FEATURES
■
Continuous Time—No Clock
■
Four 2nd Order Filter Sections, 20kHz to 300kHz
Center Frequency
■
Butterworth, Chebyshev, Elliptic or Equiripple
Delay Response
■
Lowpass, Bandpass, Highpass Responses
■
99dB Typical S/N, ±5V Supply (Q = 1)
■
93dB Typical S/N, Single 5V Supply (Q = 1)
■
Rail-to-Rail Input and Output Voltages
■
DC Accurate to 3mV (Typ)
■
±0.5% Typical Center Frequency Accuracy
■
“Zero-Power” Shutdown Mode
■
Single or Dual Supply, 5V to 10V Total
■
Resistor-Programmable fO, Q, Gain
U
APPLICATIO S
■
High Resolution Systems (14 Bits to 18 Bits)
■
Antialiasing/Reconstruction Filters
■
Data Communications, Equalizers
■
Dual or I-and-Q Channels (Two Matched 4th Order
Filters in One Package)
■
Linear Phase Filtering
■
Replacing LC Filter Modules
LTC1562-2
Very Low Noise, Low Distortion
Active RC Quad Universal Filter
U
DESCRIPTIO
The LTC®1562-2 is a low noise, low distortion continuous
time filter with rail-to-rail inputs and outputs, optimized for a
center frequency (fO) of 20kHz to 300kHz. Unlike most
monolithic filters, no clock is needed. Four independent 2nd
order filter blocks can be cascaded in any combination, such
as one 8th order or two 4th order filters. Each block’s
response is programmed with three external resistors for
center frequency, Q and gain, using simple design formulas.
Each 2nd order block provides lowpass and bandpass outputs. Highpass response is available if an external capacitor
replaces one of the resistors. Allpass, notch and elliptic
responses can also be realized.
The LTC1562-2 is designed for applications where dynamic
range is important. For example, by cascading 2nd order
sections in pairs, the user can configure the IC as a dual 4th
order Butterworth lowpass filter with 90dB signal-to-noise
ratio from a single 5V power supply. Low level signals can
exploit the built-in gain capability of the LTC1562-2. Varying
the gain of a section can achieve a dynamic range as high as
114dB with a ±5V supply.
Other cutoff frequency ranges can be provided upon request.
Please contact LTC Marketing.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Dual 4th Order 200kHz Butterworth Lowpass Filter, SNR 96dB
R
7.87k
IN1
V
IN1
5V
R
IN3
V
IN2
*V– ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
7.87k
RQ1 4.22k
R21 7.87k
0.1µF
R23 7.87k
R
4.22k
Q3
1
INV B
2
V1 B
3
V2 B
5
+
LTC1562-2
V
6
SHDN
8
V2 A
9
V1 A
10
INV A
U
Amplitude Response
R
7.87k
IN2
V
0.1µF
OUT1
–5V*
V
OUT2
INV C
V1 C
V2 C
AGND
V2 D
V1 D
INV D
20
10.2k
R
Q2
19
R22 7.87k
18
16
–
V
15
R24 7.87k
13
10.2k
R
Q4
12
11
R
7.87k
IN4
1562-2 TA01
10
0
–10
–20
–30
–40
GAIN (dB)
–50
–60
–70
–80
50k
100k
1.5M1M
FREQUENCY (Hz)
1562-2 TA02
1
LTC1562-2
WW
W
U
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Total Supply Voltage (V+ to V–) .............................. 11V
Maximum Input Voltage
at Any Pin ....................(V– – 0.3V) ≤ V ≤ (V+ + 0.3V)
Storage Temperature Range ................. –65°C to 150°C
Operating Temperature Range
LTC1562C-2 ............................................ 0°C to 70°C
LTC1562I-2 ........................................ –40°C to 85°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at T
specs are for a single 2nd order section, R
= 25°C. VS = ±5V, outputs unloaded, SHDN pin to logic “low”, unless otherwise noted. AC
A
= R2 = 10.4k ±0.1%, RQ = 9.09k ±0.1%, f
IN
The ● denotes specifications that apply over the full operating temperature
U
W
PACKAGE/ORDER INFORMATION
TOP VIEW
INV B
1
V1 B
2
V2 B
3
–*
V
4
+
V
5
SHDN
6
–*
V
7
V2 A
8
V1 A
9
INV A
10
G PACKAGE
20-LEAD PLASTIC SSOP
*G PACKAGE PINS 4, 7, 14, 17 ARE
SUBSTRATE/SHIELD CONNECTIONS
AND MUST BE TIED TO V
T
= 150°C, θJA = 136°C/W
JMAX
Consult factory for Military grade parts.
= 175kHz.
