Rainbow Electronics MAX7425 User Manual

For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.

General Description

The MAX7418–MAX7425 5th-order, low-pass, switched­capacitor filters (SCFs) operate from a single +5V
(MAX7418–MAX7421) or +3V (MAX7422–MAX7425)
supply. These devices draw only 3mA of supply current
and allow corner frequencies from 1Hz to 45kHz, mak­ing them ideal for low-power post-DAC filtering and anti­aliasing applications. They feature a shutdown mode
that reduces supply current to 0.2µA.

Two clocking options are available: self-clocking
(through the use of an external capacitor), or external
clocking for tighter corner-frequency control. An offset
adjust pin allows for adjustment of the DC output level.

The MAX7418/MAX7422 deliver 53dB of stopband
rejection and a sharp rolloff with a 1.6 transition ratio.
The MAX7421/MAX7425 achieve a sharper rolloff with a

1.25 transition ratio while still providing 37dB of stop­band rejection. The MAX7419/MAX7423 Bessel filters
provide low overshoot and fast settling, and the
MAX7420/MAX7424 Butterworth filters provide a maxi­mally flat passband response. Their fixed response sim­plifies the design task of selecting a clock frequency.

Applications

ADC Anti-Aliasing CT2 Base Stations

DAC Postfiltering Speech Processing

Features

♦ 5th-Order, Lowpass Filters

Elliptic Response (MAX7418/MAX7421/
MAX7422/MAX7425)
Bessel Response (MAX7419/MAX7423)
Butterworth Response (MAX7420/MAX7424)

♦ Clock-Turnable Corner Frequency (1Hz to 45kHz)

♦ Single-Supply Operation

+5V (MAX7418–MAX7421)
+3V (MAX7422–MAX7425)

♦ Low Power

3mA (Operating Mode)

0.2µA (Shutdown Mode)

♦ Available in 8-Pin µMAX Package

♦ Low Output Offset: ±4mV

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

________________________________________________________________ Maxim Integrated Products 1

Typical Operating Circuit

19-1821; Rev 0; 11/00

Pin Configuration

Ordering Information

Selector Guide

PART

MAX7418EUA -40°C to +85°C

TEMP. RANGE PIN-PACKAGE

8 µMAX

Selector Guide continued at end of data sheet.

Ordering Information continued at end of data sheet.

MAX7418CUA

0°C to +70°C 8 µMAX

MAX7419CUA

0°C to +70°C 8 µMAX

MAX7419EUA -40°C to +85°C 8 µMAX

MAX7420CUA

0°C to +70°C 8 µMAX

MAX7420EUA -40°C to +85°C 8 µMAX

MAX7421CUA

0°C to +70°C 8 µMAX

MAX7421EUA -40°C to +85°C 8 µMAX

PART FILTER RESPONSE

OPERATING

VOLTAGE (V)

MAX7418 r = 1.6 +5

MAX7419 Bessel +5

MAX7420 Butterworth +5

MAX7421 r = 1.25 +5

87CLK

SHDNIN

OS

6

OUTV

5

TOP VIEW

COM

GND

1

2

3

4

DD

MAX7418–

MAX7425

V

SUPPLY

0.1µF

V

DD

SHDN

INPUT

CLOCK

IN

MAX7418–

MAX7425

CLK

GND

OUT

COM

OUTPUT

OS

0.1µF

µMAX

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS—MAX7418–MAX7421

(VDD= +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF capacitor from COM to GND,
SHDN = V

DD

, f

CLK

= 2.2MHz, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.

VDDto GND..............................................................-0.3V to +6V

IN, OUT, COM, OS, CLK, SHDN ................-0.3V to (V

DD

+ 0.3V)

OUT Short-Circuit Duration.......................................................1s

Continuous Power Dissipation (T

A

= +70°C)

8-Pin µMAX (derate 4.1mW/°C above +70°C) .............330mW

Operating Temperature Ranges

MAX74 _ _C_A ...................................................0°C to +70°C

MAX74 _ _E_A ................................................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +160°C

Lead Temperature (soldering, 10s) .................................+300°C

CLOCK

FILTER CHARACTERISTICS

86 110 135

±50 ±75

MAX7419/MAX7420

MAX7418/MAX7421

MAX7419/MAX7420

MAX7418/MAX7421

-0.2 0 +0.2MAX7419/MAX7420

MAX7418/MAX7421

0.001 to 30VIN= 4Vp-p (Note 1)

