LTC1562-2

Very Low Noise, Low Distortion

Active RC Quad Universal Filter

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 (f

) 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 out-
puts. 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.

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

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 f

, Q, Gain

Dual 4th Order 200kHz Butterworth Lowpass Filter, SNR 96dB

IN2

7.87k

10.2k

R22 7.87k

R24 7.87k

10.2k

IN4

7.87k

4.22k

R23 7.87k

IN3

7.87k

4.22k

R21 7.87k

IN1

7.87k

IN2

IN1

0.1

5V*

OUT1

OUT2

1562-2 TA01

ALSO AT PINS 4, 7, 14 & 17

ALL RESISTORS 1% METAL FILM

INV C

V1 C

V2 C

AGND

V2 D

V1 D

INV D

INV B

V1 B

V2 B

SHDN

V2 A

V1 A

INV A

LTC1562-2

GAIN (dB)

50k

1.5M

1562-2 TA02

FREQUENCY (Hz)

100k

Amplitude Response

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

LTC1562-2

ABSOLUTE

AXI

RATINGS

ORDER PART

NUMBER

LTC1562CG-2
LTC1562IG-2

PACKAGE/ORDER I

FOR

ATIO

(Note 1)

Total Supply Voltage (V

to V

) .............................. 11V

Maximum Input Voltage

at Any Pin .................... (V

0.3V)

+ 0.3V)

Storage Temperature Range ................. 65

C to 150

Operating Temperature Range

LTC1562C-2 ............................................ 0

C to 70

LTC1562I-2 ........................................ 40

C to 85

Lead Temperature (Soldering, 10 sec).................. 300

Consult factory for Military grade parts.

JMAX

= 150

= 136

C/W

TOP VIEW

G PACKAGE

20-LEAD PLASTIC SSOP

*G PACKAGE PINS 4, 7, 14, 17 ARE

SUBSTRATE/SHIELD CONNECTIONS

AND MUST BE TIED TO V

1
2
3
4
5
6
7
8
9

20
19
18
17
16
15
14
13
12
11

INV B

V1 B
V2 B

SHDN

V2 A
V1 A

INV A

INV C
V1 C
V2 C
V

AGND
V

V2 D
V1 D
INV D

ELECTRICAL CHARACTERISTICS

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Total Supply Voltage

4.75

10.5

Supply Current

2.375V, R

= 5k, C

= 30pF, Outputs at 0V

23.5

5V, R

= 5k, C

= 30pF, Outputs at 0V

22.5

2.375V, R

= 5k, C

= 30pF, Outputs at 0V

5V, R

= 5k, C

= 30pF, Outputs at 0V

Output Voltage Swing, V2 Outputs

2.375V, R

= 5k, C

= 30pF

4.2

4.6

P-P

5V, R

= 5k, C

= 30pF

9.3

9.8

P-P

Output Voltage Swing, V1 Outputs

2.375V, R

= 5k, C

= 30pF, f = 250kHz

4.5

P-P

5V, R

= 5k, C

= 30pF, f = 250kHz

8.4

9.7

P-P

DC Offset Magnitude, V2 Outputs

2.375V, Input at AGND Voltage

5V, Input at AGND Voltage

DC AGND Reference Point

= Single 5V Supply

2.5

Center Frequency (f

) Error (Notes 2, 3)

5V, V2 Output Has R

= 5k, C

= 30pF

0.5

1.7

Lowpass Passband Gain at V2 Output

2.375V, f

= 10kHz,

+ 0.05

+ 0.1

V2 Output Has R

= 5k, C

= 30pF

Q Accuracy

2.375V, V2 Output Has R

= 5k, C

= 30pF

+ 2

Wideband Output Noise

2.375V, BW = 400kHz, Input AC GND

RMS

5V, BW = 400kHz, Input AC GND

RMS

Input-Referred Noise, Gain = 100

BW = 400kHz, f

= 200kHz, Q = 1, Input AC GND

7.3

RMS

The

denotes specifications that apply over the full operating temperature

range, otherwise specifications are at T

= 25

C. V

5V, outputs unloaded, SHDN pin to logic "low", unless otherwise noted. AC

specs are for a single 2nd order section, R

= R2 = 10.4k

0.1%, R

= 9.09k

0.1%, f

= 175kHz.

