1
LTC1565-31
650kHz Continuous Time,
Linear Phase Lowpass Filter
March 2000
The LTC
1565-31 is a 7th order, continuous time, linear
phase lowpass filter. The selectivity of the LTC1565-31,
combined with its linear phase and dynamic range, make it
suitable for filtering in data communications or data acqui-
sition systems. The filter attenuation is 37dB at 2
f
CUTOFF
and at least 72dB for frequencies above 3
f
CUTOFF
. Unlike
comparable LC filters, the LTC1565-31 achieves this selec-
tivity with a linear phase response in the passband.
With 5% accuracy of the cutoff frequency, the LTC1565-31
can be used in applications requiring pairs of matched filters,
such as transceiver I and Q channels. Furthermore, the
differential inputs and outputs provide a simple interface for
these wireless systems.
With a single 5V supply and a 2V
P-P
input, the LTC1565-31
features an impressive spurious free dynamic range of 75dB.
The maximum signal-to-noise ratio is 78dB and it is achieved
with a 2.5V
P-P
input signal.
The LTC1565-31 features a shutdown mode where power
supply current is reduced to less than 10
A.
Other cutoff frequencies and single-ended I/O can be pro-
vided upon request. Please contact LTC Marketing.
s
CDMA Base Stations
s
Data Communications
s
Antialiasing Filters
s
Smoothing or Reconstruction Filters
s
Matched Filter Pairs
s
Replacement for LC Filters
s
7th Order, 650kHz Linear Phase Filter in an SO-8
s
Differential Inputs and Outputs
s
Operates on a Single 5V or a
5V Supply
s
Low Offset: 5mV Typical
s
75dB THD and SNR
s
78dB SNR
s
Shutdown Mode
s
Requires No External Components
s
Requires No External Clock Signal
Single 5V Supply, Differential 650kHz Lowpass Filter
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
+IN
IN
GND
V
+OUT
OUT
V
+
SHDN
1
2
3
4
8
7
6
5
LTC1565-31
0.1
F
0.1
F
15645-31 TA01
V
OUT
+
V
OUT
V
IN
+
V
IN
5V
5V
, LTC and LT are registered trademarks of Linear Technology Corporation.
Final Electrical Specifications
FREQUENCY (Hz)
10
4
10
5
10
6
10
7
100
ATTENUATION (dB)
DELAY (
s)
20
0
20
1565-31 TA02
40
60
80
0.8
1.6
1.8
2.0
1.4
1.2
1.0
GAIN
GROUP DELAY
Frequency Response
2
LTC1565-31
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Operating Supply Voltage
4.75
11
V
Filter Gain
V
IN
= 1V
P-P
, f
IN
= 25kHz
q
0.3
0
0.3
dB
f
IN
= 200kHz (Gain Relative to 25kHz)
q
0.2
0
0.1
dB
f
IN
= 300kHz (Gain Relative to 25kHz)
q
0.7
0.4
0.1
dB
f
IN
= 500kHz (Gain Relative to 25kHz)
q
2.2
1.6
1.0
dB
f
IN
= 650kHz (Gain Relative to 25kHz)
q
4
3
2
dB
f
IN
= 900kHz (Gain Relative to 25kHz)
q
11
7
dB
f
IN
= 1.3MHz (Gain Relative to 25kHz)
q
36
31
dB
f
IN
= 2.3MHz (Gain Relative to 25kHz)
72
dB
Filter Phase
V
IN
= 1V
P-P
, f
IN
= 25kHz
13
Deg
f
IN
= 200kHz
101
Deg
f
IN
= 300kHz
q
162
150
138
Deg
f
IN
= 500kHz
113
Deg
f
IN
= 600kHz
q
34
60
85
Deg
f
IN
= 650kHz
36
Deg
f
IN
= 900kHz
92
Deg
Phase Linearity
Ratio of 600kHz Phase/300kHz Phase
q
1.97
2
2.03
Wideband Noise
Noise BW = DC to 2 f
CUTOFF
115
V
RMS
THD
f
IN
= 100kHz, 1V
P-P
(Note 2)
85
dB
Filter Differential DC Swing
Maximum Difference Between Pins 7 and 8
V
S
= 5V
q
1.4
1.7
1.9
V
P
V
S
=
5V
q
2.2
2.3
2.5
V
P
Filter DC Common Mode Range (Note 4)
Upper
V
+
1.5
V
Lower
V
+ 0.8
V
Input Bias Current
0.1
0.3
0.6
A
Input Offset Current
25
nA
Input Resistance
Common Mode, V
IN
= 1.5V to 3.5V
>50
M
Differential
>50
M
Input Capacitance
2
pF
Output DC Offset (Note 3)
V
S
= 5V
5
12
mV
V
S
=
5V (Note 6)
5
12
mV
Total Supply Voltage ............................................... 11V
Power Dissipation ............................................. 500mW
Operating Temperature Range
LTC1565-31CS8 ..................................... 0
C to 70
C
LTC1565-31IS8 ................................. 40
C to 85
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................. 300
C
(Note 1)
ORDER PART
NUMBER
LTC1565-31CS8
LTC1565-31IS8
T
JMAX
= 150
C,
JA
= 80
C/ W (NOTE 5)
Consult factory for Military grade parts.
