MAX7420CUA-T - Hytronics - Datasheet.Directory

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General Description

The MAX7418–MAX7425 5th-order, low-pass, switched-

capacitor 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, mak-

ing them ideal for low-power post-DAC filtering and anti-

aliasing 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 stop-

band rejection. The MAX7419/MAX7423 Bessel filters

provide low overshoot and fast settling, and the

MAX7420/MAX7424 Butterworth filters provide a maxi-

mally flat passband response. Their fixed response sim-

plifies the design task of selecting a clock frequency.

Applications

ADC Anti-Aliasing CT2 Base Stations

DAC Postfiltering Speech Processing

Features

o5th-Order, Lowpass Filters

Elliptic Response (MAX7418/MAX7421/

MAX7422/MAX7425)

Bessel Response (MAX7419/MAX7423)

Butterworth Response (MAX7420/MAX7424)

oClock-Turnable Corner Frequency (1Hz to 45kHz)

oSingle-Supply Operation

+5V (MAX7418–MAX7421)

+3V (MAX7422–MAX7425)

oLow Power

3mA (Operating Mode)

0.2µA (Shutdown Mode)

oAvailable in 8-Pin µMAX Package

oLow Output Offset: ±4mV

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

________________________________________________________________

Maxim Integrated Products

OUTVDD

CLK

SHDNIN

GND

COM

µMAX

TOP VIEW

MAX7418–

MAX7425

VDD

CLK

OUT

GND

INPUT

0.1µF

CLOCK

SHDN

OUTPUT

VSUPPLY

COM

MAX7418–

MAX7425

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

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 = VDD, fCLK = 2.2MHz, TA= TMIN to TMAX, 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.

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

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

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

Continuous Power Dissipation (TA= +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/V

kΩ

mVp-p

kΩ

µA

kHz

µA

Clock Output Current

(Internal Oscillator Mode) ICLK

±40 ±60

Clock Input High VIH 4.5

Clock Input Low VIL 0.5

VCLK = 0 or 5V

PARAMETER SYMBOL MIN TYP MAX

DC Insertion Gain with

Output Offset Removed

0 0.2 0.4

Output Offset Voltage VOFFSET ±4 ±25

Output Voltage Range 0.25 VDD - 0.25

Clock-to-Corner Tempco 10

Offset Voltage Gain AOS 1

VCOM

Corner Frequency fc

Clock-to-Corner Ratio fCLK /f

C100:1

2.3 2.5 2.7

VOS VCOM ±0.1

Input Resistance at COM RCOM 100 140

Clock Feedthrough 5

Resistive Output Load Drive RL10 1

CL50 500

Input Leakage Current at COM ±0.1 ±10

Input Leakage Current at OS ±0.1 ±10

Internal Oscillator Frequency fOSC

68 87 106

CONDITIONS

Output, COM unconnected

VCOM = VDD / 2

(Note 2)

VIN = VCOM = VDD / 2

Input, OS externally driven

OS to OUT

Input, COM externally driven

SHDN = GND, VCOM = 0 to VDD

VOS = 0 to VDD

COSC = 1000pF (Note

-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

0.001 to 45

MAX7418–MAX7425

_______________________________________________________________________________________ 3

5th-Order, Lowpass,

Switched-Capacitor Filters

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 = VDD, fCLK = 2.2MHz, TA= TMIN to TMAX, 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 = VDD, fCLK = 2.2MHz, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

CONDITIONS MIN TYP MAXSYMBOLPARAMETER

IN = COM (Note 4)