Input Voltage Range at OS

2.0 2.5 3.0

COM Voltage Range

Maximum Capacitive Output
Load Drive

UNITS

kHz

ppm/°C

V

mV

dB

V/V

V

V

kΩ

mVp-p

kΩ

pF

µA

µA

kHz

µA

V

V

Clock Output Current
(Internal Oscillator Mode)

I

CLK

±40 ±60

Clock Input High V

IH

4.5

Clock Input Low V

IL

0.5

V

CLK

= 0 or 5V

PARAMETER SYMBOL MIN TYP MAX

DC Insertion Gain with
Output Offset Removed

0 0.2 0.4

Output Offset Voltage V

OFFSET

±4 ±25

Output Voltage Range 0.25 V

DD

-

0.25

Clock-to-Corner Tempco 10

Offset Voltage Gain A

OS

1

V

COM

Corner Frequency f

c

Clock-to-Corner Ratio f

CLK/fC

100:1

2.3 2.5 2.7

V

OS

V

COM

±0.1

Input Resistance at COM R

COM

100 140

Clock Feedthrough 5

Resistive Output Load Drive R

L

10 1

C

L

50 500

Input Leakage Current at COM ±0.1 ±10

Input Leakage Current at OS ±0.1 ±10

Internal Oscillator Frequency f

OSC

68 87 106

CONDITIONS

Output, COM unconnected

V

COM

= V

DD

/ 2

(Note 2)

VIN= V

COM

= V

DD

/ 2

Input, OS externally driven

OS to OUT

Input, COM externally driven

SHDN = GND, V

COM

= 0 to V

DD

VOS= 0 to V

DD

C

OSC

= 1000pF

(Note 3)

-78MAX7419

MAX7418 -76

-78MAX7421

MAX7420

dB

Total Harmonic Distortion
plus Noise

THD+N

-67

fIN= 2kHz,
VIN= 4Vp-p,
measurement
bandwidth = 80kHz

FILTER CHARACTERISTICS

CLOCK

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 3

Corner-Frequency Range

0.001 to 45

5th-Order, Lowpass,

Switched-Capacitor Filters

ELECTRICAL CHARACTERISTICS—MAX7418–MAX7421 (continued)

(VDD= +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF capacitor from COM to GND,
SHDN = V

DD

, f

CLK

= 2.2MHz, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

ELECTRICAL CHARACTERISTICS—MAX7422–MAX7425

(VDD= +3V, filter output measured at OUT pin, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF capacitor from COM to GND,
SHDN = V

DD

, f

CLK

= 2.2MHz, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

FILTER CHARACTERISTICS

0.001 to 45

MAX7423/MAX7424

MAX7422/MAX7425

-0.2 0 +0.2MAX7423/MAX7424

MAX7422/MAX7425

UNITS

kHz

ppm/°C

V

mV

dB

V/V

V

kΩ

mVp-p

kΩ

pF

µA

µA

PARAMETER SYMBOL MIN TYP MAX

DC Insertion Gain with Output
Offset Removed

0 0.2 0.4

Output Offset Voltage V

OFFSET

±4 ±25

Output Voltage Range 0.25 VDD- 0.25

Clock-to-Corner Tempco 10

Offset Voltage Gain A

OS

1

V

COM

f

C

Clock-to-Corner Ratio f

CLK/fC

100:1

Input Voltage Range at OS V

OS

V

COM

±0.1

Input Resistance at COM R

COM

100 140

Clock Feedthrough 3

Resistive Output Load Drive R

L

10 1

C

L

50 500

Input Leakage Current at COM ±0.1 ±10

Input Leakage Current at OS ±0.1 ±10

CONDITIONS

V

COM

= V

DD

/ 2

(Note 2)

VIN= V

COM

= V

DD

/ 2

Measured with respect to COM

OS to OUT

VIN= 2.5Vp-p
(Note 1)

SHDN = GND, V

COM

= 0 to V

DD

VOS= 0 to V

DD

Maximum Capacitive Load
at OUT

-81MAX7423

MAX7422 -80

-80MAX7425

MAX7424

dB

Total Harmonic Distortion plus
Noise

THD+N

-70

fIN= 2kHz,
VIN= 2.5Vp-p,
measurement
bandwidth = 80kHz

1.4 1.5 1.6Input, COM externally driven
VCOM Voltage Range

1.4 1.5 1.6Output, COM internally driven

FILTER CHARACTERISTICS

SHUTDOWN

POWER REQUIREMENTS

3.4 4.1MAX7419/MAX7420

MAX7418/MAX7421

µA±0.2 ±10

V

SHDN

= 0 to V

DD

SHDN Input Leakage Current

UNITS

V

mA

µA

dB

V

V

Supply Voltage V

DD

4.5 5.5

I

DD

Supply Current

2.9 3.6

Shutdown Current I

SHDN

0.2 1

Power-Supply Rejection Ratio PSRR 70

SHDN Input High

V

SDH

4.5

SHDN Input Low

V

SDL

0.5

Operating mode,
no load

SHDN = GND

IN = COM (Note 4)

