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19-1821; Rev 0; 11/00
5th-Order, Lowpass, Switched-Capacitor Filters
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 0.2A. 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.
Features
o 5th-Order, Lowpass Filters Elliptic Response (MAX7418/MAX7421/ MAX7422/MAX7425) Bessel Response (MAX7419/MAX7423) Butterworth Response (MAX7420/MAX7424) o Clock-Turnable Corner Frequency (1Hz to 45kHz) o Single-Supply Operation +5V (MAX7418-MAX7421) +3V (MAX7422-MAX7425) o Low Power 3mA (Operating Mode) 0.2A (Shutdown Mode) o Available in 8-Pin MAX Package o Low Output Offset: 4mV
MAX7418-MAX7425
Ordering Information
PART MAX7418CUA MAX7418EUA MAX7419CUA MAX7419EUA MAX7420CUA MAX7420EUA MAX7421CUA MAX7421EUA TEMP. RANGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C 0C to +70C -40C to +85C 0C to +70C -40C to +85C PIN-PACKAGE 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX
Applications
ADC Anti-Aliasing DAC Postfiltering CT2 Base Stations Speech Processing
Selector Guide
PART MAX7418 MAX7419 MAX7420 MAX7421 FILTER RESPONSE r = 1.6 Bessel Butterworth r = 1.25 OPERATING VOLTAGE (V) +5 +5 +5 +5
Ordering Information continued at end of data sheet.
Typical Operating Circuit
VSUPPLY
Selector Guide continued at end of data sheet.
Pin Configuration
TOP VIEW
0.1F VDD INPUT IN SHDN OUT OUTPUT
COM IN GND
1 2 3
8 7
CLK SHDN OS OUT CLOCK CLK
MAX7418- MAX7425
COM GND OS 0.1F
MAX7418- MAX7425
6 5
VDD 4
MAX ________________________________________________________________ Maxim Integrated Products 1
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.
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
ABSOLUTE MAXIMUM RATINGS
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 = +70C) 8-Pin MAX (derate 4.1mW/C above +70C) .............330mW Operating Temperature Ranges MAX74 _ _C_A ...................................................0C to +70C MAX74 _ _E_A ................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10s) .................................+300C
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.
ELECTRICAL CHARACTERISTICS--MAX7418-MAX7421
(VDD = +5V, filter output measured at OUT, 10k || 50pF load to GND at OUT, OS = COM, 0.1F capacitor from COM to GND, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER FILTER CHARACTERISTICS Corner Frequency Clock-to-Corner Ratio Clock-to-Corner Tempco Output Voltage Range Output Offset Voltage DC Insertion Gain with Output Offset Removed VOFFSET VIN = VCOM = VDD / 2 VCOM = VDD / 2 (Note 2) fIN = 2kHz, VIN = 4Vp-p, measurement bandwidth = 80kHz OS to OUT Input, COM externally driven Output, COM unconnected Input, OS externally driven 100 10 50 SHDN = GND, VCOM = 0 to VDD VOS = 0 to VDD COSC = 1000pF (Note 3) VCLK = 0 or 5V MAX7418/MAX7421 MAX7419/MAX7420 MAX7418/MAX7421 MAX7419/MAX7420 4.5 0.5 68 86 2.0 2.3 MAX7418/MAX7421 MAX7419/MAX7420 MAX7418 MAX7419 MAX7420 MAX7421 0 -0.2 0.25 4 0.2 0 -76 -78 -67 -78 1 2.5 2.5 VCOM 0.1 140 5 RL CL 1 500 0.1 0.1 87 110 40 50 10 10 106 135 60 75 3.0 2.7 V/V V V k mVp-p k pF A A dB fc fCLK / fC VIN = 4Vp-p (Note 1) 0.001 to 30 100:1 10 VDD - 0.25 25 0.4 +0.2 ppm/C V mV dB kHz SYMBOL CONDITIONS MIN TYP MAX UNITS