O
20
INV C
19
V1 C
18
V2 C
–*
17
V
–
16
V
15
AGND
–*
14
V
13
V2 D
12
V1 D
11
INV D
–
ORDER PART
NUMBER
LTC1562CG-2
LTC1562IG-2
U
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
S
I
S
V
OS
H
L
Total Supply Voltage 4.75 10.5 V
Supply Current VS = ±2.375V, RL = 5k, CL = 30pF, Outputs at 0V 21 23.5 mA
= ±5V, RL = 5k, CL = 30pF, Outputs at 0V 22.5 25 mA
V
S
VS = ±2.375V, RL = 5k, CL = 30pF, Outputs at 0V ● 28 mA
V
= ±5V, RL = 5k, CL = 30pF, Outputs at 0V ● 30 mA
S
Output Voltage Swing, V2 Outputs VS = ±2.375V, RL = 5k, CL = 30pF ● 4.2 4.6 V
VS = ±5V, RL = 5k, CL = 30pF ● 9.3 9.8 V
Output Voltage Swing, V1 Outputs VS = ±2.375V, RL = 5k, CL = 30pF, f = 250kHz 4.5 V
VS = ±5V, RL = 5k, CL = 30pF, f = 250kHz 8.4 9.7 V
DC Offset Magnitude, V2 Outputs VS = ±2.375V, Input at AGND Voltage 3 17 mV
= ±5V, Input at AGND Voltage 3 17 mV
V
S
DC AGND Reference Point VS = Single 5V Supply 2.5 V
Center Frequency (fO) Error (Notes 2, 3) VS = ±5V, V2 Output Has RL = 5k, CL = 30pF 0.5 1.7 %
Lowpass Passband Gain at V2 Output VS = ±2.375V, fIN = 10kHz, ● 0 +0.05 +0.1 dB
V2 Output Has R
Q Accuracy VS = ±2.375V, V2 Output Has RL = 5k, CL = 30pF +2 %
Wideband Output Noise VS = ±2.375V, BW = 400kHz, Input AC GND 39 µV
VS = ±5V, BW = 400kHz, Input AC GND 39 µV
Input-Referred Noise, Gain = 100 BW = 400kHz, fO = 200kHz, Q = 1, Input AC GND 7.3 µV
= 5k, CL = 30pF
L
P-P
P-P
P-P
P-P
RMS
RMS
RMS
2
LTC1562-2
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at T
specs are for a single 2nd order section, R
= 25°C. VS = ±5V, outputs unloaded, SHDN pin to logic “low”, unless otherwise noted. AC
A
= R2 = 10.4k ±0.1%, RQ = 9.09k ±0.1%, f
IN
The ● denotes specifications that apply over the full operating temperature
= 175kHz.
O
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
THD Total Harmonic Distortion, V2 Output fIN = 20kHz, 2.8V
R
= 5k, CL = 30pF
L
fIN = 20kHz, 9V
= 5k, CL = 30pF
R
L
Shutdown Supply Current SHDN Pin to V
SHDN Pin to V
, V1 and V2 Outputs Have –100 dB
P-P
, V1 and V2 Outputs Have – 82 dB
P-P
+
+
, VS = ±2.375V 1.0 µA
1.5 15 µA
Shutdown-Input Logic Threshold 2.5 V
Shutdown-Input Bias Current SHDN Pin to 0V –10 – 20 µA
Shutdown Delay SHDN Pin Steps from 0V to V
+
20 µs
Shutdown Recovery Delay SHDN Pin Steps from V+ to 0V 100 µs
Inverting Input Bias Current, Each Biquad 5 pA
Note 1: Absolute Maximum Ratings are those values beyond which the life
Note 3: Tighter frequency tolerance is available, consult factory.
of a device may be impaired.
Note 2: f
change from ±5V to ±2.375 supplies is – 0.2% typical,
O
temperature coefficient magnitude, 25°C to 85°C, is
f
O
50ppm/°C typical.
As with the LTC1562, fO decreases with increasing temperature.
UW
TYPICAL PERFOR A CE CHARACTERISTICS
fO Error vs Nominal fO (V
3.0
TA = 25°C
2.5
= R
R
IN
120
Q
160 200 240 280260140 180 220
NOMINAL fO (kHz)
2.0
1.5
1.0
0.5
0
–0.5
ERROR (%)
O
f
–1.0
–1.5
–2.0
–2.5
–3.0
= ±5V)
S
Q = 5
Q = 2.5
Q = 1
1562-2 G01
fO Error vs Nominal fO (V
3.0
TA = 25°C
2.5
= R
R
IN
120
Q
160 200 240 280260140 180 220
NOMINAL fO (kHz)
2.0
1.5
1.0
0.5
0
–0.5
ERROR (%)
O
f
–1.0
–1.5
–2.0
–2.5
–3.0
S
Q = 5
Q = 2.5
Q = 1
= ±2.5V)
1562-2 G02
Q Error vs Nominal fO (V
45
40
35
30
25
20
15
Q ERROR (%)
10
5
0
–5
100
TA = 70°C
= 25°C
T
A
= RQ
R
IN
140 180 220 300240120 160 200 280
NOMINAL fO (kHz)
= ±5V)
S
Q = 5
Q = 2.5
Q = 1
260
1562-2 G03
3
LTC1562-2
UW
TYPICAL PERFOR A CE CHARACTERISTICS
Q Error vs Nominal fO (V
55
50
45
40
35
30
25
20
Q ERROR (%)
15
10
5
0
–5
100
TA = 70°C
= 25°C
T
A
= RQ
R
IN
140 180 220 300240120 160 200 280
NOMINAL fO (kHz)
LP Noise vs Nominal f
(V
= ±5V, 25°C) (Figure 3,
S
V2 Output) (RIN = R2)
100
90
80
)
70
RMS
60
50
40
LP NOISE (µV
30
20
10
120
140 180
160
NOMINAL fO (kHz)
Q = 5
Q = 2.5
Q = 1
200
Q = 5
Q = 2.5
O
220
= ±2.5V)
S
Q = 1
260
240
1562-2 G04
260
1562-2 G07
280
Peak BP Gain vs Nominal f
(V
= ±5V) (Figure 3, V1 Output)
S
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
PEAK BP GAIN (dB)
0.75
0.50
0.25
0
100
TA = 70°C
T
R
= RQ
IN
120 160
140
BP Noise vs Nominal f
(V
= ±5V, 25°C) (Figure 3,
S
V1 Output) (RIN = RQ)
100
90
80
)
70
RMS
60
50
40
BP NOISE (µV
30
20
10
120
140 180
160
NOMINAL fO (kHz)
= 25°C
A
200 280
180
NOMINAL fO (kHz)
Q = 5
Q = 2.5
Q = 1
200
220
Q = 2.5
220
O
240
Q = 5
Q = 1
240
O
260
260
1562-2 G08
Peak BP Gain vs Nominal f
(V
= ±2.5V) (Figure 3, V1 Output)
S
1562-2 G5
300
4.00
3.75
3.50
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
PEAK BP GAIN (dB)
1.00
0.75