PARAMETER SYMBOL MIN TYP MAXCONDITIONS

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

4 _______________________________________________________________________________________

FILTER CHARACTERISTICS

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425 filter output measured at OUT, 10kΩ

|| 50pF load to GND at

OUT, SHDN = V

DD

, f

CLK

= 2.2MHz, TA = T

MIN

to T

MAX

, unless otherwise noted.)

ELECTRICAL CHARACTERISTICS—MAX7422–MAX7425 (continued)

(VDD= +3V, filter output measured at OUT pin, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF capacitor from COM to GND,
SHDN = V

DD

, f

CLK

= 2.2MHz, TA= T

MIN

to T

MAX

, unless otherwise noted. Typical values are at TA= +25°C.)

CONDITIONS MIN TYP MAXSYMBOLPARAMETER

Measured at DC

SHDN = GND

0.5V

SDL

SHDN Input Low

2.5V

SDH

SHDN Input High

70PSRRPower-Supply Rejection Ratio

0.2 1I

SHDN

Shutdown Current

2.6 3.4

2.7 3.6V

DD

Supply Voltage

V

V

dB

µA

V

UNITS

SHDN Input Leakage Current

V

SHDN

= 0 to V

DD

±0.2 ±10 µA

C

OSC

= 1000pF

(Note 3)

68 87 106

f

OSC

Internal Oscillator Frequency kHz

0.5V

IL

Clock Input Low V

2.5V

IH

Clock Input High V

68 87 106

I

CLK

Clock Output Current (Internal
Oscillator Mode)

kHz

MAX7422/MAX7425

MAX7423/MAX7424 86 110 135

MAX7422/MAX7425

MAX7423/MAX7424 86 110 135

Operating mode,
no load

3.0 3.8

Supply Current I

DD

mA

MAX7422/MAX7425

MAX7423/MAX7424

MIN TYP MAXCONDITIONSPARAMETER

-0.4 ±0.2 0.4fIN= 0.38f

C

UNITS

Insertion Gain with DC Gain
Error Removed (Note 4)

dB

fIN= 0.68f

C

-0.4 ±0.2 0.4

-0.4 ±0.2 0.4fIN= 0.87f

C

-36 -33fIN= 1.25f

C

fIN= 0.97f

C

-0.4 ±0.2 0.4

-0.7 ±0.2 0.2fIN= f

C

fIN= 1.43f

C

-37.2 -35

-1 -0.74fIN= 0.5f

C

fIN= 3.25f

C

-37.2 -35

Insertion Gain Relative to
DC Gain

dB

fIN= f

C

-3.6 -3.0 -2.4

fIN= 4f

C

-41.0 -35

-67 -60fIN= 7f

C

CLOCK

POWER REQUIREMENTS

ELLIPTIC, r = 1.2—MAX7421/MAX7425

BESSEL FILTERS—MAX7419/MAX7423

SHUTDOWN

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 5

Note 1: The maximum fCis defined as the clock frequency f

CLK

= 100 x fCat which the peak S / (THD+N) drops to 68dB with a

sinusoidal input at 0.2f

C

. Maximum fC increases as VINsignal amplitude decreases.

Note 2: DC insertion gain is defined as ∆V

OUT

/ ∆VIN.

Note 3: MAX7418/MAX7421/MAX7422/MAX7425: f

OSC

(kHz) ≅ 87x103/ C

OSC

(pF).

MAX7419/MAX7420/MAX7423/MAX7424: f

OSC

(kHz) ≅ 110x103/ C

OSC

(pF).

Note 4: PSRR is the change in output voltage from a V

DD

of 4.5V and a VDDof 5.5V.

-70 -65fIN= 5f

C

-47.5 -43

-3.6 -3.0 -2.4fIN= f

C

fIN= 3f

C

-0.3 0

dB

fIN= 0.5f

C

PARAMETER MIN TYP MAX UNITSCONDITIONS

BUTTERWORTH FILTERS—MAX7420/MAX7424BUTTERWORTH FILTERS—MAX7420/MAX7424

FILTER CHARACTERISTICS

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425 filter output measured at OUT, 10kΩ

|| 50pF load to GND at

OUT, SHDN = V

DD

, f

CLK

= 2.2MHz, TA = T

MIN

to T

MAX

, unless otherwise noted.)