LTC1562-2

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

THD

Total Harmonic Distortion, V2 Output

= 20kHz, 2.8V

P-P

, V1 and V2 Outputs Have

100

= 5k, C

= 30pF

= 20kHz, 9V

P-P

, V1 and V2 Outputs Have

= 5k, C

= 30pF

Shutdown Supply Current

SHDN Pin to V

1.5

SHDN Pin to V

, V

2.375V

1.0

Shutdown-Input Logic Threshold

2.5

Shutdown-Input Bias Current

SHDN Pin to 0V

Shutdown Delay

SHDN Pin Steps from 0V

to V

Shutdown Recovery Delay

SHDN Pin Steps from V

to 0V

100

Inverting Input Bias Current, Each Biquad

ELECTRICAL CHARACTERISTICS

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: f

change from

5V to

2.375 supplies is 0.2% typical,

temperature coefficient magnitude, 25

C to 85

C, is

50ppm/

C typical.

As with the LTC1562, f

decreases with increasing temperature.

Note 3: Tighter frequency tolerance is available, consult factory.

The

denotes specifications that apply over the full operating temperature

range, otherwise specifications are at T

= 25

C. V

5V, outputs unloaded, SHDN pin to logic "low", unless otherwise noted. AC

specs are for a single 2nd order section, R

= R2 = 10.4k

0.1%, R

= 9.09k

0.1%, f

= 175kHz.

TYPICAL PERFOR A CE CHARACTERISTICS

Error vs Nominal f

5V)

Error vs Nominal f

2.5V)

NOMINAL f

(kHz)

100

Q ERROR (%)

260

1562-2 G03

140

180

220

300

240

120

160

200

280

= 70

= 25

= R

Q = 5

Q = 2.5

Q = 1

Q Error vs Nominal f

5V)

NOMINAL f

(kHz)

120

ERROR (%)

1.0

2.0

1.5

3.0

2.5

1562-2 G01

1.0

2.0

0.5

1.5

2.5

3.0

160

200

240

280

260

140

180

220

Q = 5

Q = 2.5

Q = 1

= 25

= R

NOMINAL f

(kHz)

120

ERROR (%)

1.0

2.0

1.5

3.0

2.5

1562-2 G02

1.0

2.0

0.5

1.5

2.5

3.0

160

200

240

280

260

140

180

220

Q = 5

Q = 2.5

Q = 1

= 25

= R

LTC1562-2

TYPICAL PERFOR A CE CHARACTERISTICS

Peak BP Gain vs Nominal f

5V) (Figure 3, V1 Output)

Q Error vs Nominal f

2.5V)

Peak BP Gain vs Nominal f

2.5V) (Figure 3, V1 Output)

NOMINAL f

(kHz)

100

Q ERROR (%)

260

1562-2 G04

140

180

220

300

240

120

160

200

280

Q = 5

Q = 2.5

Q = 1

= 70

= 25

= R

NOMINAL f

(kHz)

100

PEAK BP GAIN (dB)

0.25

0.50

1.00

3.00

2.00

140

180 200

280

2.50

1.50

0.75

1.25

2.25

2.75

1.75

120

160

220 240 260

300

1562-2 G5

Q = 5

Q = 2.5

Q = 1

= 70

= 25

= R

NOMINAL f

(kHz)

100

PEAK BP GAIN (dB)

0.25

0.50

1.00

3.00

2.00

140

180 200

280

2.50

1.50

0.75

1.25

2.25

2.75

4.00

3.50
3.25

3.75

1.75

120

160

220 240 260

300

1562-2 G6

Q = 5

Q = 2.5

Q = 1

= 70

= 25

= R

LP Noise vs Nominal f

5V, 25

C) (Figure 3,

V2 Output) (R

= R2)

NOMINAL f

(kHz)

120

LP NOISE (

RMS

)

100

160

200

220

1562-2 G07

140

180

240 260

280

Q = 5

Q = 2.5

Q = 1

BP Noise vs Nominal f

5V, 25

C) (Figure 3,

V1 Output) (R

= R

)

NOMINAL f

(kHz)