SHDN = GND

Operating mode, no

load

0.5VSDL

SHDN Input Low

4.5VSDH

SHDN Input High

70PSRRPower-Supply Rejection Ratio

0.2 1ISHDN

Shutdown Current

2.9 3.6

Supply Current IDD

4.5 5.5VDD

Supply Voltage

µA

UNITS

SHDN Input Leakage Current VSHDN = 0 to VDD ±0.2 ±10 µA

MAX7418/MAX7421

MAX7419/MAX7420 3.4 4.1

POWER REQUIREMENTS

SHUTDOWN

FILTER CHARACTERISTICS

100:1 to 45

MAX7423/MAX7424

MAX7422/MAX7425

-0.2 0 +0.2MAX7423/MAX7424

MAX7422/MAX7425

UNITS

kHz

ppm/°C

V/V

kΩ

mVp-p

kΩ

µA

PARAMETER SYMBOL MIN TYP MAX

DC Insertion Gain with Output

Offset Removed

0 0.2 0.4

Output Offset Voltage VOFFSET ±4 ±25

Output Voltage Range 0.25 VDD - 0.25

Clock-to-Corner Tempco 10

Offset Voltage Gain AOS 1

VCOM

Corner-Frequency Range fC

Clock-to-Corner Ratio fCLK/fC100:1

Input Voltage Range at OS VOS VCOM ±0.1

Input Resistance at COM RCOM 100 140

Clock Feedthrough 3

Resistive Output Load Drive RL10 1

CL50 500

Input Leakage Current at COM ±0.1 ±10

Input Leakage Current at OS ±0.1 ±10

CONDITIONS

VCOM = VDD / 2

(Note 2)

VIN = VCOM = VDD / 2

Measured with respect to COM

OS to OUT

VIN = 2.5Vp-p

(Note 1)

SHDN = GND, VCOM = 0 to VDD

VOS = 0 to VDD

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

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

4 _______________________________________________________________________________________

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 = VDD, fCLK = 2.2MHz, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

CONDITIONS MIN TYP MAXSYMBOLPARAMETER

Measured at DC

SHDN = GND

0.5VSDL

SHDN Input Low

2.5VSDH

SHDN Input High

70PSRRPower-Supply Rejection Ratio

0.2 1ISHDN

Shutdown Current

2.6 3.4

2.7 3.6VDD

Supply Voltage

µA

UNITS

SHDN Input Leakage Current VSHDN = 0 to VDD ±0.2 ±10 µA

COSC = 1000pF

(Note 3)

68 87 106

fOSC

Internal Oscillator Frequency kHz

0.5VIL

Clock Input Low

2.5VIH

Clock Input High

68 87 106

ICLK

Clock Output Current (Internal

Oscillator Mode) kHz

MAX7422/MAX7425

MAX7423/MAX7424 86 110 135

MAX7422/MAX7425

MAX7423/MAX7424 86 110 135

POWER REQUIREMENTS

Operating mode,

no load 3.0 3.8

Supply Current IDD mA

CLOCK

SHUTDOWN

MAX7422/MAX7425

MAX7423/MAX7424

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 5

FILTER CHARACTERISTICS

(VDD = +5V for MAX7418-MAX7420, VDD = +3V for MAX7422-MAX7425 filter output measured at OUT, 10kΩ|| 50pF load to GND at

OUT, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted.)

Note 1: The maximum fCis defined as the clock frequency fCLK = 100 x fCat which the peak S / (THD+N) drops to 68dB with a

sinusoidal input at 0.2fC. Maximum fC increases as VIN signal amplitude decreases.

Note 2: DC insertion gain is defined as ΔVOUT / ΔVIN.

Note 3: MAX7418/MAX7421/MAX7422/MAX7425: fOSC (kHz) ≅ 87x103/ COSC (pF).

MAX7419/MAX7420/MAX7423/MAX7424: fOSC (kHz) ≅ 110x103/ COSC (pF).

Note 4: PSRR is the change in output voltage from a VDD of 4.5V and a VDD of 5.5V.

BESSEL FILTERS—MAX7409/MAX7413

-67 -60fIN = 7fC

Insertion Gain Relative to

DC Gain -41.0 -35fIN = 7fC

-3.6 -3.0 -2.4

-1.0 -0.74fIN = 0.5fC

fIN = 4fC

Insertion Gain Relative to

DC Gain

-70 -65fIN = 5fC

-47.5 -43

Insertion Gain Relative to

DC Gain

-3.6 -3.0 -2.4fIN = fC

fIN = 3fC

-0.3 0

fIN = 0.5fC

-67 -60

fIN = 7fC

-41.0 -35fIN = 4fC

-3.6 -3.0 -2.4fIN = fC

-1 -0.74fIN = 0.5fC

PARAMETER MIN TYP MAX UNITSCONDITIONS

-0.4 -0.2 0.4fIN = 0.68fC

-0.4 -0.2 0.4fIN = 0.87fC

-0.4 -0.2 0.4fIN = 0.97fC

-0.7 -0.2 0.2fIN = fC

-36 -33fIN = 1.25fC

-37.2 -35fIN = 1.43fC

Insertion Gain with DC Gain

Error Removed (Note 4)

-37.2 -35

fIN = 3.25fC

-0.4 -0.2 0.4fIN = 0.38fC

BESSEL FILTERS—MAX7419/MAX7423

BUTTERWORTH FILTERS—MAX7420/MAX7424

BESSEL FILTERS—MAX7419/MAX7423

ELLIPTIC, R = 1.2—MAX7421/MAX7425

__________________________________________Typical Operating Characteristics

(VDD = +5V for MAX7418–MAX7421, VDD = +3V for MAX7422–MAX7425; fCLK = 2.2MHz; SHDN = VDD; VCOM = VOS = VDD / 2;

TA= +25°C; unless otherwise noted.)

