AD6620AS datasheet(3/44 Pages) AD

3 / 44 page

AD6620

–3–

REV. A

Following CIC2 is the second fixed-coefficient decimating filter.

This filter, CIC5, further reduces the sample rate by a program-

mable ratio from 1 to 32. The data rate out of CIC5, fSAMP5, is

determined by the decimation factors of MCIC5 and MCIC2.

Each CIC stage is a FIR filter whose response is defined by the

decimation rate. The purpose of these filters is to reduce the

data rate of the incoming signal so that the final filter stage, a FIR

RAM coefficient sum-of-products filter (RCF), can calculate

more taps per output. As shown in Figure 1, on-chip multiplex-

ers allow both CIC filters to be bypassed if a multirate clock

is used.

The fourth stage is a sum-of-products FIR filter with program-

mable 20-bit coefficients, and decimation rates programmable

from 1 to 32. The RAM Coefficient FIR Filter (RCF in Figure

1) can handle a maximum of 256 taps.

The overall filter response for the AD6620 is the composite of

all three cascaded decimating filters: CIC2, CIC5, and RCF. Each

successive filter stage is capable of narrower transition band-

widths but requires a greater number of CLK cycles to calculate

the output. More decimation in the first filter stage will minimize

overall power consumption. Data comes out via a parallel port

or a serial interface.

Figure 2 illustrates the basic function of the AD6620: to select

and filter a single channel from a wide input spectrum. The

frequency translator “tunes” the desired carrier to baseband.

CIC2 and CIC5 have fixed order responses; the RCF filter

provides the sharp transitions. More detail is provided in later

sections of the data sheet.

–fS/2

–3fS/8

–5fS/16

–fS/4

–3fS/16

–fS/8

–fS/16

fS/16

fS/8

3fS/16

fS/4

5fS/16

fS/2

3fS/8

SIGNAL OF

INTEREST

SIGNAL OF INTEREST "IMAGE"

WIDEBAND INPUT SPECTRUM

(–fsamp/2 TO fsamp/2)

Figure 2a. Wideband Input Spectrum (e.g., 30 MHz from High-Speed ADC)

–fS/2

–3fS/8

–5fS/16

–fS/4

–3fS/16

–fS/8

–fS /16

fS/16

fS/8

3fS/16

fS/4

5fS/16

fS/2

3fS/8

AFTER FREQUENCY TRANSLATION

NCO "TUNES" SIGNAL TO BASEBAND

Figure 2b. Frequency Translation (e.g., Single 1 MHz Channel Tuned to Baseband)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

CIC2, CIC5, AND RCF

FREQUENCY

Figure 2c. Baseband Signal is Decimated and Filtered by CIC2, CIC5, RCF

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