120

BP NOISE (

RMS

)

100

160

200

220

1562-2 G08

140

180

240 260

280

Q = 5

Q = 2.5

Q = 1

CTIO

Distortion vs External Load
Resistance and Frequency
(V

5V, 25

C) (Figure 8)

EXTERNAL LOAD RESISTANCE (

)

10k

100

THD (AMPLITUDE BELOW FUNDAMENTAL) (dB)

THD (AMPLITUDE BELOW FUNDAMENTAL) (%)

1562-2 G09

0.001

0.01

0.1

100

= 20kHz

= 50kHz

2nd ORDER LOWPASS
f

= 200kHz

Q = 0.7
OUTPUT LEVEL 1V

RMS

(2.83V

P-P

)

5V SUPPLIES

= 100kHz

Power Supply Pins: The V

and V

pins should be

bypassed with 0.1

F 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.

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

LTC1562-2

CTIO

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 effec-
tively 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.

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 0.1

F capacitor (at least

F for best AC performance) (Figure 2).

0.1

1562-2 F01

DIGITAL

GROUND PLANE

(IF ANY)

LTC1562-2

0.1

ANALOG
GROUND
PLANE

SINGLE-POINT
SYSTEM GROUND

Figure 1. Dual Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)

1562-2 F01

DIGITAL

GROUND PLANE

(IF ANY)

LTC1562-2

REFERENCE

0.1

ANALOG
GROUND
PLANE

SINGLE-POINT
SYSTEM GROUND

Figure 2. Single Supply Ground Plane Connection
(Including Substrate Pins 4, 7, 14, 17)

INV

*R1 AND C ARE PRECISION
INTERNAL COMPONENTS

1/4 LTC1562-2

1562-2 F03

sR1C*

TYPE

R
C

RESPONSE

AT V1

BANDPASS

HIGHPASS

RESPONSE

AT V2

LOWPASS

BANDPASS

7958

IN EACH CASE,

Q =

= (200kHz)

(

)

200kHz

( )

Figure 3. Equivalent Circuit of a Single 2nd Order Section
(Inside Dashed Line) Shown in Typical Connection. Form of
Z

Determines Response Types at the Two Outputs (See Table)

LTC1562-2

SHDN

1562-2 BD

2ND ORDER SECTIONS

INV

SHUTDOWN
SWITCH

AGND

INV

BLOCK DIAGRA

CTIO

INV A, INV B, INV C, INV D: Each of the INV pins is a virtual-
ground summing point for the corresponding 2nd order
section. For each section, all three external components
Z

, R2, R

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 cir-
cuitry (see Applications Information section). Each V1 pin

also connects to the R

resistor of the corresponding 2nd

order filter section (see Figure 3 and Applications Informa-
tion). Each output is designed to drive a nominal net load
of 4k

and 30pF, which includes the loading due to the

external R

. 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 cir-
cuitry (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 Informa-
tion). Each output is designed to drive a nominal net load
of 4k

and 30pF, which includes the loading due to the

external R2. Distortion performance improves when the
outputs are loaded as lightly as possible.

Overall Block Diagram Showing Four 3-Terminal 2nd Order Sections

LTC1562-2

APPLICATIO

S I

FOR

ATIO

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 Z

). The three external resistors

program f

, Q and gain. A combination of internal preci-

sion components and external resistor R2 sets the center
frequency f

of each 2nd order block. The LTC1562-2 is

trimmed at manufacture so that f

will be 200kHz

0.5%

if the external resistor R2 is exactly 7958

. The LTC1562-

2 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 Z

in Figure 3 becomes a capacitor C

(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). More-
over, the virtual-ground input gives each 2nd order sec-
tion the built-in capability for analog operations such as
gain (preamplification), summing and weighting of mul-
tiple inputs, or accepting current or charge signals di-
rectly. These Operational Filter

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 de-
signing custom filters, is the general-purpose analog filter
design software FilterCAD

for Windows

. This software

includes tools for finding the necessary f

, Q and gain

parameters to meet target filter specifications such as
frequency response.

Setting f

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 Z

. These transfer func-

tions all have the same denominator, a complex pole pair
with center frequency

= 2

and quality parameter Q.

