19-1821; Rev 0; 11/00 5th-Order, Lowpass, Switched-Capacitor Filters Applications ADC Anti-Aliasing CT2 Base Stations DAC Postfiltering Speech Processing Selector Guide PART FILTER RESPONSE OPERATING VOLTAGE (V) MAX7418 r = 1.6 +5 MAX7419 Bessel +5 MAX7420 Butterworth +5 MAX7421 r = 1.25 +5 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 Ordering Information PART TEMP. RANGE MAX7418CUA 0°C to +70°C 8 µMAX PIN-PACKAGE MAX7418EUA -40°C to +85°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 Ordering Information continued at end of data sheet. Typical Operating Circuit VSUPPLY Selector Guide continued at end of data sheet. Pin Configuration 0.1µF VDD TOP VIEW INPUT COM 1 8 CLK IN 2 7 SHDN GND 3 6 OS 5 OUT MAX7418– MAX7425 VDD 4 IN SHDN OUT OUTPUT MAX7418– MAX7425 CLOCK COM CLK GND OS 0.1µF µMAX ________________________________________________________________ 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. MAX7418–MAX7425 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. MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters 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 = +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 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, 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.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS FILTER CHARACTERISTICS Corner Frequency Clock-to-Corner Ratio fc fCLK / fC VIN = 4Vp-p (Note 1) 0.001 to 30 Clock-to-Corner Tempco 10 Output Voltage Range Output Offset Voltage VIN = VCOM = VDD / 2 VCOM = VDD / 2 (Note 2) Total Harmonic Distortion plus Noise fIN = 2kHz, VIN = 4Vp-p, measurement bandwidth = 80kHz THD+N Offset Voltage Gain AOS COM Voltage Range VCOM Input Resistance at COM VOS ppm/°C VDD - 0.25 0.25 VOFFSET DC Insertion Gain with Output Offset Removed Input Voltage Range at OS kHz 100:1 ±4 ±25 MAX7418/MAX7421 0 0.2 0.4 MAX7419/MAX7420 -0.2 0 +0.2 MAX7418 -76 MAX7419 -78 MAX7420 -67 MAX7421 -78 OS to OUT V/V Input, COM externally driven 2.0 2.5 3.0 Output, COM unconnected 2.3 2.5 2.7 RCOM VCOM ±0.1 100 Clock Feedthrough dB dB 1 Input, OS externally driven V mV V V 140 kΩ 5 mVp-p Resistive Output Load Drive RL 10 1 kΩ Maximum Capacitive Output Load Drive CL 50 500 pF Input Leakage Current at COM SHDN = GND, VCOM = 0 to VDD ±0.1 ±10 µA Input Leakage Current at OS VOS = 0 to VDD ±0.1 ±10 µA CLOCK Internal Oscillator Frequency fOSC COSC = 1000pF (Note 3) Clock Output Current (Internal Oscillator Mode) ICLK VCLK = 0 or 5V Clock Input High VIH Clock Input Low VIL 2 MAX7418/MAX7421 68 87 106 MAX7419/MAX7420 86 110 135 MAX7418/MAX7421 ±40 ±60 MAX7419/MAX7420 ±50 ±75 4.5 _______________________________________________________________________________________ kHz µA V 0.5 V 5th-Order, Lowpass, Switched-Capacitor Filters (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.) PARAMETER POWER REQUIREMENTS Supply Voltage Supply Current SYMBOL CONDITIONS MIN TYP MAX UNITS 5.5 V MAX7418/MAX7421 2.9 3.6 MAX7419/MAX7420 3.4 4.1 1 VDD IDD 4.5 Operating mode, no load Shutdown Current I SHDN SHDN = GND 0.2 Power-Supply Rejection Ratio PSRR IN = COM (Note 4) 70 SHUTDOWN SHDN Input High VSDH SHDN Input Low VSDL SHDN Input Leakage Current µA dB 4.5 V ±0.2 V SHDN = 0 to VDD mA 0.5 V ±10 µA 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.