__________________________________________Typical Operating Characteristics

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425, f

CLK

= 2.2MHz, SHDN = VDD, V

COM

= VOS= VDD/ 2,

T

A

= +25°C, unless otherwise noted.)

-90

-60

-70

-80

-50

-40

-30

-20

-10

0

10

04020 60 80 100

MAX7418/MAX7422

FREQUENCY RESPONSE (ELLIPTIC, r = 1.6)

MAX7418 toc01

INPUT FREQUENCY (kHz)

GAIN (dB)

-60

-40

-50

-20

-30

0

-10

10

04020 60 80 100

MAX7419/MAX7423

FREQUENCY RESPONSE (BESSEL)

MAX7418 toc02

INPUT FREQUENCY (kHz)

GAIN (dB)

-80

-60

-70

-30

-40

-50

0

-10

-20

10

04020 60 80 100

MAX7420/MAX7424

FREQUENCY RESPONSE (BUTTERWORTH)

MAX7418 toc03

INPUT FREQUENCY (kHz)

GAIN (dB)

Insertion Gain Relative to
DC Gain

____________________________Typical Operating Characteristics (continued)

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425, f

CLK

= 2.2MHz, SHDN = VDD, V

COM

= VOS= VDD/ 2,

T

A

= +25°C, unless otherwise noted.

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

6 _______________________________________________________________________________________

)

)

)

MAX7421/MAX7425

FREQUENCY RESPONSE (ELLIPTIC, r = 1.25)

10

0

-10

-20

-30

-40

GAIN (dB)

-50

-60

-70

-80

-90
04020 60 80 100

INPUT FREQUENCY (kHz)

MAX7418 toc04

0.4

0.2

-0.2

GAIN (dB)

-0.4

-0.6

-0.8

-1.0

MAX7420/MAX7424

PASSBAND FREQUENCY RESPONSE

0.5

0

-0.5

-1.0

-1.5

-2.0

GAIN (dB)

-2.5

-3.0

-3.5

-4.0

09.04.5 13.5 18.0 22.5

0

-50

-100

-150

PHASE SHIFT (DEGREES)

-200

-250
084 1216202428

(BUTTERWORTH)

INPUT FREQUENCY (kHz)

MAX7419/MAX7423

PHASE RESPONSE (BESSEL)

INPUT FREQUENCY (kHz

0.4

0.2

MAX7418 toc07

-0.2

GAIN (dB)

-0.4

-0.6

-0.8

-1.0

0

-50

MAX7418 toc10

-100

-150

-200

-250

PHASE SHIFT (DEGREES)

-300

-350

0

0 9.04.5 13.5 18.0 22.5

0

0 9.04.5 13.5 18.0 22.5

08124 16202428

MAX7418/MAX7422

PASSBAND FREQUENCY RESPONSE

(ELLIPTIC, r = 1.6)

INPUT FREQUENCY (kHz)

MAX7421/MAX7425

PASSBAND FREQUENCY RESPONSE

(ELLIPTIC, r = 1.25)

INPUT FREQUENCY (kHz)

MAX7420/MAX7424

PHASE RESPONSE (BUTTERWORTH)

INPUT FREQUENCY (kHz

0.5

MAX7418 toc05

MAX7418 toc08

MAX7418 toc11

0

-0.5

-1.0

-1.5

-2.0

GAIN (dB)

-2.5

-3.0

-3.5

-4.0

0

-50

-100

-150

-200

-250

-300

PHASE SHIFT (DEGREES)

-350

-400

-450

0

-50

-100

-150

-200

-250

-300

PHASE SHIFT (DEGREES)

-350

-400

-450

MAX7419/MAX7423

PASSBAND FREQUENCY RESPONSE

(BESSEL)

09.04.5 13.5 18.0 22.5
INPUT FREQUENCY (kHz)

MAX7418/MAX7422

PHASE RESPONSE (ELLIPTIC, r = 1.6)

08124 16202428

INPUT FREQUENCY (kHz)

MAX7424/MAX7425

PHASE RESPONSE (ELLIPTIC, r = 1.25)

08124 16202428

INPUT FREQUENCY (kHz

MAX7418 toc06

MAX7418 toc09

MAX7418 toc12

____________________________Typical Operating Characteristics (continued)

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425, f

CLK

= 2.2MHz, SHDN = VDD, V

COM

= VOS= VDD/ 2,

T

A

= +25°C, unless otherwise noted.

