LF2242
DEVICES INCORPORATED
12/16-bit Half-Band Interpolating/
LF2242
Decimating Digital Filter
12/16-bit Half-Band Interpolating/
DEVICES INCORPORATED
Decimating Digital Filter
FEATURES
DESCRIPTION
The LF2242 is a linear-phase, half-
band (low pass) interpolating/
decimating digital filter that, unlike
intricate analog filters, requires no
tuning. The LF2242 can also signifi-
cantly reduce the complexity of
traditional analog anti-aliasing pre-
filters without compromising the
signal bandwidth or attenuation. This
can be achieved by using the LF2242
as a decimating post-filter with an
A/D converter and by sampling the
signal at twice the rate needed.
Likewise, by using the LF2242 as an
interpolating pre-filter with a D/A
converter, the corresponding analog
reconstruction post-filter circuitry can
be simplified.
sample rate. Once data is clocked in,
the 55-value output response begins
after 7 clock cycles and ends after 61
clock cycles. The pipeline latency
from the input of an impulse response
to its corresponding output peak is 34
clock cycles.
❑ 40 MHz Clock Rate
❑ Passband (0 to 0.22fS)
Ripple: ±0.02 dB
❑ Stopband (0.28fS to 0.5fS)
Rejection: 59.4 dB
❑ User-Selectable 2:1 Decimation or
1:2 Interpolation
The output data may be in either
two’s complement format or inverted
offset binary format. To avoid
truncation errors, the output data is
always internally rounded before it is
latched into the output register.
Rounding is user-selectable, and the
output data can be rounded from 16
bit values down to 8 bit values.
❑ 12-bit Two’s Complement Input
and 16-bit Output with
User-Selectable Rounding, 8- to
16-Bits
❑ User-Selectable Two’s Complement
or Inverted Offset Binary Output
Formats
❑ Three-State Outputs
❑ Replaces TRW/ Raytheon/
Fairchild TMC2242
DC gain of the LF2242 is 1.0015
(0.0126 dB) in pass-through and
decimate modes and 0.5007 (–3.004
dB) in interpolate mode. Passband
ripple does not exceed ±0.02 dB from
0 to 0.22fS with stopband attenuation
greater than 59.4 dB from 0.28fS to
0.5fS (Nyquist frequency). The
❑ Package Styles Available:
• 44-pin PLCC, J-Lead
• 44-pin PQFP
The coefficients of the LF2242 are
fixed, and the only user programming
required is the selection of the mode
(interpolate, decimate, or pass-
through) and rounding. The asyn-
chronous three-state output enable
control simplifies interfacing to a bus.
response of the filter is –6 dB at 0.25fS.
Data can be input into the LF2242 at a Full compliance with CCIR Recom-
rate of up to 40 million samples per mendation 601 (–12 dB at 0.25fS) can
second. Within the 40 MHz I/O limit, be achieved by cascading two devices
the output sample rate can be one-
half, equal to, or two times the input
serially.
LF2242 BLOCK DIAGRAM
TCO RND2–0
3
3
55-TAP
FIR
FILTER
ROUND
AND LIMIT
CIRCUIT
12
12
16
16
INTERPOLATION
CIRCUIT
DECIMATION
CIRCUIT
SI11–0
SO15–0
3
3
OE
CLK
TO ALL REGISTERS
INT DEC SYNC
Video Imaging Products
08/16/2000–LDS.2242-K
1