Total Harmonic Distortion plus Noise Offset Voltage Gain COM Voltage Range Input Voltage Range at OS Input Resistance at COM Clock Feedthrough Resistive Output Load Drive Maximum Capacitive Output Load Drive Input Leakage Current at COM Input Leakage Current at OS CLOCK Internal Oscillator Frequency Clock Output Current (Internal Oscillator Mode) Clock Input High Clock Input Low
THD+N
AOS VCOM VOS RCOM
fOSC ICLK VIH VIL
kHz A V V
2
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5th-Order, Lowpass, Switched-Capacitor Filters
ELECTRICAL CHARACTERISTICS--MAX7418-MAX7421 (continued)
(VDD = +5V, filter output measured at OUT, 10k || 50pF load to GND at OUT, OS = COM, 0.1F capacitor from COM to GND, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER POWER REQUIREMENTS Supply Voltage Supply Current Shutdown Current Power-Supply Rejection Ratio SHUTDOWN SHDN Input High SHDN Input Low SHDN Input Leakage Current SYMBOL VDD IDD I SHDN PSRR VSDH VSDL V SHDN = 0 to VDD 0.2 Operating mode, no load SHDN = GND IN = COM (Note 4) 4.5 0.5 10 MAX7418/MAX7421 MAX7419/MAX7420 CONDITIONS MIN 4.5 2.9 3.4 0.2 70 TYP MAX 5.5 3.6 4.1 1 UNITS V mA A dB V V A
MAX7418-MAX7425
ELECTRICAL CHARACTERISTICS--MAX7422-MAX7425
(VDD = +3V, filter output measured at OUT pin, 10k || 50pF load to GND at OUT, OS = COM, 0.1F capacitor from COM to GND, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER FILTER CHARACTERISTICS Corner-Frequency Range Clock-to-Corner Ratio Clock-to-Corner Tempco Output Voltage Range Output Offset Voltage DC Insertion Gain with Output Offset Removed VOFFSET VIN = VCOM = VDD / 2 VCOM = VDD / 2 (Note 2) fIN = 2kHz, VIN = 2.5Vp-p, measurement bandwidth = 80kHz OS to OUT Input, COM externally driven Output, COM internally driven Measured with respect to COM 100 10 50 SHDN = GND, VCOM = 0 to VDD VOS = 0 to VDD 1.4 1.4 MAX7422/MAX7425 MAX7423/MAX7424 MAX7422 MAX7423 MAX7424 MAX7425 0 -0.2 0.25 4 0.2 0 -80 -81 -70 -80 1 1.5 1.5 VCOM 0.1 140 3 RL CL 1 500 0.1 0.1 10 10 1.6 1.6 V/V V V k mVp-p k pF A A dB fC fCLK/fC VIN = 2.5Vp-p (Note 1) MAX7422/MAX7425 MAX7423/MAX7424 0.001 to 45 0.001 to 45 100:1 10 VDD - 0.25 25 0.4 +0.2 ppm/C V mV dB kHz SYMBOL CONDITIONS MIN TYP MAX UNITS
Total Harmonic Distortion plus Noise Offset Voltage Gain COM Voltage Range Input Voltage Range at OS Input Resistance at COM Clock Feedthrough Resistive Output Load Drive Maximum Capacitive Load at OUT Input Leakage Current at COM Input Leakage Current at OS
THD+N
AOS VCOM VOS RCOM
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3
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
ELECTRICAL CHARACTERISTICS--MAX7422-MAX7425 (continued)