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ELECTRICAL CHARACTERISTICS
1
2
3
4
8
7
6
5
TOP VIEW
+OUT
OUT
V
+
SHDN
+IN
IN
GND
V
S8 PACKAGE
8-LEAD PLASTIC SO
S8 PART MARKING
156531
56531I
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
S
= 5V (V
+
= 5V, V
= 0V), R
LOAD
= 10k from each output to AC ground,
and Pin 5 open unless otherwise specified.
3
LTC1565-31
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Input and output voltages expressed as peak-to-peak numbers are
assumed to be fully differential.
Note 3: Output DC offset is measured between Pin 8 and Pin 7 with Pin 1
and Pin 2 connected to Pin 3.
Note 4: A 250mV
P-P
, 100kHz differential signal is applied to Pins 1 and 2.
The DC voltages at Pins 1 and 2 are equal. This is the "common mode
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output DC Offset Drift
V
S
= 5V
400
V/
C
V
S
=
5V
400
V/
C
Ground Voltage (Pin 3) in
V
S
= 5V
q
2.49
2.51
2.52
V
Single Supply Applications
SHDN Pin Logic Thresholds
V
S
= 5V, Minimum Logical "1"
4.2
V
V
S
= 5V, Maximum Logical "0"
3.3
V
V
S
=
5V, Minimum Logical "1"
2.75
V
V
S
=
5V, Maximum Logical "0"
2.50
V
SHDN Pin Pull-Up Current
V
S
= 5V
5
A
V
S
=
5V
9
A
Power Supply Current
V
S
= 5V
q
24
31
mA
V
S
=
5V
q
25
33
mA
Power Supply Current in Shutdown Mode
Shutdown. Includes SHDN Pull-Up Current
V
S
= 5V
q
4
8
16
A
V
S
=
5V
q
10
20
40
A
voltage." The "common mode range" is the range of common mode
voltages for which the 250mV
P-P
differential output has better than 65dB
2nd or 3rd harmonic distortion.
Note 5: Thermal resistance varies depending upon the amount of PC board
metal attached to the device.
JA
is specified for a 3.8 square inch test
board covered with 2 oz copper on both sides.
Note 6: Output DC offset measurements are performed by automatic test
equipment approximately 0.5 seconds after application of power.
+IN, IN (Pins 1, 2): Input Pins. Signals can be applied to
either or both input pins. The DC gain from differential
inputs (Pin 1 to Pin 2) to the differential outputs (Pin 8 to
Pin 7) is 1.0V/V. The input range is described in the
Applications Information section.
GND (Pin 3): Ground. The ground pin is the reference
voltage for the filter and is internally biased to one-half the
total power supply voltage of the filter, maximizing the
dynamic range of the filter. For single supply operation,
the ground pin should be bypassed with a quality 0.1
F
ceramic capacitor to Pin 4. For dual supply operation,
connect Pin 3 to a high quality DC ground. A ground plane
should be used. A poor ground will increase noise and
distortion.
PI
N
FU
N
CTIO
N
S
U
U
U
The impedance seen at Pin 3 is 2.5k
in normal mode. In
shutdown, the pin is internally biased to the same levels
as normal mode. The impedance in shutdown mode is
typically 500k
but varies with supply voltage and
temperature.
V
, V
+
(Pins 4, 6): Power Supply Pins. For a single 5V
supply (Pin 4 grounded), a quality 0.1
F ceramic bypass
capacitor is required from the positive supply pin (Pin 6)
to the negative supply pin (Pin 4). The bypass should be as
close as possible to the IC. For dual supply applications
(Pin 3 is grounded), bypass Pin 6 to Pin 3 and Pin 4 to Pin
3 with a quality 0.1
F ceramic capacitor.
The maximum voltage difference between the ground pin
(Pin 3) and the positive supply pin (Pin 6) should not
exceed 5.5V.
ELECTRICAL CHARACTERISTICS
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
S
= 5V (V
+
= 5V, V
= 0V), R
LOAD
= 10k from each output to AC ground,
and Pin 5 open unless otherwise specified.