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS FILTER CHARACTERISTICS Corner-Frequency Range Clock-to-Corner Ratio fC VIN = 2.5Vp-p (Note 1) MAX7422/MAX7425 0.001 to 45 0.001 to 45 MAX7423/MAX7424 fCLK/fC 100:1 Clock-to-Corner Tempco 10 Output Voltage Range Output Offset Voltage 0.25 VOFFSET VIN = VCOM = VDD / 2 DC Insertion Gain with Output Offset Removed VCOM = VDD / 2 (Note 2) Total Harmonic Distortion plus Noise fIN = 2kHz, VIN = 2.5Vp-p, measurement bandwidth = 80kHz Offset Voltage Gain THD+N AOS COM Voltage Range Input Voltage Range at OS Input Resistance at COM kHz VCOM VOS ppm/°C VDD - 0.25 ±4 ±25 MAX7422/MAX7425 0 0.2 0.4 MAX7423/MAX7424 -0.2 0 +0.2 MAX7422 -80 MAX7423 -81 MAX7424 -70 MAX7425 -80 OS to OUT 1 V/V 1.4 1.5 1.6 Output, COM internally driven 1.4 1.5 1.6 Measured with respect to COM 100 Resistive Output Load Drive RL 10 Maximum Capacitive Load at OUT CL 50 Clock Feedthrough dB dB Input, COM externally driven RCOM V mV V VCOM ±0.1 V 140 kΩ 3 mVp-p 1 kΩ 500 pF Input Leakage Current at COM SHDN = GND, VCOM = 0 to VDD ±0.1 ±10 µA Input Leakage Current at OS VOS = 0 to VDD ±0.1 ±10 µA _______________________________________________________________________________________ 3 MAX7418–MAX7425 ELECTRICAL CHARACTERISTICS—MAX7418–MAX7421 (continued) MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters 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.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS CLOCK Internal Oscillator Frequency fOSC Clock Output Current (Internal Oscillator Mode) ICLK Clock Input High VIH Clock Input Low VIL COSC = 1000pF (Note 3) MAX7422/MAX7425 68 87 106 MAX7423/MAX7424 86 110 135 MAX7422/MAX7425 68 87 106 MAX7423/MAX7424 86 110 135 2.5 kHz kHz V 0.5 V 3.6 V POWER REQUIREMENTS Supply Voltage Supply Current VDD IDD 2.7 Operating mode, no load MAX7422/MAX7425 2.6 3.4 MAX7423/MAX7424 3.0 3.8 1 Shutdown Current I SHDN SHDN = GND 0.2 Power-Supply Rejection Ratio PSRR Measured at DC 70 mA µA dB SHUTDOWN SHDN Input High VSDH SHDN Input Low VSDL SHDN Input Leakage Current 2.5 VSHDN = 0 to VDD V ±0.2 0.5 V ±10 µA 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 = VDD, fCLK = 2.2MHz, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX fIN = 0.38fC -0.4 ±0.2 0.4 fIN = 0.68fC -0.4 ±0.2 0.4 fIN = 0.87fC -0.4 ±0.2 0.4 fIN = 0.97fC -0.4 ±0.2 0.4 fIN = fC -0.7 ±0.2 0.2 UNITS ELLIPTIC, r = 1.2—MAX7421/MAX7425 Insertion Gain with DC Gain Error Removed (Note 4) fIN = 1.25fC -36 -33 fIN = 1.43fC -37.2 -35 fIN = 3.25fC -37.2 -35 dB BESSEL FILTERS—MAX7419/MAX7423 fIN = 0.5fC Insertion Gain Relative to DC Gain 4 fIN = fC -1 -3.6 -0.74 -3.0 -2.4 fIN = 4fC -41.0 -35 fIN = 7fC -67 -60 _______________________________________________________________________________________ dB 5th-Order, Lowpass, Switched-Capacitor Filters (VDD = +5V for MAX7418–MAX7421, 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.) PARAMETER CONDITIONS MIN TYP MAX UNITS BUTTERWORTH FILTERS—MAX7420/MAX7424 Insertion Gain Relative to DC Gain fIN = 0.5fC -0.3 0 fIN = fC -3.6 -3.0 -2.4 fIN = 3fC -47.5 -43 fIN = 5fC -70 -65 dB 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 = +25°C, unless otherwise noted.) MAX7419/MAX7423 FREQUENCY RESPONSE (BESSEL) 0 -10 0 0 -10 -10 GAIN (dB) -30 -40 -50 -60 -20 GAIN (dB) -20 GAIN (dB) 10 MAX7418 toc02 10 MAX7418 toc01 10 MAX7420/MAX7424 FREQUENCY RESPONSE (BUTTERWORTH) MAX7418 toc03 MAX7418/MAX7422 FREQUENCY RESPONSE (ELLIPTIC, r = 1.6) -20 -30 -30 -40 -50 -40 -60 -70 -50 -80 -70 -60 -90 0 20 40 60 INPUT FREQUENCY (kHz) 80 100 -80 0 20 40 60 INPUT FREQUENCY (kHz) 80 100 0 20 40 60 80 100 INPUT FREQUENCY (kHz) _______________________________________________________________________________________ 5 MAX7418–MAX7425 FILTER CHARACTERISTICS ____________________________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. -10 0.2 0 -0.5 0 GAIN (dB) -30 -40 -50 -1.0 GAIN (dB) -20 -0.2 -0.4 -0.6 -3.0 -70 -0.8 -80 -90 -3.5 -4.0 -1.0 0 20 40 60 80 100 0 4.5 9.0 13.5 18.0 0 22.5 4.5 9.0 13.5 18.0 22.5 INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz) MAX7420/MAX7424 PASSBAND FREQUENCY RESPONSE (BUTTERWORTH) MAX7421/MAX7425 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, r = 1.25) MAX7418/MAX7422 PHASE RESPONSE (ELLIPTIC, r = 1.6) 0.2 0 GAIN (dB) -1.0 -1.5 -2.0 -0.2 -0.4 -2.5 -0.6 MAX7418 toc09 -0.5 0 -50 PHASE SHIFT (DEGREES) 0 MAX7418 toc08 0.4 MAX7418 toc07 0.5 GAIN (dB) -2.0 -2.5 -60 -100 -150 -200 -250 -300 -350 -3.0 -3.5 -0.8 -4.0 -1.0 0 4.5 9.0 13.5 18.0 22.5 -400 -450 0 4.5 9.0 13.5 18.0 0 22.5 4 8 12 16 20 24 28 INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz) MAX7419/MAX7423 PHASE RESPONSE (BESSEL) MAX7420/MAX7424 PHASE RESPONSE (BUTTERWORTH) MAX7424/MAX7425 PHASE RESPONSE (ELLIPTIC, r = 1.25) -100 -150 -50 PHASE SHIFT (DEGREES) -50 PHASE SHIFT (DEGREES) -50 0 MAX7418 toc11 0 MAX7418 toc10 0 -100 -150 -200 -250 -100 -150 -200 -250 -300 -350 -200 -300 -400 -350 -250 0 4 8 12 16 20 INPUT FREQUENCY (kHz) 6 -1.5 MAX7418 toc12 GAIN (dB) 0.5 MAX7418 toc06 0 MAX7418 toc05 0.4 MAX7418 toc04 10 MAX7419/MAX7423 PASSBAND FREQUENCY RESPONSE (BESSEL) MAX7418/MAX7422 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, r = 1.6) MAX7421/MAX7425 FREQUENCY RESPONSE (ELLIPTIC, r = 1.25) PHASE SHIFT (DEGREES) MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters 24 28 -450 0 4 8 12 16 20 INPUT FREQUENCY (kHz) 24 28 0 4 8 12 16 20 INPUT FREQUENCY (kHz) _______________________________________________________________________________________ 24 28 5th-Order, Lowpass, Switched-Capacitor Filters MAX7419 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BESSEL) -20 -40 -50 -20 D -70 -40 -50 -20 0 1 2 4 0 5 -50 D -80 E -90 3 -40 -70 -80 E -90 -30 -60 D -70 -80 E -90 1 2 3 4 5 0 1 2 3 4 5 MAX7421 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.25) MAX7422 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.6) MAX7423 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BESSEL) 0 0 -10 -20 -10 -20 THD + N (dB) -30 -40 -50 SEE TABLE A -20 -30 -40 -50 -60 -60 B -70 E -90 0 2 3 4 0 5 -50 A B -80 C -90 1 -40 -70 -80 -80 -30 -60 A D -70 SEE TABLE A -10 THD + N (dB) SEE TABLE A MAX7418 toc18 AMPLITUDE (Vp-p) MAX7418 toc17 AMPLITUDE (Vp-p) MAX7418 toc16 AMPLITUDE (Vp-p) 0 0.5 C -90 1.0 1.5 2.0 2.5 3.0 0 0.5 1.0 1.5 2.0 2.5 3.0 AMPLITUDE (Vp-p) AMPLITUDE (Vp-p) MAX7424 