(VDD = +3V, filter output measured at OUT pin, 10k || 50pF load to GND at OUT, OS = COM, 0.1F capacitor from COM to GND, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER CLOCK Internal Oscillator Frequency Clock Output Current (Internal Oscillator Mode) Clock Input High Clock Input Low POWER REQUIREMENTS Supply Voltage Supply Current Shutdown Current Power-Supply Rejection Ratio SHUTDOWN SHDN Input High SHDN Input Low SHDN Input Leakage Current VSDH VSDL VSHDN = 0 to VDD 0.2 2.5 0.5 10 V V A VDD IDD I SHDN PSRR Operating mode, no load SHDN = GND Measured at DC MAX7422/MAX7425 MAX7423/MAX7424 2.7 2.6 3.0 0.2 70 3.6 3.4 3.8 1 V mA A dB fOSC ICLK VIH VIL COSC = 1000pF (Note 3) MAX7422/MAX7425 MAX7423/MAX7424 MAX7422/MAX7425 MAX7423/MAX7424 68 86 68 86 2.5 0.5 87 110 87 110 106 135 106 135 kHz kHz V V SYMBOL CONDITIONS MIN TYP MAX UNITS
FILTER CHARACTERISTICS
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425 filter output measured at OUT, 10k || 50pF load to GND at OUT, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER ELLIPTIC, r = 1.2--MAX7421/MAX7425 fIN = 0.38fC fIN = 0.68fC fIN = 0.87fC Insertion Gain with DC Gain Error Removed (Note 4) fIN = 0.97fC fIN = fC fIN = 1.25fC fIN = 1.43fC fIN = 3.25fC BESSEL FILTERS--MAX7419/MAX7423 fIN = 0.5fC Insertion Gain Relative to DC Gain fIN = fC fIN = 4fC fIN = 7fC -1 -3.6 -0.74 -3.0 -41.0 -67 -2.4 -35 -60 dB -0.4 -0.4 -0.4 -0.4 -0.7 0.2 0.2 0.2 0.2 0.2 -36 -37.2 -37.2 0.4 0.4 0.4 0.4 0.2 -33 -35 -35 dB CONDITIONS MIN TYP MAX UNITS
4
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5th-Order, Lowpass, Switched-Capacitor Filters
FILTER CHARACTERISTICS
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425 filter output measured at OUT, 10k || 50pF load to GND at OUT, SHDN = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER BUTTERWORTH FILTERS--MAX7420/MAX7424 fIN = 0.5fC Insertion Gain Relative to DC Gain fIN = fC fIN = 3fC fIN = 5fC -0.3 -3.6 0 -3.0 -47.5 -70 -2.4 -43 -65 dB CONDITIONS MIN TYP MAX UNITS
MAX7418-MAX7425
Note 1: The maximum fC is defined as the clock frequency fCLK = 100 x fC at 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 VOUT / VIN. 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.
__________________________________________Typical Operating Characteristics
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425, fCLK = 2.2MHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.)
MAX7418/MAX7422 FREQUENCY RESPONSE (ELLIPTIC, r = 1.6)
MAX7418 toc01
MAX7419/MAX7423 FREQUENCY RESPONSE (BESSEL)
MAX7418 toc02
0 -10 -20 GAIN (dB)
0 -10 GAIN (dB) -20 -30 -40 -50 -60
0 -10 -20 GAIN (dB) -30 -40 -50 -60 -70 -80
-30 -40 -50 -60 -70 -80 -90 0 20 40 60 80 100 INPUT FREQUENCY (kHz)
0
20
40
60
80
100
0
20
40
60
80
100
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (kHz)
_______________________________________________________________________________________
MAX7418 toc03
10
MAX7420/MAX7424 FREQUENCY RESPONSE (BUTTERWORTH)
10
10
5
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
____________________________Typical Operating Characteristics (continued)
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425, fCLK = 2.2MHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.