0.50
0.25
0
100
TA = 70°C
T
A
R
= RQ
IN
120 160
140
= 25°C
Distortion vs External Load
Resistance and Frequency
(V
= ±5V, 25°C) (Figure 8)
S
0
2nd ORDER LOWPASS
–10
= 200kHz
f
O
Q = 0.7
–20
OUTPUT LEVEL 1V
±5V SUPPLIES
–30
–40
–50
–60
–70
–80
–90
THD (AMPLITUDE BELOW FUNDAMENTAL) (dB)
–100
280
10k
EXTERNAL LOAD RESISTANCE (Ω)
RMS
fIN = 100kHz
f
= 50kHz
IN
= 20kHz
f
IN
5k
Q = 5
Q = 2.5
Q = 1
200 280
220
180
NOMINAL fO (kHz)
(2.83V
)
P-P
2k
240
1562-2 G09
260
O
300
1562-2 G6
THD (AMPLITUDE BELOW FUNDAMENTAL) (%)
100
10
1
0.1
0.01
0.001
1k
UUU
PIN FUNCTIONS
Power Supply Pins: The V+ and V– pins should be
bypassed with 0.1µF capacitors to an adequate analog
ground or ground plane. These capacitors should be
connected as closely as possible to the supply pins. Pins
4, 7, 14 and 17 are internally connected to V– (Pin 16) and
should also be tied to the same point as Pin 16 for best
shielding. Low noise linear supplies are recommended.
Switching supplies are not recommended as they will
lower the filter dynamic range.
4
Analog Ground (AGND): The AGND pin is the midpoint of
a resistive voltage divider, developing a potential halfway
between the V+ and V– pins, with an equivalent series
resistance nominally 7k. This serves as an internal ground
reference. Filter performance will reflect the quality of the
analog signal ground and an analog ground plane
surrounding the package is recommended. The analog
ground plane should be connected to any digital ground at
a single point. For dual supply operation, the AGND pin
UUU
PIN FUNCTIONS
LTC1562-2
should be connected to the ground plane (Figure 1). For
single supply operation, the AGND pin should be bypassed
to the ground plane with at least a 0.1µF capacitor (at least
1µF for best AC performance) (Figure 2).
ANALOG
GROUND
PLANE
+
V
0.1µF
SINGLE-POINT
SYSTEM GROUND
10
1
2
3
4
5
6
7
8
9
LTC1562-2
20
19
18
17
16
15
14
13
12
11
GROUND PLANE
DIGITAL
(IF ANY)
–
V
0.1µF
Shutdown (SHDN): When the SHDN input goes high or is
open-circuited, the LTC1562-2 enters a “zero-power”
shutdown state and only junction leakage currents flow.
The AGND pin and the amplifier outputs (see Figure 3)
assume a high impedance state and the amplifiers effectively disappear from the circuit. (If an input signal is
applied to a complete filter circuit while the LTC1562-2 is
in shutdown, some signal will normally flow to the output
through passive components around the inactive op amps.)
A small pull-up current source at the SHDN input
defaults
the LTC1562-2 to the shutdown state if the SHDN pin is left
floating
. Therefore, the user
must
connect the SHDN pin
to a logic “low” (0V for ±5V supplies, V– for 5V total
supply) for normal operation of the LTC1562-2. (This
convention permits true “zero-power” shutdown since not
even the driving logic must deliver current while the part
is in shutdown.) With a single supply voltage, use V– for
logic “low,” do not connect SHDN to the AGND pin.
1562-2 F01
Figure 1. Dual Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
ANALOG
GROUND
PLANE
+
V
0.1µF
SINGLE-POINT
SYSTEM GROUND
10
1
2
3
4
5
6
7
8
9
LTC1562-2
20
19
18
17
16
15
14
13
12
+
V
11
GROUND PLANE
/2
REFERENCE
DIGITAL
(IF ANY)
1µF
1562-2 F01
Figure 2. Single Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)
1/4 LTC1562-2
1
sR1C*
*R1 AND C ARE PRECISION
INTERNAL COMPONENTS
C
–
+
V2 V1
ZIN TYPE
R
C
R2
RESPONSE
AT V1
BANDPASS
HIGHPASS
RESPONSE
AT V2
LOWPASS
BANDPASS
INV
Z
IN
+
V
IN
–
R
Q
IN EACH CASE,
= (200kHz)
f
O
RQ
Q =
R2
7958Ω
()
R2
200kHz
()
f
O
1562-2 F03
Figure 3. Equivalent Circuit of a Single 2nd Order Section
(Inside Dashed Line) Shown in Typical Connection. Form of
ZIN Determines Response Types at the Two Outputs (See Table)
5
LTC1562-2
PIN FUNCTIONS
UUU
INV A, INV B, INV C, INV D: Each of the INV pins is a virtualground summing point for the corresponding 2nd order
section. For each section, all three external components
ZIN, R2, RQ connect to the INV pin as shown in Figure 3 and
described further in the Applications Information. Note
that the INV pins are sensitive internal nodes of the filter
and will readily receive any unintended signals that are
capacitively coupled into them. Capacitance to the INV
nodes will also affect the frequency response of the filter
sections. For these reasons, printed circuit connections to
the INV pins must be kept as short as possible, less than
one inch (2.5cm) total and surrounded by a ground plane.