4
LTC1565-31
PI
N
FU
N
CTIO
N
S
U
U
U
SHDN (Pin 5): Shutdown. When the Pin 5 voltage is low,
the LTC1565-31 goes into the current saving shutdown
mode. Pin 5 has a 4
A pull-up current. Leaving Pin 5 open
will place the LTC1565-31 in its normal operating mode.
OUT, + OUT (Pins 7, 8): Output Pins. Pins 7 and 8 are the
filter differential output. Each pin can drive 1k
or 300pF
loads. The common mode voltage at the output pins is the
same as the voltage at Pin 3.
BLOCK DIAGRA
W
1
+
R
INPUT BUFFERS
WITH COMMON MODE
TRANSLATION CIRCUIT
R
5k
5k
~1M
SHUTDOWN
SWITCH
+IN
IN
7th ORDER
LINEAR
PHASE
FILTER
NETWORK
2
GND
V
+
+
~1M
V
V
+
SHUTDOWN
SWITCH
3
6
4
5
8 +OUT
7 OUT
V
+
OUTPUT
BUFFER
OUTPUT
BUFFER
SHUTDOWN
SHDN
1565-31 BD
4
A
V
+
5
LTC1565-31
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
Interfacing to the LTC1565-31
The difference between the voltages at Pin 1 and Pin 2 is
the "differential input voltage." The average of the voltages
at Pin 1 and Pin 2 is the "common mode input voltage."
The difference between the voltages at Pin 7 and Pin 8 is
the "differential output voltage." The average of the volt-
ages at Pin 7 and Pin 8 is the "common mode output
voltage." The input and output common mode voltages
are independent. The input common mode voltage is set
by the signal source, if DC coupled, or by the biasing
network if AC coupled (Figures 1 and 2). The output
common mode voltage is equal to the voltage of Pin 3, the
GND pin. The GND pin is biased to one-half of the supply
voltage by an internal resistive divider (see Block Dia-
gram). To alter the common mode output voltage, Pin 3
can be driven with an external voltage source or resistor
network. If external resistors are used, it is important to
note that the internal 5k resistors can vary
20% (their
ratio only varies
2%). The output can also be AC coupled.
Input Common Mode and Differential Voltage Range
The range of voltage each input can support while operat-
ing in its linear region is typically 0.8V to 3.5V for a single
5V supply and 4.2V to 3.2V for a
5V supply. Therefore,
the filter can accept a variety of common mode input
voltages. Figures 3 and 4 show the THD of the filter versus
common mode input voltage with a 2V
P-P
differential input
signal.
+
+IN
IN
GND
V
+OUT
OUT
V
+
SHDN
1
2
3
4
8
7
6
5
LTC1565-31
0.1
F
0.1
F
15645-31 F01
V
OUT
+
V
OUT
5V
V
IN
+
V
IN
+
V
IN
+
+ V
IN
2
DC COUPLED INPUT
V
IN
(COMMON MODE) =
V
OUT
+
+ V
OUT
2
V
OUT
(COMMON MODE) =
=
V
+
2
+
+IN
IN
GND
V
+OUT
OUT
V
+
SHDN
1
2
3
4
8
7
6
5
LTC1565-31
1
F
0.1
F
0.1
F
100k
100k
0.1
F
15645-31 F02
V
OUT
+
V
OUT
5V
V
IN
+
V
IN
+
AC COUPLED INPUT
V
IN
(COMMON MODE) = V
OUT
(COMMON MODE)
=
V
+
2
Figure 1
Figure 2
INPUT COMMON MODE VOLTAGE (V)
90
THD (dB)
70
50
30
80
60
40
3
1
1
3
1565-31 F03
5
4
5
2
0
2
4
V
IN
= 2V
P-P
V
S
=
5V
f
IN
= 100kHz
INPUT COMMON MODE VOLTAGE (V)
0.5
80
THD (dB)
70
60
50
1.0
1.5
2.0
2.5
1565-31 F04
3.0
40
30
3.5
V
IN
= 2V
P-P
V
S
=
5V
f
IN
= 100kHz
Figure 3. THD vs Common Mode Input Voltage
Figure 4. THD vs Common Mode Input Voltage
Figure 5 shows the THD and S/N ratio versus differential
input voltage level for both a single 5V supply and a
5V
supply. The common mode voltage of the input signal is
one-half the total power supply voltage of the filter. The
spurious free dynamic range, where the THD and S/N ratio
are equal, is 75dB to 77dB when the differential input
voltage level is 2V
P-P
; that is, for a single 5V supply, the
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