Feb 2000 New Continuous-Time Lowpass Filters Feature Differential I/O, Linear Phase and Wide Dynamic Range

DESIGN INFORMATION
New Continuous-Time Lowpass Filters
Feature Differential I/O, Linear Phase
and Wide Dynamic Range
by Michael Kultgen and Nello Sevastopoulos
DESIGN INFORMATION
New Continuous-Time Lowpass
Filters Feature Differential I/O,
Linear Phase and Wide Dynamic
Range ..................................... 32
Michael Kultgen and Nello Sevastopoulos
LTC1546 Multiprotocol Chip
Simplifies NET1-, NET2- and TBR2Compliant Serial Interfaces
............................................... 33
Dan Eddleman
LTC2050, Zero-Drift Operational
Amplifier in SOT-23 Package
Minimizes Board Area without
Compromising Specs ............... 34
David Hutchinson
+
VIN
–
0.01µF
5
1
IN+
OUT+
2
IN–
6
OUT–
LTC1565-31
7
3
GND
V+
0.1µF 4
8
–
V
SHDN
20
2.0
ATTENUATION
0
ATENUATION (dB)
requiring linear-phase lowpass filtering with a cutoff frequency in the
vicinity of 650kHz.
Figure 1 shows the extremely
simple hookup of the LTC1565-31.
The filter is packaged in an SO-8 and
it operates on a single 5V supply.
Aside from power supply and analog
ground bypass capacitors, no other
external components are required.
The device is designed to be driven
differentially, yet, for single-ended
applications, either input can be
shorted to the analog ground (pin 3).
Differential signals with a common
mode range of 1.3V to 2.7V and up to
2VP-P amplitude can be DC coupled
into the input pins of the filter without any significant performance
degradation. The maximum dynamic
range (THD plus S/N ratio) is 78dB.
This is obtained with a 2.5VP-P differential signal biased at a 2VDC
common mode voltage.
The filter output should be taken
differentially, as indicated in Figure
1. The common mode voltage of the
differential output is the voltage of
pin 3, which is one-half of the power
supply of the filter. Figure 2 shows the
frequency response of the filter, which
features an almost flat group delay in
the filter’s passband. The amplitude
1.8
–20
1.6
GROUP DELAY
–40
1.4
–60
1.2
–80
1.0
–100
10k
100k
1M
FREQUENCY (Hz)
DELAY (µs)
The new LTC1565-XX family of
monolithic, continuous-time lowpass
filters is tailored for data communication systems requiring band-limiting
in their differential signal paths.
Because it eliminates the need for
external components, The LTC1565XX offers a compact solution to high
frequency, differential filter requirements. Each LTC1565-XX is
optimized for a single cutoff frequency.
The first member of the LTC1565-XX
family, the LTC1565-31, is a 7th order,
linear-phase lowpass filter with a
650kHz cutoff frequency. It is tailored primarily for CDMA cellular base
stations or any other application
0.8
10M
Figure 2. LTC1565-31 frequency response
response has a –3dB gain at 650kHz,
–20dB at 1MHz and in excess of
–70dB at 2MHz.
The out-of-band rejection of the
filter is quite impressive. Depending
on the input signal amplitude, the
stopband attenuation above 3MHz is
between 80dB and 100dB (see Figure
2). The LTC1565-31 achieves these
attenuation levels without prefiltering.
This is highly desirable in applications where strong interfering signals
are present at the inputs of the filter.
Other responses and configurations will be available in the near
future. Contact LTC marketing for
details regarding other cutof f
frequencies.
+
VOUT
–
5V
5V
0.1µF
0.01µF
ON
for
the latest information
on LTC products,
visit
www.linear-tech.com
SHUTDOWN
Figure 1. The LTC1565-31 is extremely simple to hook up.
32
Linear Technology Magazine • February 2000