristics of
the CIC5. Note that the CIC5 stage can protect a much wider
band than the CIC2 for any given rejection.
M
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
This table helps to calculate an upper bound on decimation,
M
RAM COEFFICIENT FILTER
The final signal processing stage is a sum-of-products decimat-
ing filter with programmable coefficients (see Figure 27). The
data memories I-RAM and Q-RAM store the 160 most recent
complex samples from the previous filter stage with 20-bit reso-
lution. The coefficient memory, CMEM, stores up to 256
coefficients with 20-bit resolution. On every CLK cycle, one tap
for I and one tap for Q are calculated using the same coeffi-
cients. The RCF output consists of 24-bit data bits.
CIC5
CIC5
Table IV. SSB CIC5 Alias Rejection Table (f
, given the desired filter characteristics.
–50 dB –60 dB –70 dB –80 dB –90 dB –100 dB
10.227 8.078
7.924
6.213
5.068
4.267
3.68
3.233
2.881
2.598
2.365
2.17
2.005
1.863
1.74
1.632
1.536
1.451
1.375
1.307
1.245
1.188
1.137
1.09
1.046
1.006
0.969
0.934
0.902
0.872
0.844
0.818
6.367
5.022
4.107
3.463
2.989
2.627
2.342
2.113
1.924
1.765
1.631
1.516
1.416
1.328
1.25
1.181
1.119
1.064
1.013
0.967
0.925
0.887
0.852
0.819
0.789
0.761
0.734
0.71
0.687
0.666
6.393
5.11
4.057
3.326
2.808
2.425
2.133
1.902
1.716
1.563
1.435
1.326
1.232
1.151
1.079
1.016
0.96
0.91
0.865
0.824
0.786
0.752
0.721
0.692
0.666
0.641
0.618
0.597
0.577
0.559
0.541
5.066
4.107
3.271
2.687
2.27
1.962
1.726
1.54
1.39
1.266
1.162
1.074
0.998
0.932
0.874
0.823
0.778
0.737
0.701
0.667
0.637
0.61
0.584
0.561
0.54
0.52
0.501
0.484
0.468
0.453
0.439
4.008
3.297
2.636
2.17
1.836
1.588
1.397
1.247
1.125
1.025
0.941
0.87
0.809
0.755
0.708
0.667
0.63
0.597
0.568
0.541
0.516
0.494
0.474
0.455
0.437
0.421
0.406
0.392
0.379
0.367
0.355
SAMP2
3.183
2.642
2.121
1.748
1.48
1.281
1.128
1.007
0.909
0.828
0.76
0.703
0.653
0.61
0.572
0.539
0.509
0.483
0.459
0.437
0.417
0.399
0.383
0.367
0.353
0.34
0.328
0.317
0.306
0.297
0.287
= 1)
REV. B