MAX7421/MAX7425 FREQUENCY RESPONSE (ELLIPTIC, r = 1.25)
MAX7418 toc04
MAX7418/MAX7422 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, r = 1.6)
MAX7418 toc05
MAX7419/MAX7423 PASSBAND FREQUENCY RESPONSE (BESSEL)
0 -0.5 -1.0 GAIN (dB) -1.5 -2.0 -2.5
MAX7418 toc06
10 0 -10 -20 GAIN (dB)
0.4 0.2 0 GAIN (dB) -0.2 -0.4 -0.6
0.5
-30 -40 -50 -60 -70 -80 -90 0 20 40 60 80 100 INPUT FREQUENCY (kHz)
-3.0 -0.8 -1.0 0 4.5 9.0 13.5 18.0 22.5 INPUT FREQUENCY (kHz) -3.5 -4.0 0 4.5 9.0 13.5 18.0 22.5 INPUT FREQUENCY (kHz)
MAX7418 toc07
MAX7418 toc08
0 -0.5 -1.0 GAIN (dB)
0.2 0 GAIN (dB) -0.2 -0.4 -0.6
-50 PHASE SHIFT (DEGREES) -100 -150 -200 -250 -300 -350
-1.5 -2.0 -2.5 -3.0 -3.5 -4.0 0 4.5 9.0 13.5 18.0 22.5 INPUT FREQUENCY (kHz)
-0.8 -1.0 0 4.5 9.0 13.5 18.0 22.5 INPUT FREQUENCY (kHz)
-400 -450 0 4 8 12 16 20 24 28 INPUT FREQUENCY (kHz)
MAX7419/MAX7423 PHASE RESPONSE (BESSEL)
MAX7418 toc10
MAX7420/MAX7424 PHASE RESPONSE (BUTTERWORTH)
MAX7418 toc11
MAX7424/MAX7425 PHASE RESPONSE (ELLIPTIC, r = 1.25)
-50 PHASE SHIFT (DEGREES) -100 -150 -200 -250 -300 -350
MAX7418 toc12
0
0 -50 PHASE SHIFT (DEGREES) -100 -150 -200 -250 -300
0
PHASE SHIFT (DEGREES)
-50
-100
-150
-200
-400 -450 0 4 8 12 16 20 24 28 0 4 8 12 16 20 24 28 INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz)
-250 0 4 8 12 16 20 24 28 INPUT FREQUENCY (kHz)
-350
6
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MAX7418 toc09
0.5
MAX7420/MAX7424 PASSBAND FREQUENCY RESPONSE (BUTTERWORTH)
0.4
MAX7421/MAX7425 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, r = 1.25)
0
MAX7418/MAX7422 PHASE RESPONSE (ELLIPTIC, r = 1.6)
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 = +25C, unless otherwise noted.
MAX7418 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.6)
MAX7418 toc13
MAX7418-MAX7425
MAX7419 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BESSEL)
MAX7418 toc14
MAX7420 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BUTTERWORTH)
SEE TABLE A -10 -20 THD + N (dB) -30 -40 -50 -60 D
MAX7418 toc15
0 -10 -20 THD + N (dB) -30 -40 -50 -60 -70 -80 -90 0
SEE TABLE A
0 -10 -20 THD + N (dB) -30 -40 -50 -60
SEE TABLE A
0
D E 1 2 3 4 5
-70 -80 -90 0 E 1
D
-70 -80 -90 E 0 1
2
3
4
5
2
3
4
5
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
MAX7421 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.25)
MAX7418 toc16
MAX7422 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.6)
MAX7418 toc17
MAX7423 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BESSEL)
SEE TABLE A -10 -20 THD + N (dB) -30 -40 -50 -60 A B
MAX7418 toc18
0 -10 -20 THD + N (dB) -30 -40 -50 -60
SEE TABLE A
0 -10 -20 THD + N (dB) -30 -40 -50 -60
SEE TABLE A
0
D -70 -80 -90 0 E 1 2 3 4 5
A -70 -80 -90 0 0.5 C 1.0 1.5 2.0
B
-70 -80 -90 2.5 3.0 0 0.5 C 1.0 1.5
2.0
2.5
3.0
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
MAX7424 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BUTTERWORTH)
MAX7418 toc19
MAX7425 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.25)
SEE TABLE A -10 -20 THD + N (dB) -30 -40 -50 -60 -70 -80 A B C 0 0.5 1.0 1.5 2.0 2.5 3.0
MAX7418 toc20
INTERNAL OSCILLATOR FREQUENCY vs. SMALL CAPACITANCE (pF)
MAX7418 toc21
0 SEE TABLE A -10 -20 THD + N (dB) -30 -40 -50 -60 -70 -80 -90 0 0.5 1.0 C 1.5 2.0 2.5 A B
0
7000 OSCILLATOR FREQUENCY (kHz) 6000 5000 4000 3000 ELLIPTIC 2000 1000 0 1 10 100 1000 CAPACITANCE ( pF) BESSEL/BUTTERWORTH
-90 3.0
10000
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
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7
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
____________________________Typical Operating Characteristics (continued)
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425, fCLK = 2.2MHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.