(The numerators depend on the response type as de-
scribed below.) External resistors R2 and R

set f

and Q

as follows:

C R R

kHz

R R

kHz

(

)

(

)

1 2

7958

200

1 2

7958

200

( )

R1 = 7958

and C = 100pF are internal to the LTC1562-2

while R2 and R

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 f

R Ideal

kHz

200

7958

( )

(

)

Then select a practical R2 value from the available finite-
tolerance resistors. Use the actual R2 value to find the
desired R

, which also will be approximated with finite

tolerance:

(

)

7958

The f

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 f

. This relationship desensitizes f

to R2's

tolerance (by a factor of 2 incrementally), but it also
implies that R2 has a wider range than f

. (R

and R

also

Operational Filter and FilterCAD are trademarks of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.

LTC1562-2

tend to scale with R2.) At high f

these resistors fall below

4k, heavily loading the outputs of the LTC1562-2 and
leading to increased THD and other effects. At the other
extreme, a lower f

limit of 20kHz reflects an arbitrary

upper resistor limit of 1M

. The LTC1562-2's MOS input

circuitry can accommodate higher resistor values than
this, but junction leakage current from the input protection
circuitry may cause DC errors.

The 2nd order transfer functions H

(s), H

(s) and

(s) (below) are all inverting so that, for example, at DC

the lowpass gain is H

. If two such sections are cas-

caded, 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.

Basic Lowpass

When Z

of Figure 3 is a resistor of value R

, a standard

2nd order lowpass transfer function results from V

to V2

(Figure 5):

V s

Q s

( )

= R2/R

is the DC gain magnitude. (Note that the

transfer function includes a sign inversion.) Parameters

APPLICATIO

S I

FOR

ATIO

GAIN (V/

0.707 H

f (LOG SCALE)

BANDPASS RESPONSE

GAIN (V/

0.707 H

f (LOG SCALE)

LOWPASS RESPONSE

1562-2 F04

GAIN (V/

0.707 H

f (LOG SCALE)

HIGHPASS RESPONSE

f f

L H

;

Figure 4. Characteristics of Standard 2nd Order Filter Responses

INV

2nd ORDER

1/4 LTC1562-2

1562 F05

OUT

Figure 5. Basic Lowpass Configuration

LTC1562-2

APPLICATIO

S I

FOR

ATIO

(= 2

) and Q are set by R2 and R

as above. For a 2nd

order lowpass response the gain magnitude becomes QH

at frequency f

, and for Q > 0.707, a gain peak occurs at

a frequency below f

, 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:

Q s

( )

The value of the gain parameter H

depends on the circuit

configuration as follows. When Z

is a resistor of value

, a bandpass response results at the V1 output (Figure

6a) with a gain parameter H

= R

. Alternatively, a

capacitor of value C

gives a bandpass response at the V2

output (Figure 6b), with the same H

(s) expression, and

the gain parameter now H

= (R

/7958

)(C

/100pF). This

transfer function has a gain magnitude of H

(its peak value)

when the frequency equals f

and has a phase shift of 180

at that frequency. Q measures the sharpness of the peak
(the ratio of f

to 3dB bandwidth) in a 2nd order bandpass

function, as illustrated in Figure 4.

= 2

and Q are set

by R2 and R

as described previously in Setting f

and Q.

= C

/100pF is the highpass gain parameter. Param-

eters

= 2

and Q are set by R2 and R

as above. For

a 2nd order highpass response the gain magnitude at
frequency f

is QH

, and approaches H

at high frequen-

cies (f >> f

). For Q > 0.707, a gain peak occurs at a

frequency above f

as shown in Figure 4. The transfer

function includes a sign inversion.

INV

2nd ORDER

1/4 LTC1562-2

(b) Capacitive Input

(a) Resistive Input

1562-2 F06

OUT

INV

2nd ORDER

1/4 LTC1562-2

OUT

Figure 6. Basic Bandpass Configurations

Basic Highpass

When Z

of Figure 3 is a capacitor of value C

, a highpass

response appears at the V1 output (Figure 7).