V1 A, V1 B, V1 C, V1 D: Output Pins. Provide a bandpass,
highpass or other response depending on external circuitry (see Applications Information section). Each V1 pin
W
BLOCK DIAGRA
Overall Block Diagram Showing Four 3-Terminal 2nd Order Sections
also connects to the RQ resistor of the corresponding 2nd
order filter section (see Figure 3 and Applications Information). Each output is designed to drive a nominal net load
of 4kΩ and 30pF, which includes the loading due to the
external RQ. Distortion performance improves when the
outputs are loaded as lightly as possible.
V2 A, V2 B, V2 C, V2 D: Output Pins. Provide a lowpass,
bandpass or other response depending on external circuitry (see Applications Information section). Each V2 pin
also connects to the R2 resistor of the corresponding 2nd
order filter section (see Figure 3 and Applications Information). Each output is designed to drive a nominal net load
of 4kΩ and 30pF, which includes the loading due to the
external R2. Distortion performance improves when the
outputs are loaded as lightly as possible.
SHDN
INV V1 V2
AB
–
+
+
V
–
V
V
SHUTDOWN
SWITCH
R
R
SHUTDOWN
SWITCH
–
V
AGND
+
DC
+
–
INV V1 V2 INV V1 V2
C
∫∫
2ND ORDER SECTIONS
∫∫
CC
INV V1 V2
–
+
+
–
C
1562-2 BD
6
LTC1562-2
U
WUU
APPLICATIONS INFORMATION
The LTC1562-2 contains four matched, 2nd order,
3-terminal universal continuous-time filter blocks, each
with a virtual-ground input node (INV) and two rail-to-rail
outputs (V1, V2). In the most basic application, one such
block and three external resistors provide 2nd order
lowpass and bandpass responses simultaneously (Figure
3, with a resistor for ZIN). The three external resistors
program fO, Q and gain. A combination of internal precision components and external resistor R2 sets the center
frequency fO of each 2nd order block. The LTC1562-2 is
trimmed at manufacture so that fO will be 200kHz ±0.5%
if the external resistor R2 is exactly 7958Ω. The LTC15622 is a higher frequency, pin compatible variant of the
LTC1562, with different internal R and C values and higher
speed amplifiers.
However, lowpass/bandpass filtering is only one specific
application for the 2nd order building blocks in the
LTC1562-2. Highpass response results if the external
impedance ZIN in Figure 3 becomes a capacitor CIN (whose
value sets only gain, not critical frequencies) as described
below. Responses with zeroes (e.g, elliptic or notch
responses) are available by feedforward connections with
multiple 2nd order blocks (see Typical Applicatons). Moreover, the virtual-ground input gives each 2nd order section the built-in capability for analog operations such as
gain (preamplification), summing and weighting of multiple inputs, or accepting current or charge signals directly. These Operational FilterTM frequency-selective
building blocks are nearly as versatile as operational
amplifiers.
The user who is not copying exactly one of the Typical
Applications schematics shown later in this data sheet is
urged to read carefully the next few sections through at
least Signal Swings, for orientation about the LTC1562-2,
before attempting to design custom application circuits.
Also available free from LTC, and recommended for designing custom filters, is the general-purpose analog filter
design software FilterCADTM for Windows®. This software
includes tools for finding the necessary f0, Q and gain
parameters to meet target filter specifications such as
frequency response.
Setting fO and Q
Each of the four 2nd order sections in the LTC1562-2 can
be programmed for a standard filter function (lowpass,
bandpass or highpass) when configured as in Figure 3
with a resistor or capacitor for ZIN. These transfer functions all have the same denominator, a complex pole pair
with center frequency ωO = 2πfO and quality parameter Q.
(The numerators depend on the response type as described below.) External resistors R2 and RQ set fO and Q
as follows:
f
==
O
==
Q
R1 = 7958Ω and C = 100pF are internal to the LTC1562-2
while R2 and RQ are external.
A typical design procedure proceeds from the desired f
and Q as follows, using finite-tolerance fixed resistors.
First find the ideal R2 value for the desired fO:
R Ideal
2
()
Then select a practical R2 value from the available finitetolerance resistors. Use the actual R2 value to find the
desired RQ, which also will be approximated with finite
tolerance:
RQ R
= ()7958 2Ω
Q
The fO range is approximately 20kHz to 300kHz, limited
mainly by the magnitudes of the external resistors
required. As shown above, R2 varies with the inverse
square of fO. This relationship desensitizes fO to R2’s
tolerance (by a factor of 2 incrementally), but it also
implies that R2 has a wider range than fO. (RQ and RIN also
Operational Filter and FilterCAD are trademarks of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
1
π ()
CRR
212
R
QQQ
12
()
RR
200
=
f
7958
R
Ω
7958 2
()
2
kHz
O
7958
()
Ω
200
kHz
()
2
R
R
=
R
Ω
200
2
R
f
O
kHz
O
7
LTC1562-2
U
WUU
APPLICATIONS INFORMATION
tend to scale with R2.)