INTERNAL OSCILLATOR FREQUENCY vs. LARGE CAPACITANCE (nF)
MAX7418 toc22
ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE
MAX7418 toc23
6 OSCILLATOR FREQUENCY (Hz) 5 4 3 2 ELLIPTIC 1 0 10 100 CAPACITANCE (nF) BESSEL/BUTTERWORTH
87.0 OSCILLATOR FREQUENCY (kHz) 86.5 86.0 85.5 85.0 84.5 84.0 COSC = 1000PF 2.5 3.0 3.5 4.0 4.5 5.0
1000
5.5
SUPPLY VOLTAGE (V)
ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE
VDD = 3V
MAX7418 toc24
ELLIPTIC SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX7418 toc25
87.0 OSCILLATOR FREQUENCY (kHz) 86.5 86.0 85.5 85.0 84.5 84.0 COSC = 1000pF -40 -15 10 35 60 VDD = 5V
3.3
3.1 SUPPLY CURRENT (A)
2.9
2.7
2.5
2.3 85 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
TEMPERATURE (C)
ELLIPTIC SUPPLY CURRENT vs. TEMPERATURE
MAX7418 toc26
3.0
Table A.
LABEL A B C fIN (kHz) 2 2 1 2 1 fC (kHz) 30 22 10 22 10 fCLK (kHz) 3000 2200 1000 2200 1000 BW (kHz) 80 80 22 80 22
2.9 SUPPLY CURRENT (mA)
VDD = 5V
2.8
2.7 VDD = 3V 2.6
D E
2.5 -40 -15 10 35 60 85 TEMPERATURE (C)
8
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5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
Typical Operating Characteristics (continued)
(VDD = +5V for MAX7418-MAX7421, VDD = +3V for MAX7422-MAX7425, fCLK = 2.2MHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.
DC OFFSET VOLTAGE vs. TEMPERATURE
MAX7418 toc27
DC OFFSET VOLTAGE vs. SUPPLY VOLTAGE
MAX7418 toc28
3.0 2.5 DC OFFSET VOLTAGE (mV) 2.0 1.5 1.0 VDD = 3V 0.5 0 -40 -15 10 35 60 VDD = 5V
2.5
DC OFFSET VOLTAGE (mV)
2.0
1.5
1.0
0.5
0 85 2.5 3.0 3.5 4.0 4.5 5.0 5.5 TEMPERATURE (C) SUPPLY VOLTAGE (V)
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME COM IN GND VDD OUT OS SHDN CLK FUNCTION Common Input Pin. Biased internally at midsupply. Bypass COM externally to GND with a 0.1F capacitor. To override internal biasing, drive COM with an external supply. Filter Input Ground Positive Supply Input: +5V for MAX7418-MAX7421, +3V for MAX7422-MAX7425. Bypass VDD to GND with a 0.1F capacitor. Filter Output 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. Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDD for normal operation. 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.
_______________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.
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9
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
RS + C1 L2 C3 L4 C5
VIN
RL
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
Figure 1. 5th-Order Ladder Filter Network
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.
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.
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 (providing a steeper rolloff) and typically 37dB of stopband rejection.