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BUTTERWORTH) MAX7425 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.25) INTERNAL OSCILLATOR FREQUENCY vs. SMALL CAPACITANCE (pF) 0 0 -10 -20 -10 -20 THD + N (dB) -30 SEE TABLE A -40 -50 -60 A -30 -40 -50 -60 B -70 A -70 -80 B -90 0 0.5 1.0 C -90 1.5 2.0 AMPLITUDE (Vp-p) 2.5 3.0 6000 5000 BESSEL/BUTTERWORTH 4000 3000 ELLIPTIC 2000 1000 -80 C 7000 MAX7418 toc21 SEE TABLE A OSCILLATOR FREQUENCY (kHz) MAX7418 toc19 AMPLITUDE (Vp-p) MAX7418 toc20 THD + N (dB) -30 SEE TABLE A -10 -60 -60 THD + N (dB) 0 MAX7418 toc15 -10 THD + N (dB) -30 SEE TABLE A THD + N (dB) SEE TABLE A -10 THD + N (dB) 0 MAX7418 toc13 0 MAX7420 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (BUTTERWORTH) MAX7418 toc14 MAX7418 THD + NOISE vs. INPUT SIGNAL AMPLITUDE (ELLIPTIC, r = 1.6) 0 0.5 1.0 0 1.5 2.0 AMPLITUDE (Vp-p) 2.5 3.0 1 10 100 1000 CAPACITANCE ( pF) 10000 _______________________________________________________________________________________ 7 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 = +25°C, unless otherwise noted. ____________________________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. ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE INTERNAL OSCILLATOR FREQUENCY vs. LARGE CAPACITANCE (nF) 4 BESSEL/BUTTERWORTH 3 2 ELLIPTIC 86.0 85.5 85.0 84.0 0 10 100 COSC = 1000PF 2.5 1000 3.0 3.5 4.0 4.5 5.0 CAPACITANCE (nF) SUPPLY VOLTAGE (V) ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE ELLIPTIC SUPPLY CURRENT vs. SUPPLY VOLTAGE 86.0 85.5 VDD = 5V 85.0 5.5 MAX7418 toc25 3.1 SUPPLY CURRENT (µA) VDD = 3V 86.5 3.3 MAX7418 toc24 87.0 OSCILLATOR FREQUENCY (kHz) 86.5 84.5 1 2.9 2.7 2.5 84.5 84.0 MAX7418 toc23 5 87.0 OSCILLATOR FREQUENCY (kHz) MAX7418 toc22 OSCILLATOR FREQUENCY (Hz) 6 COSC = 1000pF -40 -15 2.3 10 35 60 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 2.9 SUPPLY CURRENT (mA) MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters VDD = 5V 2.8 2.7 VDD = 3V 2.6 Table A. LABEL fIN (kHz) fC (kHz) fCLK (kHz) BW (kHz) A 2 30 3000 80 B 2 22 2200 80 C 1 10 1000 22 D 2 22 2200 80 E 1 10 1000 22 2.5 -40 -15 10 35 60 85 TEMPERATURE (°C) 8 _______________________________________________________________________________________ 5th-Order, Lowpass, Switched-Capacitor Filters DC OFFSET VOLTAGE vs. SUPPLY VOLTAGE DC OFFSET VOLTAGE vs. TEMPERATURE DC OFFSET VOLTAGE (mV) VDD = 5V 2.0 1.5 1.0 VDD = 3V MAX7418 toc28 2.5 DC OFFSET VOLTAGE (mV) 2.5 MAX7418 toc27 3.0 2.0 1.5 1.0 0.5 0.5 0 0 -40 -15 10 35 60 85 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) TEMPERATURE (°C) Pin Description PIN NAME FUNCTION 1 COM 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. 2 IN 3 GND Ground 4 VDD Positive Supply Input: +5V for MAX7418–MAX7421, +3V for MAX7422–MAX7425. Bypass VDD to GND with a 0.1µF capacitor. 5 OUT Filter Output 6 OS 7 SHDN 8 CLK Filter Input 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. _______________________________________________________________________________________ 9 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 = +25°C, unless otherwise noted. RS + - VIN L2 C1 L4 C3 C5 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 Clock Signal 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). 