4k, heavily loading the outputs of the LTC1562-2 and
leading to increased THD and other effects. At the other
extreme, a lower fO limit of 20kHz reflects an arbitrary
upper resistor limit of 1MΩ. The LTC1562-2’s MOS input
circuitry can accommodate higher resistor values than
this, but junc
tion leakage current from the input protection
circuitry may cause DC errors.
The 2nd order transfer functions HLP(s), HBP(s) and
HHP(s) (below) are all inverting so that, for example, at DC
the lowpass gain is –HL. If two such sections are cascaded, these phase inversions cancel. Thus, the filter in the
application schematic on the first page of this data sheet
is a dual DC preserving, noninverting, rail-to-rail lowpass
filter, approximating two “straight wires with frequency
selectivity.”
Figure 4 shows further details of 2nd order lowpass,
bandpass and highpass responses. Configurations to
obtain these responses appear in the next three sections.
At high fO these resistors fall below
Basic Lowpass
When ZIN of Figure 3 is a resistor of value RIN, a standard
2nd order lowpass transfer function results from VIN to V2
(Figure 5):
2
Vs
2
()
Vs
()
IN
HL = R2/R
Hs
()
==
LP
is the DC gain magnitude. (Note that the
IN
2
sQs
+
ω
H
–
LO
2
ωω
()
OO
+
/
transfer function includes a sign inversion.) Parameters
R
IN
V
IN
R2R
Q
V
OUT
INV V1
2nd ORDER
1/4 LTC1562-2
Figure 5. Basic Lowpass Configuration
V2
1562 F05
0.707 H
GAIN (V/V)
BANDPASS RESPONSE
H
B
B
f
f
f
L
O
f (LOG SCALE)
f
O
Q
==
–
ff
HL
–121
ff
=
L
O
=+
ff
HO
H
;
fff
OLH
2
+
+
2
QQ
2
121
QQ
+
2
1
1
0.707 H
GAIN (V/V)
H
P
H
L
L
LOWPASS RESPONSE
1
ff
=
CO
1
=
–
ff
PO
=
HH
PL
111
Q
f
f
P
f (LOG SCALE)
1
––
2
QQ
1
2
2
Q
1
–
C
+
22
2
4
Q
2
1
1
1
+
2
0.707 H
GAIN (V/V)
H
P
H
H
H
HIGHPASS RESPONSE
ff
=
CO
1
=
ff
PO
=
HH
PH
111
Q
f
f
C
P
f (LOG SCALE)
1
1
+
––
22
2
QQ
–
1
1
–
2
2
Q
1
–
2
4
Q
2
1
1
2
1562-2 F04
1
–
1
+
Figure 4. Characteristics of Standard 2nd Order Filter Responses
8
LTC1562-2
U
WUU
APPLICATIONS INFORMATION
ωO (= 2πfO) and Q are set by R2 and RQ as above. For a 2nd
order lowpass response the gain magnitude becomes QH
at frequency fO, and for Q > 0.707, a gain peak occurs at
a frequency below fO, as shown in Figure 4.
Basic Bandpass
There are two different ways to obtain a bandpass function
in Figure 3, both of which give the following transfer
function form:
ω
HQs
–/
()
Hs
()
BP
=
sQs
BO
2
+
ωω
/
()
OO
2
+
The value of the gain parameter HB depends on the circuit
configuration as follows. When ZIN is a resistor of value
RIN, a bandpass response results at the V1 output (Figure
6a) with a gain parameter HB = RQ/RIN. Alternatively, a
capacitor of value CIN gives a bandpass response at the V2
output (Figure 6b), with the same HBP(s) expression, and
the gain parameter now HB = (RQ/7958Ω)(CIN/100pF). This
transfer function has a gain magnitude of HB (its peak value)
when the frequency equals fO and has a phase shift of 180°
at that frequency. Q measures the sharpness of the peak
(the ratio of fO to – 3dB bandwidth) in a 2nd order bandpass
function, as illustrated in Figure 4. ωO = 2πfO and Q are set
by R2 and RQ as described previously in Setting fO and Q.
C
R
IN
V
IN
R
R2
Q
V
OUT
INV V1
2nd ORDER
1/4 LTC1562-2
Figure 6. Basic Bandpass Configurations
V2
IN
V
IN
Q
INV V1
2nd ORDER
1/4 LTC1562-2
(b) Capacitive Input(a) Resistive Input
V2
Basic Highpass
When ZIN of Figure 3 is a capacitor of value CIN, a highpass
response appears at the V1 output (Figure 7).
Vs
1
()
Hs
==
Vs
()
IN
HP
()
2
sQs
+
()
2
Hs
–
H
2
+ωω
/
OO
R2R
V
OUT
1562-2 F06
L
HH = CIN/100pF is the highpass gain parameter. Parameters ωO = 2πfO and Q are set by R2 and RQ as above. For
a 2nd order highpass response the gain magnitude at
frequency fO is QHH, and approaches HH at high frequencies (f >> fO). For Q > 0.707, a gain peak occurs at a
frequency above fO as shown in Figure 4. The transfer
function includes a sign inversion.
