AD6620SSLASHPCB datasheet(25/43 Pages) AD

25 / 43 page

AD6620

–25–

REV. 0

The plots following (Figure 43) represent the CIC5 transfer

function with respect to the CIC5 output rate for a decimation

of 4. The first plot is referenced to the input sample rate and

shows the complex spectrum from –fSAMP/2 to +fSAMP/2. The

second plot is referenced to the CIC5 output rate; the complex

spectrum ranges from –fSAMP5/2 to +fSAMP5/2. Aliased images in

CIC5 “fold back” toward the edge of the desired filter pass

band. It is the level of these aliases as they move into the desired

pass band that are of concern. The improved roll-off of CIC5

over CIC2 can be seen when these plots are compared to those

previously shown for CIC2.

–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/fSAMP5

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

Figure 43. CIC5 Alias Rejection, MCIC5 = 4

The set of plots below (Figure 44) represents a decimation of 32

in the CIC5 filter. It can be seen that the lobes of the filter drop

as the decimation rate increases, but the aliased frequencies

increase due to the reduction of the output rate.

–120

–100

–80

–60

–40

–20

f/fSAMP

–0.3

–0.2

–0.1

0.1

0.2

0.3

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP5

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

Figure 44. CIC5 Alias Rejection, MCIC5 = 32

RAM COEFFICIENT FILTER

The final signal processing stage is a sum-of-products decimat-

ing filter with programmable coefficients. Figure 45 shows a

simplified block diagram. The data memories I-RAM and Q-RAM

store the 256 most recent complex samples from the previous

filter stage with 18-bit resolution. The coefficient memory,

C-RAM, stores up to 256 coefficients with 20-bit resolution. On

each CLK cycle one tap for I and one tap for Q is calculated

using the same coefficients. The I and Q accumulators provide

3 bits of headroom. This headroom allows the output of the

RCF filter to contain 23 significant bits.

IIN

QIN

IOUT

QOUT

256 18b

I-RAM

256 20b

C-RAM

256 18b

Q-RAM

Figure 45. RAM Coefficient Filter Block Diagram

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