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

6 _______________________________________________________________________________________

-90

-60

-70

-80

-50

-40

-30

-20

-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

-10

04020 60 80 100

MAX7419/MAX7423

FREQUENCY RESPONSE (BESSEL)

MAX7418 toc02

INPUT FREQUENCY (kHz)

GAIN (dB)

-80

-60

-70

-30

-40

-50

-10

-20

04020 60 80 100

MAX7420/MAX7424

FREQUENCY RESPONSE (BUTTERWORTH)

MAX7418 toc03

INPUT FREQUENCY (kHz)

GAIN (dB)

-90

-60

-70

-80

-50

-40

-30

-20

-10

04020 60 80 100

MAX7421/MAX7425

FREQUENCY RESPONSE (ELLIPTIC, R = 1.25)

MAX7418 toc04

INPUT FREQUENCY (kHz)

GAIN (dB)

-4.0

-3.0

-3.5

-1.5

-2.0

-2.5

-0.5

-1.0

0.5

09.04.5 13.5 18.0 22.5

MAX7420/MAX7424

PASSBAND FREQUENCY RESPONSE

(BUTTERWORTH)

MAX7418 toc07

INPUT FREQUENCY (kHz)

GAIN (dB)

MAX7418 toc05

-1.0

-0.6

-0.8

-0.2

-0.4

0.2

0.4

09.04.5 13.5 18.0 22.5

MAX7418/MAX7422

PASSBAND FREQUENCY RESPONSE

(ELLIPTIC, R = 1.6)

INPUT FREQUENCY (kHz)

GAIN (dB)

-4.0

-3.0

-3.5

-1.5

-2.0

-2.5

-0.5

-1.0

0.5

09.04.5 13.5 18.0 22.5

MAX7419/MAX7423

PASSBAND FREQUENCY RESPONSE

(BESSEL)

MAX7418 toc06

INPUT FREQUENCY (kHz)

GAIN (dB)

-1.0

-0.6

-0.8

-0.2

-0.4

0.2

0.4

09.04.5 13.5 18.0 22.5

MAX7421/MAX7425

PASSBAND FREQUENCY RESPONSE

(ELLIPTIC, R = 1.25)

MAX7418 toc08

INPUT FREQUENCY (kHz)

GAIN (dB)

-450

-350

-400

-200

-250

-300

-50

-100

-150

08124 16202428

MAX7418/MAX7422

PHASE RESPONSE (ELLIPTIC, R = 1.6)

INPUT FREQUENCY (kHz)

PHASE SHIFT (DEGREES)

MAX7418 toc09

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________

____________________________Typical Operating Characteristics (continued)

(VDD = +5V for MAX7418–MAX7421, VDD = +3V for MAX7422–MAX7425; fCLK = 2.2MHz; SHDN = VDD; VCOM = VOS = VDD / 2;

TA= +25°C; unless otherwise noted.)

-250

-200

-100

-150

-50

084 1216202428

MAX7419/MAX7423

PHASE RESPONSE (BESSEL)

MAX7418 toc10

INPUT FREQUENCY (kHz)

PHASE SHIFT (DEGREES)

-350

-250

-300

-150

-200

-50

-100

08124 16202428

MAX7420/MAX7424

PHASE RESPONSE (BUTTERWORTH)

MAX7418 toc11

INPUT FREQUENCY (kHz)

PHASE SHIFT (DEGREES)

-450

-350

-400

-200

-250

-300

-50

-100

-150

08124 16202428

MAX7424/MAX7425

PHASE RESPONSE (ELLIPTIC, R = 1.25)

MAX7418 toc12

INPUT FREQUENCY (kHz)

PHASE SHIFT (DEGREES)

-90

-70

-80

-40

-50

-60

-10

-20

-30

021 345

MAX7418

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, R = 1.6)

MAX7418 toc13

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-10

-20

-30

021 345

MAX7421

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, R = 1.25)

MAX7418 toc16

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-10

-20

-30

021 345

MAX7419

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BESSEL)

MAX7418 toc14

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-10

-20

-30

021 345

MAX7420

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BUTTERWORTH)

MAX7418 toc15

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-10

-20

-30

01.00.5 1.5 2.0 2.5 3.0

MAX7422

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, R = 1.6)

MAX7418 toc17

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-10

-20

-30

01.00.5 1.5 2.0 2.5 3.0

MAX7423

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BESSEL)

MAX7418 toc18

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

____________________________Typical Operating Characteristics (continued)

(VDD = +5V for MAX7418–MAX7421, VDD = +3V for MAX7422–MAX7425; fCLK = 2.2MHz; SHDN = VDD; VCOM = VOS = VDD / 2;

TA= +25°C; unless otherwise noted.)