C
IN
V
IN
R2R
Q
V
OUT
INV V1
2nd ORDER
1/4 LTC1562-2
Figure 7. Basic Highpass Configuration
V2
1562-2 F07
Signal Swings
The V1 and V2 outputs are capable of swinging to within
roughly 100mV of each power supply rail. As with any
analog filter, the signal swings in each 2nd order section
must be scaled so that no output overloads (saturates),
even if it is not used as a signal output. (Filter literature
often calls this the “dynamics” issue.) When an unused
output has a larger swing than the output of interest, the
section’s gain or input amplitude must be scaled down to
avoid overdriving the unused output. The LTC1562-2 can
still be used with high performance in such situations as
long as this constraint is followed.
For an LTC1562-2 section as in Figure 3, the magnitudes
of the two outputs V2 and V1, at a frequency ω = 2πf, have
the ratio,
|()|
2
|()|
Vj
1
ω
()Vj
kHz
200ω
=
f
regardless of the details of ZIN. Therefore, an input frequency above or below 200kHz produces larger output
amplitude at V1 or V2, respectively. This relationship can
guide the choice of filter design for maximum dynamic
range in situations (such as bandpass responses) where
there is more than one way to achieve the desired frequency response with an LTC1562-2 section.
9
LTC1562-2
U
WUU
APPLICATIONS INFORMATION
Because 2nd order sections with Q ≥ 1 have response
peaks near fO, the gain ratio above implies some rules of
thumb:
fO < 200kHz ⇒ V2 tends to have the larger swing
fO > 200kHz ⇒ V1 tends to have the larger swing.
The following situations are convenient because the
relative swing issue does not arise. The unused output’s
swing is naturally the smaller of the two in these cases:
Lowpass response (resistor input, V2 output, Figure 5)
with fO < 200kHz
Bandpass response (capacitor input, V2 output, Figure
6b) with fO < 200kHz
Bandpass response (resistor input, V1 output, Figure
6a) with fO > 200kHz
Highpass response (capacitor input, V1 output, Figure
7) with fO > 200kHz
The LTC1562, a lower frequency variant of the LTC1562 -2,
has a design center fO of 100kHz compared to 200kHz in the
LTC1562-2. The rules summarized above apply to the
LTC1562 but with 100kHz replacing the 200kHz limits.
Thus, an LTC1562 highpass filter section with fO above
100kHz automatically satisfies the desirable condition of the
unused output carrying the smaller signal swing.
R
IN
7.87k
V
IN
Figure 8. 200kHz, Q = 0.7 Lowpass Circuit
for Distortion vs Loading Test
R
Q
5.49k
INV V1
2nd ORDER
1/4 LTC1562-2
R2
7.87k
V2
C
L
30pF
V
OUT
R
L
(EXTERNAL
LOAD RESISTANCE)
1562-2 F08
require further dynamic range, reducing the value of Z
IN
boosts the signal gain while reducing the input referred
noise. This feature can increase the SNR for low level
signals. Varying or switching ZIN is also an efficient way to
effect automatic gain control (AGC). From a system viewpoint, this technique boosts the ratio of maximum signal
to minimum noise, for a typical 2nd order lowpass response (Q = 1, fO = 200kHz), to 114dB.
Input Voltages Beyond the Power Supplies
Properly used, the LTC1562-2 can accommodate input
voltage excursions well beyond its supply voltage. This
requires care in design but can be useful, for example,
when large out-of-band interference is to be removed from
a smaller desired signal. The flexibility for different input
voltages arises because the INV inputs are at virtual
ground potential, like the inverting input of an op amp with
negative feedback. The LTC1562-2 fundamentally responds
to input
current
and the external voltage VIN appears only
across the external impedance ZIN in Figure 3.
To accept beyond-the-supply input voltages, it is impor-
tant to keep the LTC1562-2 powered on, not in shutdown
mode, and to avoid saturating the V1 or V2 output of the
2nd order section that receives the input. If any of these
conditions is violated, the INV input will depart from a
virtual ground, leading to an overload condition whose
recovery timing depends on circuit details. In the event
that this overload drives the INV input beyond the supply
voltages, the LTC1562-2 could be damaged.
The most subtle part of preventing overload is to consider
the possible input signals or spectra and take care that
none of them can drive either V1 or V2 to the supply limits.
Note that neither output can be allowed to saturate, even
if it is not used as the signal output. If necessary the
passband gain can be reduced (by increasing the impedance of ZIN in Figure 3) to reduce output swings.
Low Level or Wide Range Input Signals
The LTC1562-2 contains a built-in capability for low noise
amplification of low level signals. The ZIN impedance in
each 2nd order section controls the block’s gain. When set
for unity passband gain, a 2nd order section can deliver an
output signal 99dB above the noise level. If low level inputs
10
The final issue to be addressed with beyond-the-supply
inputs is current and voltage limits. Current entering the
virtual ground INV input flows eventually through the
output circuitry that drives V1 and V2. The input current
magnitude (VIN/ZIN in Figure 3) should be limited by
design to less than 1mA for good distortion performance.
On the other hand, the input voltage VIN appears across the
LTC1562-2
1
2
1
2
4
2
CCCC
P P IN P
±
()
–
U
WUU
APPLICATIONS INFORMATION
external component ZIN, usually a resistor or capacitor.
This component must of course be rated to sustain the
magnitude of voltage imposed on it.
Lowpass “T” Input Circuit
The virtual ground INV input in the Operational Filter
block provides a means for adding an “extra” lowpass
pole to any resistor-input application (such as the basic
lowpass, Figure 5, or bandpass, Figure 6a). The resistor
that would otherwise form ZIN is split into two parts and
a capacitor to ground added, forming an R-C-R “T”
network (Figure 9). This adds an extra, independent real
pole at a frequency:
f
=
P
where CT is the new external capacitor and RP is the
parallel combination of the two input resistors R
R
. This pair of resistors must normally have a pre-
INB
scribed series total value RIN to set the filter’s gain as
described above. The parallel value RP can however be set
arbitrarily (to RIN/4 or less) which allows choosing a
convenient standard capacitor value for CT and fine tuning
the new pole with RP.