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 7

MAX

8

THD

N

(dB)

E

MAX7424

MAX7425

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

741

THD + NOISE vs. INPUT SIGNAL AMPLITUD

(ELLIPTIC, r = 1.6)

0

SEE TABLE A

-10

-20

-30

-40

+

-50

-60

-70

-80

-90
021 345

AMPLITUDE (Vp-p)

D

E

MAX7418 toc13

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90
021 345

MAX7421

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, r = 1.25)

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90
021 345

D

E

AMPLITUDE (Vp-p)

MAX7418 toc16

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90
0 1.00.5 1.5 2.0 2.5 3.0

MAX7419

(BESSEL)

D

E

AMPLITUDE (Vp-p)

MAX7422

(ELLIPTIC, r = 1.6)

C

AMPLITUDE (Vp-p)

MAX7418 toc14

MAX7418 toc17

A

B

MAX7420

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BUTTERWORTH)

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90
021 345

D

E

AMPLITUDE (Vp-p)

MAX7423

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BESSEL)

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90

01.00.5 1.5 2.0 2.5 3.0

A

B

C

AMPLITUDE (Vp-p)

MAX7418 toc15

MAX7418 toc18

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BUTTERWORTH)

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90

C

0 0.5 1.0 1.5 2.0 2.5 3.0

AMPLITUDE (Vp-p)

B

A

MAX7418 toc19

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, r = 1.25)

0

SEE TABLE A

-10

-20

-30

-40

-50

THD + N (dB)

-60

-70

-80

-90
0 0.5 1.0 1.5 2.0 2.5 3.0

A

B

C

AMPLITUDE (Vp-p)

INTERNAL OSCILLATOR FREQUENCY

vs. SMALL CAPACITANCE (pF)

7000

6000

MAX7418 toc20

5000

4000

3000

ELLIPTIC

2000

OSCILLATOR FREQUENCY (kHz)

1000

0

1 10 100 1000 10000

MAX7418 toc21

BESSEL/BUTTERWORTH

CAPACITANCE ( pF)

____________________________Typical Operating Characteristics (continued)

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425, f

CLK

= 2.2MHz, SHDN = VDD, V

COM

= VOS= VDD/ 2,

T

A

= +25°C, unless otherwise noted.

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

8 _______________________________________________________________________________________

1

f

IN

(kHz)

2

2

1

2

10

f

C

(kHz)

E

30

22

10

22

1000

LABEL

A

B

f

CLK

(kHz)

3000

2200

1000C

D 2200

22

BW

(kHz)

80

80

22

80

Table A.

INTERNAL OSCILLATOR FREQUENCY

vs. LARGE CAPACITANCE (nF)

6

ELLIPTIC INTERNAL OSCILLATOR

FREQUENCY vs. SUPPLY VOLTAGE

87.0

5

4

3

2

ELLIPTIC

OSCILLATOR FREQUENCY (Hz)

1

0

10 100 1000

BESSEL/BUTTERWORTH

CAPACITANCE (nF)

ELLIPTIC INTERNAL OSCILLATOR

FREQUENCY vs. TEMPERATURE

87.0

86.5

86.0

85.5

85.0

OSCILLATOR FREQUENCY (kHz)

84.5

C

= 1000pF

OSC

84.0

-40 10-15 356085

ELLIPTIC SUPPLY CURRENT

3.0

VDD = 3V

VDD = 5V

TEMPERATURE (°C)

vs. TEMPERATURE

MAX7418 toc22

MAX7418 toc24

86.5

86.0

85.5

85.0

OSCILLATOR FREQUENCY (kHz)

84.5

C

= 1000PF

OSC

84.0

2.5 3.5 4.03.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)

ELLIPTIC SUPPLY CURRENT

vs. SUPPLY VOLTAGE

3.3

3.1

2.9

2.7

SUPPLY CURRENT (µA)

2.5

2.3

2.5 3.53.0 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)

MAX7418 toc23

MAX7418 toc25

2.9

2.8

2.7

SUPPLY CURRENT (mA)

2.6

2.5

-40 10-15 35 60 85

VDD = 5V

VDD = 3V

TEMPERATURE (°C)

MAX7418 toc26

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 9

Pin Description

_______________Detailed Description

The MAX7418/MAX7421/MAX7422/MAX7425 elliptic
lowpass filters provide sharp rolloff with good stopband
rejection. The MAX7419/MAX7423 Bessel filters provide
low overshoot and fast settling responses, and the
MAX7420/MAX7424 Butterworth filters provide a maxi­mally flat passband response. All parts operate with a
100:1 clock-to-corner frequency ratio.

