AD6620
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
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.
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.
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.
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.
(–f
2 TO f
2)
samp/
WIDEBAND INPUT SPECTRUM
SIGNAL OF INTEREST "IMAGE"
samp/
SIGNAL OF
INTEREST
C'
C
A
A'
B
B'
D
D'
–f /2
–3f /8 –5f /16 –f /4 –3f /16 –f /8
–f /16
f
/16
f
/8
3f /16
f
/4
5f /16
3f /8
S
f /2
S
DC
S
S
S
S
S
S
S
S
S
S
S
S
Figure 2a. Wideband Input Spectrum (e.g., 30 MHz from High-Speed ADC)
NCO "TUNES" SIGNAL TO BASEBAND
AFTER FREQUENCY TRANSLATION
C'
C
A'
A
B'
B
D'
D
–f /2
–3f /8 –5f /16
–f /4 –3f /16 –f /8
–f /16
DC
f
/16
f
/8
3f /16
f
/4
5f /16
3f /8
f /2
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Figure 2b. Frequency Translation (e.g., Single 1 MHz Channel Tuned to Baseband)
CIC2, CIC5, AND RCF
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
FREQUENCY
Figure 2c. Baseband Signal is Decimated and Filtered by CIC2, CIC5, RCF
REV. A
–3–
ADI [ ADI ]