V s

H s

Q s

( )

INV

2nd ORDER

1/4 LTC1562-2

1562-2 F07

OUT

Figure 7. Basic Highpass Configuration

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

= 2

f, have

the ratio,

(

)

V j

kHz

200

regardless of the details of Z

. Therefore, an input fre-

quency 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 fre-
quency response with an LTC1562-2 section.

LTC1562-2

Because 2nd order sections with Q

1 have response

peaks near f

, the gain ratio above implies some rules of

thumb:

< 200kHz

V2 tends to have the larger swing

> 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 f

< 200kHz

Bandpass response (capacitor input, V2 output, Figure
6b) with f

< 200kHz

Bandpass response (resistor input, V1 output, Figure
6a) with f

> 200kHz

Highpass response (capacitor input, V1 output, Figure
7) with f

> 200kHz

The LTC1562, a lower frequency variant of the LTC1562 -2,
has a design center f

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 f

above

100kHz automatically satisfies the desirable condition of the
unused output carrying the smaller signal swing.

require further dynamic range, reducing the value of Z

boosts the signal gain while reducing the input referred
noise. This feature can increase the SNR for low level
signals. Varying or switching Z

is also an efficient way to

effect automatic gain control (AGC). From a system view-
point, this technique boosts the ratio of maximum signal
to minimum noise, for a typical 2nd order lowpass re-
sponse (Q = 1, f

= 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 V

appears only

across the external impedance Z

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 imped-
ance of Z

in Figure 3) to reduce output swings.

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 (

in Figure 3) should be limited by

design to less than 1mA for good distortion performance.
On the other hand, the input voltage V

appears across the

APPLICATIO

S I

FOR

ATIO

INV

2nd ORDER

1/4 LTC1562-2

1562-2 F08

R2
7.87k

30pF

(EXTERNAL
LOAD RESISTANCE)

5.49k

7.87k

OUT

Figure 8. 200kHz, Q = 0.7 Lowpass Circuit
for Distortion vs Loading Test

Low Level or Wide Range Input Signals

The LTC1562-2 contains a built-in capability for low noise
amplification of low level signals. The Z

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

LTC1562-2

APPLICATIO

S I

FOR

ATIO

external component Z

, 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 Z

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:

R C

P T

where C

is the new external capacitor and R

is the

parallel combination of the two input resistors R

INA

and

INB

. This pair of resistors must normally have a pre-

scribed series total value R

to set the filter's gain as

described above. The parallel value R

can however be set

arbitrarily (to R

/4 or less) which allows choosing a

convenient standard capacitor value for C

and fine tuning

the new pole with R

INV

2nd ORDER

1/4 LTC1562-2

1562-2 F10

INB

INA

Figure 10. Highpass "T" Input Circuit

A practical limitation of this technique is that the C

capaci-

tor 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

INA

and R

B are unequal, preferably the

larger of the two should be placed in the R

INB

position.

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 C

into

two series parts C

INA

and C

INB

, with a resistor R

to ground

between them (Figure 10). This adds an extra 1st order
highpass corner with a zero at DC and a pole at the
frequency:

R C

T P

where C

= C

INA

+ C

INB

is the parallel combination of the

two capacitors. At the same time, the total series capaci-
tance C

will control the filter's gain parameter (H

Basic Highpass). For a given series value C

, the parallel

value C

can still be set arbitrarily (to 4C

or greater).

INV

2nd ORDER

1/4 LTC1562-2

1562-2 F09

INB

INA

Figure 9. Lowpass "T" Input Circuit

The procedure therefore is to begin with the target extra
pole frequency f

. Determine the series value R

from the

gain requirement. Select a capacitor value C

such that R

= 1/(2

) is no greater than R

/4, and then choose

INA

and R

INB

that will simultaneously have the parallel

value R

and the series value R

. Such R

INA

and R

INB

can

be found directly from the expression:

R R

IN P

(

)

The procedure then is to begin with the target corner (pole)
frequency f

. Determine the series value C

from the gain

requirement (for example, C

= H

(100pF) for a high-

pass). Select a resistor value R

such that C

= 1/(2

)

is at least 4C

, and select C

INA

and C

INB

that will simulta-

neously have the parallel value C

and the series value C

Such C

INA

and C

INB

can be found directly from the

expression:

C C

IN P

(

)