Most switch capacitor filters (SCFs) are designed with
biquadratic sections. Each section implements two
pole-zero pairs, and the sections can be cascaded to

produce higher order filters. The advantage to this
approach is ease of design. However, this type of
design is highly sensitive to component variations if any
section’s Q is high. The MAX7418–MAX7425 use an
alternative approach, which is to emulate a passive net­work using switched-capacitor integrators with sum­ming and scaling. The passive network may be
synthesized using CAD programs, or may be found in
many filter books. Figure 1 shows a basic 5th-order lad­der filter structure.

Typical Operating Characteristics (continued)

(VDD= +5V for MAX7418–MAX7421, VDD= +3V for MAX7422–MAX7425, f

CLK

= 2.2MHz, SHDN = VDD, V

COM

= VOS= VDD/ 2,

T

A

= +25°C, unless otherwise noted.

0

0.5

1.5

1.0

2.0

2.5

2.5 3.53.0 4.0 4.5 5.0 5.5

DC OFFSET VOLTAGE

vs. SUPPLY VOLTAGE

MAX7418 toc28

SUPPLY VOLTAGE (V)

DC OFFSET VOLTAGE (mV)

0

1.0

0.5

2.0

1.5

2.5

3.0

-40 10-15 356085

DC OFFSET VOLTAGE

vs. TEMPERATURE

MAX7418 toc27

TEMPERATURE (°C)

DC OFFSET VOLTAGE (mV)

VDD = 5V

VDD = 3V

Filter OutputOUT5

Offset Adjust Input. To adjust output offset, connect OS to an external supply through a resistive voltage­divider (Figure 4). Connect OS to COM if no offset adjustment is needed. See the Offset and Common-Mode
Input Adjustment section.

OS6

Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDDfor normal operation.

SHDN

7

Clock Input. Connect an external capacitor (C

OSC

) from CLK to ground. To override the internal oscillator,

connect CLK to an external clock: f

C

= f

CLK

/100.

CLK8

Positive Supply Input: +5V for MAX7418–MAX7421, +3V for MAX7422–MAX7425. Bypass V

DD

to GND with

a 0.1µF capacitor.

V

DD

4

Ground GND3

PIN

Filter InputIN2

Common Input Pin. Biased internally at midsupply. Bypass COM externally to GND with a 0.1µF capacitor.
To override internal biasing, drive COM with an external supply.

COM1

FUNCTIONNAME

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

10 ______________________________________________________________________________________

An SCF that emulates a passive ladder filter retains
many of the same advantages. The component sensi­tivity of a passive ladder filter is low when compared to
a cascaded biquadratic design because each compo­nent affects the entire filter shape rather than a single
pole-zero pair. In other words, a mismatched compo­nent in a biquadratic design has a concentrated error
on its respective poles, while the same mismatch in a
ladder filter design spreads its error over all poles.

Elliptic Characteristics

Lowpass elliptic filters such as the MAX7418/MAX7421/
MAX7422/MAX7425 provide the steepest possible
rolloff with frequency of the four most common filter
types (Butterworth, Bessel, Chebyshev, and elliptic).
The high-Q value of the poles near the passband edge
combined with the stopband zeros allow for the sharp
attenuation characteristic of elliptic filters, making these
devices ideal for anti-aliasing and post-DAC filtering in
single-supply systems (see Anti-Aliasing and Post-DAC
Filtering).

In the frequency domain, the first transmission zero
causes the filter’s amplitude to drop to a minimum level
(Figure 2). Beyond this zero, the response rises as the
frequency increases until the next transmission zero.
The stopband begins at the stopband frequency, f

S

. At

frequencies above f

S

, the filter’s gain does not exceed

the gain at f

S

. The corner frequency, fC, is defined as
the point at which the filter output attenuation falls just
below the passband ripple. The transition ratio (r) is
defined as the ratio of the stopband frequency to the
corner frequency:

r = f

S

/ f

C

The MAX7418/MAX7422 have a transition ratio of 1.6
and typically 53dB of stopband rejection. The
MAX7421/MAX7425 have a transition ratio of 1.25 (pro­viding a steeper rolloff) and typically 37dB of stopband
rejection.

Bessel Characteristics

Lowpass Bessel filters such as the MAX7419/MAX7423

delay all frequency components equally, preserving the
line up shape of step inputs (subject to the attenuation
of the higher frequencies). Bessel filters settle quickly—
an important characteristic in applications that use a
multiplexer (mux) to select an input signal for an ana­log-to-digital converter (ADC). An anti-aliasing filter
placed between the mux and the ADC must settle
quickly after a new channel is selected.