RC
π12
PT
INA
and
A practical limitation of this technique is that the CT capacitor values that tend to be required (hundreds or thousands
of pF) can destabilize the op amp in Figure 3 if R
INB
is too
small, leading to AC errors such as Q enhancement. For this
reason, when R
larger of the two should be placed in the R
and RINB are unequal, preferably the
INA
position.
INB
Highpass “T” Input Circuit
A method similar to the preceding technique adds an
“extra” highpass pole to any capacitor-input application
(such as the bandpass of Figure 6b or the highpass of
Figure 7). This method splits the input capacitance CIN into
two series parts C
INA
and C
, with a resistor RT to ground
INB
between them (Figure 10). This adds an extra 1st order
highpass corner with a zero at DC and a pole at the
frequency:
f
=
P
where CP = C
RC
π12
TP
INA
+ C
is the parallel combination of the
INB
two capacitors. At the same time, the total series capacitance CIN will control the filter’s gain parameter (HH in
Basic Highpass). For a given series value CIN, the parallel
value CP can still be set arbitrarily (to 4CIN or greater).
R
V
INA
IN
Figure 9. Lowpass “T” Input Circuit
R
C
T
INB
INV V1
2nd ORDER
1/4 LTC1562-2
R2R
Q
V2
1562-2 F09
The procedure therefore is to begin with the target extra
pole frequency fP. Determine the series value RIN from the
gain requirement. Select a capacitor value CT such that R
P
= 1/(2πfPCT) is no greater than RIN/4, and then choose
R
and R
INA
value RP and the series value RIN. Such R
that will simultaneously have the parallel
INB
and R
INA
INB
can
be found directly from the expression:
1
RRRR
±
IN IN IN P
2
2
2
1
–
4
()
C
INA
V
IN
Figure 10. Highpass “T” Input Circuit
C
INB
R
T
INV V1
2nd ORDER
1/4 LTC1562-2
R2R
Q
V2
1562-2 F10
The procedure then is to begin with the target corner (pole)
frequency fP. Determine the series value CIN from the gain
requirement (for example, CIN = HH(100pF) for a highpass). Select a resistor value RT such that CP = 1/(2πRTfP)
is at least 4CIN, and select C
INA
and C
that will simulta-
INB
neously have the parallel value CP and the series value CIN.
Such C
and C
INA
can be found directly from the
INB
expression:
11
LTC1562-2
This procedure can be iterated, adjusting the value of RT,
to find convenient values for C
INA
and C
since resistor
INB
values are generally available in finer increments than
capacitor values.
LTC1562/LTC1562-2 Demo Board
The LTC demonstration board DC266 is assembled with
an LTC1562 or LTC1562-2 in a 20-pin SSOP package and
power supply decoupling capacitors. Jumpers on the
board configure the filter chip for dual or single supply
operation and power shutdown. Pads for surface mount
U
TYPICAL APPLICATIONS
175kHz 8th Order Elliptic Highpass Filter
C
82pF
IN2
R
20.5k
IN2
C
220pF
IN1
V
IN
5V
R
9.09k
Q1
R21 7.15k
0.1µF
R23 11.3k
R
59k
Q3
*V– ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
ALL CAPACITORS 5% STANDARD VALUES
1
INV B
2
V1 B
3
V2 B
5
+
LTC1562-2
V
6
SHDN
8
V2 A
9
V1 A
10
INV A
40.2k
R
IN4
INV C
V1 C
V2 C
AGND
V2 D
V1 D
INV D
20
R
100pF
Q2
R22 10k
R24 4.02k
3.24k
R
Q4
19
18
16
–
V
15
13
12
11
C
IN4
C
R
26.7k
resistors and capacitors are provided to build applicationspecific filters. Also provided are terminals for inputs,
outputs and power supplies.
Notches and Elliptic Responses
Further circuit techniques appear in the LTC1562 final
data sheet under the heading “Notches and Elliptic Responses.” These techniques are directly applicable to the
LTC1562-2 with the substitution of the different values for
the internal components R1 and C. In the LTC1562-2, R1
is 7958Ω and C is 100pF.
