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AD6624 データシート(Datasheet) 20 Page - Analog Devices

部品番号. AD6624
部品情報  Four-Channel, 80 MSPS Digital Receive Signal Processor (RSP)
ダウンロード  40 Pages
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メーカー  AD [Analog Devices]
ホームページ  http://www.analog.com
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REV. B
AD6624
–20–
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 fSAMP2. The maximum input rate is given by Equa-
tion 9. NCH equals two for Diversity Channel Real input mode;
otherwise NCH equals one. In order to satisfy this equation, MCIC2
can be increased, NCH can be reduced, or fCLK can be increased
(reference fractional rate input timing described in the Input
Timing section).
f
f
N
SAMP
CLK
CH
2
(9)
The decimation ratio, MCIC5, may be programmed from 2 to 32
(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.
Hz
z
z
S
M
CIC
CIC
()


+
1
2
1
1
5
5
51
5
(10)
Hf
Mf
f
f
f
S
CIC
SAMP
SAMP
CIC
()
sin
sin
×




+
1
2
5
5
5
2
2
5
π
π
The scale factor, SCIC5 is a programmable unsigned integer
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, SCIC5 should be set to the smallest value possible
(lowest attenuation) without creating an overflow condition.
This can be safely accomplished using Equation 11, where
OLrCIC2 is the largest fraction of full scale possible at the input
to this filter stage. This value is output from the rCIC2 stage,
then pipelined into the CIC5.
S
ceil
M
OL
OL
M
OL
CIC
CIC
rCIC
CIC
CIC
S
rCIC
CIC
52
5
5
2
5
5
5
5
2
5
2
5
()
= () ×
+
log (
) –
(11)
The output rate of this stage is given by Equation 12.
f
f
M
SAMP
SAMP
CIC
5
2
5
(12)
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.
Table IV. SSB CIC5 Alias Rejection Table (fSAMP2 = 1)
MCIC5
–50 dB –60 dB
–70 dB
–80 dB –90 dB
–100 dB
2
10.227
8.078
6.393
5.066
4.008
3.183
3
7.924
6.367
5.11
4.107
3.297
2.642
4
6.213
5.022
4.057
3.271
2.636
2.121
5
5.068
4.107
3.326
2.687
2.17
1.748
6
4.267
3.463
2.808
2.27
1.836
1.48
7
3.68
2.989
2.425
1.962
1.588
1.281
8
3.233
2.627
2.133
1.726
1.397
1.128
9
2.881
2.342
1.902
1.54
1.247
1.007
10
2.598
2.113
1.716
1.39
1.125
0.909
11
2.365
1.924
1.563
1.266
1.025
0.828
12
2.17
1.765
1.435
1.162
0.941
0.76
13
2.005
1.631
1.326
1.074
0.87
0.703
14
1.863
1.516
1.232
0.998
0.809
0.653
15
1.74
1.416
1.151
0.932
0.755
0.61
16
1.632
1.328
1.079
0.874
0.708
0.572
17
1.536
1.25
1.016
0.823
0.667
0.539
18
1.451
1.181
0.96
0.778
0.63
0.509
19
1.375
1.119
0.91
0.737
0.597
0.483
20
1.307
1.064
0.865
0.701
0.568
0.459
21
1.245
1.013
0.824
0.667
0.541
0.437
22
1.188
0.967
0.786
0.637
0.516
0.417
23
1.137
0.925
0.752
0.61
0.494
0.399
24
1.09
0.887
0.721
0.584
0.474
0.383
25
1.046
0.852
0.692
0.561
0.455
0.367
26
1.006
0.819
0.666
0.54
0.437
0.353
27
0.969
0.789
0.641
0.52
0.421
0.34
28
0.934
0.761
0.618
0.501
0.406
0.328
29
0.902
0.734
0.597
0.484
0.392
0.317
30
0.872
0.71
0.577
0.468
0.379
0.306
31
0.844
0.687
0.559
0.453
0.367
0.297
32
0.818
0.666
0.541
0.439
0.355
0.287
This table helps to calculate an upper bound on decimation,
MCIC5, given the desired filter characteristics.
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.




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