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

8 _______________________________________________________________________________________

-90

-70

-80

-40

-50

-60

-30

-20

-10

0 0.5 1.0 1.5 2.0 2.5 3.0

MAX7424

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(BUTTERWORTH)

MAX7418 toc19

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

-90

-70

-80

-40

-50

-60

-30

-20

-10

0 0.5 1.0 1.5 2.0 2.5 3.0

MAX7425

THD + NOISE vs. INPUT SIGNAL AMPLITUDE

(ELLIPTIC, R = 1.25)

MAX7418 toc20

AMPLITUDE (Vp-p)

THD + N (dB)

SEE TABLE A

2000

1000

4000

3000

6000

5000

7000

INTERNAL OSCILLATOR FREQUENCY

vs. SMALL CAPACITANCE (pF)

MAX7418 toc21

CAPACITANCE ( pF)

OSCILLATOR FREQUENCY (kHz)

1 10 100 1000 10000

ELLIPTIC

BESSEL/BUTTERWORTH

10 100 1000

INTERNAL OSCILLATOR FREQUENCY

vs. LARGE CAPACITANCE (nF)

MAX7418 toc22

CAPACITANCE (nF)

OSCILLATOR REQUENCY (Hz)

ELLIPTIC

BESSEL/BUTTERWORTH

2.3

2.5

2.9

2.7

3.1

3.3

2.5 3.53.0 4.0 4.5 5.0 5.5

ELLIPTIC SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX7418 toc25

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (μA)

84.0

85.0

84.5

86.0

85.5

86.5

87.0

2.5 3.5 4.03.0 4.5 5.0 5.5

ELLIPTIC INTERNAL OSCILLATOR

FREQUENCY vs. SUPPLY VOLTAGE

MAX7418 toc23

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (kHz)

COSC = 1000PF

84.0

85.0

84.5

86.0

85.5

86.5

87.0

-40 10-15 35 60 85

ELLIPTIC INTERNAL OSCILLATOR

FREQUENCY vs. TEMPERATURE

MAX7418 toc24

TEMPERATURE (°C)

OSCILLATOR FREQUENCY (kHz)

VDD = 3V

VDD = 5V

COSC = 1000PF

2.5

2.6

2.8

2.7

2.9

3.0

-40 10-15 35 60 85

ELLIPTIC SUPPLY CURRENT

vs. TEMPERATURE

MAX7418 toc26

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

VDD = 5V

VDD = 3V

fIN

(kHz)

1000

LABEL

fCLK

(kHz)

3000

2200

1000C

D2200

(kHz)

Table A.

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________ 9

NAME FUNCTION

1COM 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.

2IN Filter Input

3GND Ground

4 VDD Positive Supply Input: +5V for MAX7418–MAX7421, +3V for MAX7422–MAX7425. Bypass VDD to GND with a

0.1µF capacitor.

8CLK Clock Input. Connect an external capacitor (COSC) from CLK to ground. To override the internal oscillator,

connect CLK to an external clock: fC= fCLK /100.

7SHDN Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDD for normal operation.

6OS

Offset Adjust Input. To adjust output offset, connect OS to an external supply through a resistive voltage-

divider (Figure 4). Connect OS to COM if no offset adjustment is needed. The

Offset and Common-Mode

Input Adjustment

section.

5OUT Filter Output

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 maxi-

mally 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 net-

work using switched-capacitor integrators with sum-

ming 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 lad-

der filter structure.

Typical Operating Characteristics (continued)

(VDD = +5V for MAX7418–MAX7421, VDD = +3V for MAX7422–MAX7425; fCLK = 2.2MHz; SHDN = VDD; VCOM = VOS = VDD / 2;

TA= +25°C; unless otherwise noted.)