Butterworth Characteristics

Lowpass Butterworth filters such as the MAX7420/
MAX7424 provide a maximally flat passband response,
making them ideal for instrumentation applications that
require minimum deviation from the DC gain throughout
the passband.

The difference between Bessel and Butterworth filters
can be observed when a 1kHz square wave is applied
to the filter input (Figure 3, trace A). With the filter cutoff
frequencies set at 5kHz, trace B shows the Bessel filter
response and trace C shows the Butterworth filter
response.

Clock Signal

External Clock

These SCFs are designed for use with external clocks
that have a 40% to 60% duty cycle. When using an
external clock, drive the CLK pin with a CMOS gate
powered from 0 to V

DD

. Varying the rate of the external

clock adjusts the corner frequency of the filter:

Figure 1. 5th-Order Ladder Filter Network

Figure 2. Elliptic Filter Response

R

S

+

V

IN

-

L2

C1

L4

C5C3

R

L

GAIN (dB)

PASSBAND STOPBAND

f

f

f

C

CLK

=

C

100

RIPPLE

TRANSITION RATIO =

f

S

f

CfS

f

S

f

C

FREQUENCY

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

______________________________________________________________________________________ 11

Internal Clock

When using the internal oscillator, the capacitance
(C

OSC

) on CLK determines the oscillator frequency:

where

k = 87 x 103 for the
MAX7418/MAX7421/MAX7422/MAX7425

and

k = 110 x 103for the
MAX7419/MAX7420/MAX7423/ MAX7424.

Since C

OSC

is in the low picofarads, minimize the stray
capacitance at CLK so that it does not affect the inter­nal oscillator frequency. Varying the rate of the internal
oscillator adjusts the filter’s corner frequency by a
100:1 clock-to-corner frequency ratio. For example, an
internal oscillator frequency of 2.2MHz produces a
nominal corner frequency of 2.2kHz.

Input Impedance vs. Clock Frequencies

The MAX7418–MAX7425s’ input impedance is effective
as a switched-capacitor resistor and is inversely propor­tional to frequency. The input impedance values deter­mined by the equation represents the average input
impedance, since the input current is not continuous.
As a rule, use a driver with an output resistance less
than 10% of the filter’s input impedance.

Estimate the input impedance of the filter by using the
following formula:

where f

CLK

= clock frequency and CIN= 1pF.

Low-Power Shutdown Mode

The MAX7418–MAX7425 have a shutdown mode that is
activated by driving SHDN low. In shutdown mode, the
filter supply current reduces to 0.2µA, and the output of
the filter becomes high impedance. For normal opera­tion, drive SHDN high or connect to VDD.

Applications Information

Offset (OS) and Common-Mode (COM)

Input Adjustment

COM sets the common-mode input voltage and is
biased at midsupply with an internal resistor-divider. If
the application does not require offset adjustment, con­nect OS to COM. For applications in which offset
adjustment is required, apply an external bias voltage
through a resistor-divider network to OS, as shown in
Figure 4. For applications that require DC level shifting,
adjust OS with respect to COM. (Note: Do not leave OS
unconnected.) The output voltage is represented by
these equations:

where (VIN- V

COM

) is lowpass filtered by the SCF and

OS is added at the output stage. See the Electrical

Characteristics table for the input voltage range of COM

Figure 4. Offset Adjustment Circuit

Figure 3. Bessel vs. Butterworth Filter Response

2V/div

A

2V/div

B

2V/div

C

200µs/div

A: 1kHz INPUT SIGNAL
B: MAX7419 BESSEL FILTER RESPONSE; f
C: MAX7420 BUTTERWORTH FILTER RESPONSE; f

= 5kHz

C

= 5kHz

C

f

OSC

(kHz)

=

C

OSC

k

(pF)

V

SUPPLY

0.1µF

V

DD

INPUT

CLOCK

IN

MAX7418–

MAX7425

CLK

GND

Z

=

IN

(f C )

CLK IN

SHDN

OUT

COM

OS

1

×

0.1µF

0.1µF

OUTPUT

50k

50k

50k

VVVV

( )

=− +

OUT IN COM OS

V

V

COM

DD

()

=

typ

2

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

12 ______________________________________________________________________________________

and OS. Changing the voltage on COM or OS signifi­cantly from midsupply reduces the dynamic range.