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 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. RIPPLE fC TRANSITION RATIO = fS fC GAIN (dB) MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters fS PASSBAND STOPBAND fC fS 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 0.1µF VDD A 2V/div INPUT IN SHDN OUT COM 0.1µF B 50k MAX7418– MAX7425 2V/div CLOCK C OUTPUT CLK 50k OS 0.1µF GND 200µs/div 50k A: 1kHz INPUT SIGNAL B: MAX7419 BESSEL FILTER RESPONSE; fC = 5kHz C: MAX7420 BUTTERWORTH FILTER RESPONSE; fC = 5kHz 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) 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. 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: ZIN = 1 (fCLK × 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 0.2µA, 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: VOUT = (VIN − VCOM ) + VOS VCOM = VDD (typ) 2 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 ______________________________________________________________________________________ 11 MAX7418–MAX7425 5th-Order, Lowpass, Switched-Capacitor Filters 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 0.1µF 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. VDD INPUT V+ V- SHDN OUT COM IN * OUTPUT MAX7418– MAX7425 CLOCK CLK OS 0.1µF 0.1µF GND 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. Anti-Aliasing and Post-DAC Filtering 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 10kΩ load at TA = +25°C. 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) fIN (kHz) 2.2 2 MAX7418 TYPICAL HARMONIC DISTORTION (dB) 2nd 3rd 4th 5th <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 <-80 4 1.5 2 2.2 2 MAX7421 4 1.5 2 4.0 4 MAX7422 2 2.2 2 4.0 4 MAX7425 2 2.2 12 VIN (Vp-p) 2 ______________________________________________________________________________________ 5th-Order, Lowpass, Switched-Capacitor Filters MAX7418–MAX7425 Table 2. MAX7420/MAX7424 Typical Harmonic Distortion FILTER fCLK (MHz) fIN (kHz) 2.2 2 MAX7420 MAX7424 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd 3rd 4th 5th -77 -67 < -80 -76 < -80 -70 < -80 < -80 < -80 -70 < -80 < -80 < -80 -77 < -80 < -80 4 1.5 2 3.5 3 2.2 2 2 Table 3. MAX7419/MAX7423 Typical Harmonic Distortion FILTER fCLK (MHz) fIN (kHz) 2.2 2 MAX7419 MAX7423 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd 3rd 4th 5th < -80 -77 < -80 < -80 < -80 -80 < -80 < -80 < -80 -75 < -80 < -80 < -80 < -80 < -80 < -80 4 1.5 2 3.5 3 2.2 2 2 Ordering Information (continued) PART TEMP. RANGE MAX7422CUA 0°C to +70°C 8 µMAX PIN-PACKAGE MAX7422EUA MAX7423CUA -40°C to +85°C 0°C to +70°C MAX7423EUA Selector Guide (continued) PART FILTER RESPONSE OPERATING VOLTAGE (V) 8 µMAX MAX7422 r = 1.6 +3 8 µMAX MAX7423 Bessel +3 -40°C to +85°C 8 µMAX MAX7424 Butterworth +3 MAX7424CUA 0°C to +70°C 8 µMAX MAX7425 r = 1.25 +3 MAX7424EUA -40°C to +85°C 8 µMAX MAX7425CUA 0°C to +70°C 8 µMAX MAX7425EUA -40°C to +85°C 8 µMAX Chip Information TRANSISTOR COUNT: 1457 PROCESS: BiCMOS ______________________________________________________________________________________ 13 ________________________________________________________Package Information 8LUMAXD.EPS 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.