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)

1.0

0.5

2.0

1.5

2.5

3.0

-40 10-15 35 60 85

DC OFFSET VOLTAGE

vs. TEMPERATURE

MAX7418 toc27

TEMPERATURE (°C)

DC OFFSET VOLTAGE (mV)

VDD = 5V

VDD = 3V

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 sensi-

tivity of a passive ladder filter is low when compared to

a cascaded biquadratic design because each compo-

nent affects the entire filter shape rather than a single

pole-zero pair. In other words, a mismatched compo-

nent 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, fS. At

frequencies above fS, the filter’s gain does not exceed

the gain at fS. 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 = fS / fC

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 (pro-

viding 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 ana-

log-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 VDD. Varying the rate of the external

clock adjusts the corner frequency of the filter:

CCLK

=100

C5C3

VIN

-RL

Figure 1. 5th-Order Ladder Filter Network

PASSBAND STOPBAND

GAIN (dB)

FREQUENCY

fCfS

TRANSITION RATIO =

RIPPLE

Figure 2. Elliptic Filter Response

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

______________________________________________________________________________________ 11

Internal Clock

When using the internal oscillator, the capacitance

(COSC) on CLK determines the oscillator frequency:

where

k=87x103 for the

MAX7418/MAX7421/MAX7422/MAX7425

and

k=110X103for the

MAX7419/MAX7420/MAX7423/ MAX7424.

Since COSC is in the low picofarads, minimize the stray

capacitance at CLK so that it does not affect the inter-

nal 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 2.2kHz produces a nom-

inal corner frequency of 2.2MHz.

Input Impedance vs. Clock Frequencies

The MAX7418–MAX7425s’ input impedance is effective-

ly that of a switched-capacitor resistor (see the following

equation), and is inversely proportional to frequency.

The input impedance values determined by the equa-

tion represent 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 fCLK = 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 opera-

tion, 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, con-

nect 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 - VCOM) is lowpass filtered by the SCF and

OS is added at the output stage. See the

Electrical

VVVV

VVtypical

OUT IN COM OS

COM DD

( )

()

=− +

Z 1

(f C )

IN CLK IN

=×

OSC(kHz) k

COSC(pF)

VDD

VSUPPLY

CLK

GND

INPUT

OUTPUT

50k

OUT

0.1μF

CLOCK

SHDN

COM

MAX7418–

MAX7425

Figure 4. Offset Adjustment Circuit

2V/div

A: 1kHz INPUT SIGNAL

B: MAX7419 BESSEL FILTER RESPONSE; fC = 5kHz

C: MAX7420 BUTTERWORTH FILTER RESPONSE; fC = 5kHz

200μs/div

Figure 3. Bessel vs. Butterworth Filter Response

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

12 ______________________________________________________________________________________

Characteristics

table for the input voltage range of COM

and OS. Changing the voltage on COM or OS signifi-

cantly from midsupply reduces the dynamic range.

Power Supplies

The MAX7418–MAX7421 operate from a single +5V

supply and the MAX7422–MAX7425 operate from a sin-

gle +3V supply. Bypass VDD to 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 equiv-

alent. For either single-supply or dual-supply operation,

drive CLK and SHDN from GND (V- in dual supply

operation) to VDD. 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 observ-

ing 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-to-

peak 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 fIN

(kHz)

VIN

(Vp-p)

MAX7418 2

MAX7421 2

2nd 4th

fCLK

(MHz)

2.2

1.5

<-80<-80

<-80

MAX7422

TYPICAL HARMONIC DISTORTION (dB)

<-80

4.0

2.2

<-80<-80

<-80

<-80 <-80

3rd 5th

2.2

1.5

<-80<-80

<-80

MAX7425 22

4.0

2.2

<-80

VDD

V+

V-

CLK

GND

INPUT

OUTPUTOUT

0.1μF

CLOCK

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

SHDN

COM

0.1μF

MAX7418–

MAX7425

V+

V-

Figure 5. Dual-Supply Operation

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 fIN

(kHz)

VIN

(Vp-p)

MAX7420 2

24-77

< -80

TYPICAL HARMONIC DISTORTION (dB)

< -80

2nd 4th

fCLK

(MHz)

2.2

1.5

MAX7424 22

3.5

2.2

< -80

-67 -76

-70

-77

< -80

3rd 5th

FILTER fIN

(kHz)

VIN

(Vp-p)

MAX7419 2

24< -80

< -80

TYPICAL HARMONIC DISTORTION (dB)

< -80

2nd 4th

fCLK

(MHz)

2.2

1.5

MAX7423 22

3.5

2.2

< -80

-77 < -80

-80

-75

< -80

3rd 5th

MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

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