Power Supplies

The MAX7418–MAX7421 operate from a single +5V
supply and the MAX7422–MAX7425 operate from a sin­gle +3V supply. Bypass VDDto GND with a 0.1µF
capacitor. If dual supplies are required, connect COM
to the system ground and GND to the negative supply.
Figure 5 shows an example of dual-supply operation.
Single-supply and dual-supply performance are equiv­alent. For either single-supply or dual-supply operation,
drive CLK and SHDN from GND (V- in dual supply
operation) to V

DD

. Use the MAX7418–MAX7421 for
±2.5, and use the MAX7422–MAX7425 for ±1.5V. For
±5V dual-supply applications, refer to the MAX291/
MAX292/MAX295/MAX296 and MAX293/MAX294/
MAX297 data sheets.

Input Signal Amplitude Range

The optimal input signal range is determined by observ­ing the voltage level at which the signal-to-noise plus
distortion (SINAD) ratio is maximized for a given corner
frequency. The Typical Operating Characteristics show
the THD + Noise response as the input signal’s peak-to­peak amplitude is varied.

Anti-Aliasing and Post-DAC Filtering

When using the MAX7418–MAX7425 for anti-aliasing or
post-DAC filtering, synchronize the DAC (or ADC) and
the filter clocks. If the clocks are not synchronized, beat
frequencies may alias into the desired passband.

Harmonic Distortion

Harmonic distortion arises from nonlinearities within the
filter. These nonlinearities generate harmonics when a
pure sine wave is applied to the filter input. Tables 1, 2,
and 3 list typical harmonic distortion values with a 10kΩ
load at TA= +25°C.

Table 1. MAX7418/MAX7421/MAX7422/MAX7425 Typical Harmonic Distortion

FILTER

f

IN

(kHz)

V

IN

(Vp-p)

MAX7418

2

MAX7421

2

2

4

2nd 4th

f

CLK

(MHz)

2.2

1.5

2

<-80<-80

<-80

MAX7422

4

TYPICAL HARMONIC DISTORTION (dB)

<-80

<-80

4

2

2

4.0

2.2

<-80<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80 <-80

3rd 5th

2.2

1.5

<-80<-80

<-80

4

<-80

<-80

MAX7425

2

2

4.0

2.2

<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80

<-80

Figure 5. Dual-Supply Operation

V+

SHDN

COM

*

OUTPUTOUT

OS

0.1µF

0.1µF

V

DD

IN

INPUT

MAX7418–

V+

V-

*CONNECT SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.

CLOCK

MAX7425

CLK

GND

V-

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

______________________________________________________________________________________ 13

Table 3. MAX7419/MAX7423 Typical Harmonic Distortion

Table 2. MAX7420/MAX7424 Typical Harmonic Distortion

Chip Information

TRANSISTOR COUNT: 1457

PROCESS: BiCMOS

Ordering Information (continued)

PART TEMP. RANGE PIN-PACKAGE

MAX7423CUA

MAX7423EUA -40°C to +85°C

0°C to +70°C 8 µMAX

8 µMAX

MAX7422CUA

0°C to +70°C 8 µMAX

MAX7422EUA -40°C to +85°C 8 µMAX

PART

FILTER RESPONSE

OPERATING

VOLTAGE (V)

MAX7422 r = 1.6 +3

MAX7423 Bessel +3

MAX7424 Butterworth +3

MAX7425 r = 1.25 +3

Selector Guide (continued)

MAX7424CUA

0°C to +70°C 8 µMAX

MAX7424EUA -40°C to +85°C 8 µMAX

MAX7425CUA

0°C to +70°C 8 µMAX

MAX7425EUA -40°C to +85°C 8 µMAX

FILTER

f

IN

(kHz)

V

IN

(Vp-p)

MAX7420

2

2

4

-77

< -80

TYPICAL HARMONIC DISTORTION (dB)

< -80

< -80

2nd

4th

f

CLK

(MHz)

2.2

1.5

3

MAX7424

2

2

3.5

2.2

< -80

< -80

< -80

< -80

-67 -76

-70

-70

-77

< -80

< -80

< -80

3rd 5th

FILTER

f

IN

(kHz)

V

IN

(Vp-p)

MAX7419

2

2

4

< -80

< -80

TYPICAL HARMONIC DISTORTION (dB)

< -80

< -80

2nd

4th

f

CLK

(MHz)

2.2

1.5

3

MAX7423

2

2

3.5

2.2

< -80

< -80

< -80

< -80

-77 < -80

-80

-75

< -80

< -80

< -80

< -80

3rd 5th

MAX7418–MAX7425

5th-Order, Lowpass,
Switched-Capacitor Filters

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

________________________________________________________Package Information

8LUMAXD.EPS