AD6620SSLASHPCB datasheet(23/44 Pages) AD

23 / 44 page

AD6620

–23–

REV. A

The frequency response of the filter is given by the following

equations. The gain and pass band droop of CIC5 should be

calculated by these equations. Both parameters may be compen-

sated for in the RCF stage.

CIC

()

–

=×









CIC

SAMP

CIC

()

sin

=×





























5 5

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 the equation below, where OLCIC2

is the largest fraction of full scale possible at the input to this

filter stage. This value is output from the CIC2 stage then pipe-

lined into the CIC5. SCIC5 is ignored when this filter is bypassed

by setting MCIC5 = 1.

ceil

CIC

=×

()

= () ×

log (

) –

when CIC5 is bypassed;

CIC

The output rate of this stage is given by the equation below.

SAMP

CIC

≤

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP2

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

Figure 37. CIC2 Alias Rejection, MCIC2 = 16

5TH 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 the

equation below. 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).

SAMP

CLK

≤

The decimation ratio, MCIC5, may be programmed from 1 to 32

(all integer values). When MCIC5 = 1, this stage is bypassed and

the CIC5 scale factor is ignored.

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