47pF
IN3
Amplitude Response
IN3
45.3k
0.1µF
1562-2 TA03a
–5V*
V
OUT
10
0
–10
–20
–30
–40
GAIN (dB)
–50
–60
–70
–80
–90
50k 900k
200k
FREQUENCY (Hz)
1562-2 TA03b
12
U
TYPICAL APPLICATIONS
Dual 5th Order 170kHz Elliptic Highpass Filter, Single 5V Supply
C
C
IN1
I1
82pF
100pF
V
IN1
R
I1
2k
5V
100pF
V
IN2
0.1µF
C
C
IN3
I3
82pF
R
I3
2k
43.2k
R
Q1
R21 11.5k
R23 11.5k
43.2k
R
Q3
LTC1562-2
220pF
C
IN2
R
15k
IN2
V
OUT1
*
V
OUT2
1562-2 TA05a
1
INV B
2
V1 B
3
V2 B
5
+
V
LTC1562-2
6
SHDN
8
V2 A
9
V1 A
10
INV A
R
15k
IN4
INV C
V1 C
V2 C
AGND
V2 D
V1 D
INV D
20
R
7.68k
Q2
19
R22 6.34k
18
16
–
V
15
13
12
11
R24 6.34k
R
Q4
C
IN4
7.68k
220pF
+
1µF
*GROUND ALSO AT PINS 4, 7, 14 & 17
Amplitude Response
10
0
–10
–20
–30
–40
GAIN (dB)
–50
–60
–70
–80
–90
10k
FREQUENCY (Hz)
100k 1M
1562-2 TA05b
13
LTC1562-2
U
TYPICAL APPLICATIONS
100kHz 8th Order Bandpass Linear Phase, –3dB BW = f
C
10pF
IN1
V
IN
5V
R
Q1
R21 31.6k
0.1µF
R23 35.7k
R
142k
Q3
C
IN3
10pF
*V– ALSO AT PINS 4, 7, 14 & 17
78.7k
1
INV B
2
V1 B
3
V2 B
5
+
V
6
SHDN
8
V2 A
9
V1 A
10
INV A
LTC1562-2
221k
R
IN4
INV C
V1 C
V2 C
V
AGND
V2 D
V1 D
INV D
R
IN2
20
R
19
R22 30.1k
18
16
–
15
R24 28.7k
13
R
12
11
Frequency Response
10
0
–10 60
–20
–30
–40
–50
AMPLITUDE RESPONSE (dB)
–60
–70
60k
80k 100k 140k
FREQUENCY (Hz)
AMPLITUDE
RESPONSE
GROUP
DELAY
120k
1562-2 TA06b
Q2
Q4
178k
76.8k
0.1µF
118k
0
CENTER
1562-2 TA6a
GROUP DELAY (µs)
–5V*
V
OUT
/10
14
U
FREQUENCY (kHz)
10
–100
GAIN (dB)
–60
–70
–80
–90
–50
–40
–30
–20
–10
100 1000
1562-2 TA07b
10
0
TYPICAL APPLICATIONS
LTC1562-2 9th Order 200kHz Lowpass Elliptic Filter
R
7.32k
IN2
R
R
IN1A
IN1B
4.02k
R
4.02k
IN3
RQ1 6.04k
180pF
R21 8.06k
0.1µF
R23 12.4k
RQ3 5.36k
10.2k
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
ALL RESISTORS ARE ±1%, ALL CAPACITORS ARE ±5%
V
IN
1
INVB
2
V1B
3
V2B
5
+
V
LTC1562-2
6
SHDN
8
V2A
9
V1A
10
INVA
C
22pF
IN4
INVC
V1C
V2C
V
AGND
V2D
V1D
INVD
LTC1562-2
27pF
C
IN2
20
RQ2 13k
19
R22 6.04k
18
16
–
R24 6.04k
RQ4 14.3k
R
6.04k
IN4
1562-2 TA07a
–
0.1µF
15
13
12
11
–5V5V
V
OUT
Amplitude Response
PACKAGE DESCRIPTION
0.205 – 0.212**
(5.20 – 5.38)
0.005 – 0.009
(0.13 – 0.22)
*
**
DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
0.022 – 0.037
(0.55 – 0.95)
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
Dimensions in inches (millimeters) unless otherwise noted.
G Package
20-Lead Plastic SSOP (0.209)
(LTC DWG # 05-08-1640)
0.278 – 0.289*
(7.07 – 7.33)
1718 14 13 12 1115161920
12345678910
° – 8°
0
0.0256
(0.65)
BSC
0.010 – 0.015
(0.25 – 0.38)
0.002 – 0.008
(0.05 – 0.21)
0.301 – 0.311
(7.65 – 7.90)
0.068 – 0.078
(1.73 – 1.99)
G20 SSOP 0595
15
LTC1562-2
U
TYPICAL APPLICATIONS
256kHz Linear Phase 6th Order Lowpass Filter with a 2nd Order
V
IN
5V
0.1µF
R23 4.12k
R
R
4.12k
Allpass Phase Equalizer, Single Supply
6.19k
R
FF1
R
1.54k
LTC1562-2
R
4.12k
IN4
B1
INV C
V1 C
V2 C
AGND
V2 D
V1 D
INV D
20
19
18
16
–
V
15
13
12
11
C
IN4
R
IN1
7.5k
R
3.24k
Q1
R21 6.81k
7.32k
Q3
IN3
1
INV B
2
V1 B
3
V2 B
5
+
V
6
SHDN
8
V2 A
9
V1 A
10
INV A
R
4.12k
Q2
R22 6.19k
R24 4.12k
R
7.32k
Q4
22pF 5%
+
1µF
*
V
OUT
Amplitude Response
10
0
–10
–20
–30
–40
GAIN (dB)
–50
–60
–70
–80
10k 1M
100k
FREQUENCY (Hz)
*GROUND ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM
1562-2 TA04b
1562-2 TA04a
Group Delay Response
8
7
6
5
4
DELAY (µs)
3
2
1
0
50 100 150 200 250 300
FREQUENCY (kHz)
350 400
1562-2 TA04c
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LTC1068-X Quad 2-Pole Switched Capacitor Building Block Clock Tuned
LTC1560-1 5-Pole Elliptic Lowpass, fC = 1MHz/0.5MHz No External Components, SO8
LTC1562 Quad 2-Pole Active RC, 10kHz to 150kHz Same Pinout as LTC1562-2
15622f LT/TP 0599 4K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1998
16
Linear T echnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear-tech.com
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