LTC1562-2

TYPICAL APPLICATIO

IN2

20.5k

26.7k

R22 10k

R24 4.02k

3.24k

IN4

40.2k

59k

R23 11.3k

9.09k

R21 7.15k

0.1

5V*

OUT

ALSO AT PINS 4, 7, 14 & 17

ALL RESISTORS 1% METAL FILM
ALL CAPACITORS 5% STANDARD VALUES

IN1

220pF

IN2

82pF

IN3

47pF

IN3

45.3k

IN4

100pF

1562-2 TA03a

INV C

V1 C

V2 C

AGND

V2 D

V1 D

INV D

INV B

V1 B

V2 B

SHDN

V2 A

V1 A

INV A

LTC1562-2

175kHz 8th Order Elliptic Highpass Filter

GAIN (dB)

50k

900k

FREQUENCY (Hz)

200k

1562-2 TA03b

Amplitude Response

This procedure can be iterated, adjusting the value of R

to find convenient values for C

INA

and C

INB

since resistor

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

resistors and capacitors are provided to build application-
specific 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 Re-
sponses." 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 7958

and C is 100pF.

LTC1562-2

PACKAGE DESCRIPTIO

Dimensions in inches (millimeters) unless otherwise noted.

G Package

20-Lead Plastic SSOP (0.209)

(LTC DWG # 05-08-1640)

G20 SSOP 0595

0.005 0.009

(0.13 0.22)

0.022 0.037

(0.55 0.95)

0.205 0.212**

(5.20 5.38)

0.301 0.311

(7.65 7.90)

2 3

6 7 8

9 10

0.278 0.289*

(7.07 7.33)

14 13 12 11

0.068 0.078

(1.73 1.99)

0.002 0.008

(0.05 0.21)

0.0256

(0.65)

BSC

0.010 0.015

(0.25 0.38)

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

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.

TYPICAL APPLICATIO

6.04k

5.36k

IN3

10.2k

IN1A

4.02k

IN1B

4.02k

13k

IN2

7.32k

IN4

6.04k

IN2

27pF

IN4

22pF

14.3k

R22 6.04k

R24 6.04k

0.1

OUT

R21 8.06k

R23 12.4k

LTC1562-2

INVB

V1B

V2B

SHDN

V2A

V1A

INVA

INVC

V1C

V2C

AGND

V2D

V1D

INVD

0.1

180pF

1562-2 TA07a

PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V

ALL RESISTORS ARE

1%, ALL CAPACITORS ARE

FREQUENCY (kHz)

100

GAIN (dB)

100

1000

1562-2 TA07b

LTC1562-2 9th Order 200kHz Lowpass Elliptic Filter

Amplitude Response

LTC1562-2

15622f LT/TP 0599 4K · PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1998

TYPICAL APPLICATIO

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

FAX: (408) 434-0507

www.linear-tech.com

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

LTC1068-X

Quad 2-Pole Switched Capacitor Building Block

Clock Tuned

LTC1560-1

5-Pole Elliptic Lowpass, f

= 1MHz/0.5MHz

No External Components, SO8

LTC1562

Quad 2-Pole Active RC, 10kHz to 150kHz

Same Pinout as LTC1562-2

DELAY (

FREQUENCY (kHz)

100

150

200

250

300

1562-2 TA04c

350

400

Group Delay Response

GAIN (dB)

10k

1562-2 TA04b

FREQUENCY (Hz)

100k

Amplitude Response

1.54k

FF1

6.19k

4.12k

R22 6.19k

R24 4.12k

7.32k

IN4

4.12k

7.32k

R23 4.12k

IN3

4.12k

IN1

7.5k

3.24k

R21 6.81k

0.1

OUT

*GROUND ALSO AT PINS 4, 7, 14 & 17
ALL RESISTORS 1% METAL FILM

IN4

22pF 5%

1562-2 TA04a

INV C

V1 C

V2 C

AGND

V2 D

V1 D

INV D

INV B

V1 B

V2 B

SHDN

V2 A

V1 A

INV A

LTC1562-2

256kHz Linear Phase 6th Order Lowpass Filter with a 2nd Order

Allpass Phase Equalizer, Single Supply

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LTC1562-2

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LTC1562-2