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: f fC = CLK 100
RIPPLE
fC GAIN (dB)
TRANSITION RATIO = fS fC fS
PASSBAND fC fS
STOPBAND FREQUENCY
Bessel Characteristics
Lowpass Bessel filters such as the MAX7419/MAX7423
10 Figure 2. Elliptic Filter Response ______________________________________________________________________________________
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
VSUPPLY
2V/div A 2V/div B 2V/div C
0.1F VDD INPUT IN SHDN OUT COM 0.1F
OUTPUT 50k
MAX7418- MAX7425
CLOCK CLK OS 0.1F 50k
200s/div A: 1kHz INPUT SIGNAL B: MAX7419 BESSEL FILTER RESPONSE; fC = 5kHz C: MAX7420 BUTTERWORTH FILTER RESPONSE; fC = 5kHz
GND
50k
Figure 4. Offset Adjustment Circuit Figure 3. Bessel vs. Butterworth Filter Response
Internal Clock When using the internal oscillator, the capacitance (COSC) on CLK determines the oscillator frequency: fOSC (kHz) = k COSC (pF)
ZIN =
1 (fCLK x CIN )
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 0.2A, and the output of the filter becomes high impedance. For normal operation, drive SHDN high or connect to VDD.
where k = 87 x 103 for the MAX7418/MAX7421/MAX7422/MAX7425 and k = 110 x 103 for 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 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.
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: VOUT = (VIN - VCOM ) + VOS VCOM = VDD (typ) 2
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 (VIN - VCOM) 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
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11
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
and OS. Changing the voltage on COM or OS significantly from midsupply reduces the dynamic range.
V+
Power Supplies
The MAX7418-MAX7421 operate from a single +5V supply and the MAX7422-MAX7425 operate from a single +3V supply. Bypass V DD to GND with a 0.1F 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.
VDD INPUT IN SHDN OUT COM * OUTPUT
V+ V-
MAX7418- MAX7425
CLOCK CLK OS 0.1F GND 0.1F
V*CONNECT SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.
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.
Figure 5. Dual-Supply Operation
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 10k load at TA = +25C.
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.
Table 1. MAX7418/MAX7421/MAX7422/MAX7425 Typical Harmonic Distortion
FILTER fCLK (MHz) 2.2 MAX7418 1.5 2.2 MAX7421 1.5 4.0 MAX7422 2.2 4.0 MAX7425 2.2 12 2 2 4 2 <-80 <-80 <-80 <-80 2 4 2 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 2 2 4 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 fIN (kHz) 2 4 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd <-80 3rd <-80 4th <-80 5th <-80
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5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
Table 2. MAX7420/MAX7424 Typical Harmonic Distortion
FILTER fCLK (MHz) 2.2 MAX7420 1.5 MAX7424 3.5 2.2 2 3 2 2 fIN (kHz) 2 4 < -80 < -80 < -80 -70 -70 -77 < -80 < -80 < -80 < -80 < -80 < -80 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd -77 3rd -67 4th < -80 5th -76
Table 3. MAX7419/MAX7423 Typical Harmonic Distortion
FILTER fCLK (MHz) 2.2 MAX7419 1.5 MAX7423 3.5 2.2 2 3 2 2 fIN (kHz) 2 4 < -80 < -80 < -80 -80 -75 < -80 < -80 < -80 < -80 < -80 < -80 < -80 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd < -80 3rd -77 4th < -80 5th < -80
Ordering Information (continued)
PART MAX7422CUA MAX7422EUA MAX7423CUA MAX7423EUA MAX7424CUA MAX7424EUA MAX7425CUA MAX7425EUA TEMP. RANGE 0C to +70C -40C to +85C 0C to +70C -40C to +85C 0C to +70C -40C to +85C 0C to +70C -40C to +85C PIN-PACKAGE 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX 8 MAX PART MAX7422 MAX7423 MAX7424 MAX7425
Selector Guide (continued)
FILTER RESPONSE r = 1.6 Bessel Butterworth r = 1.25 OPERATING VOLTAGE (V) +3 +3 +3 +3
Chip Information
TRANSISTOR COUNT: 1457 PROCESS: BiCMOS
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13
5th-Order, Lowpass, Switched-Capacitor Filters MAX7418-MAX7425
________________________________________________________Package Information
8LUMAXD.EPS
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 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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