8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX748

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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX748

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19-1416; 1/99
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
MAX7480 8th-order, lowpass, Butterworth, switched-capacitor filter (SCF) operates from single supply. device draws only 2.9mA supply current allows corner frequencies from 2kHz, making ideal low-power post-DAC filtering anti-aliasing applications. MAX7480 features shutdown mode, which reduces supply current 0.2µA. clocking options available: self-clocking (through external capacitor) external clocking tighter corner-frequency control. offset adjust allows adjustment output level. MAX7480 Butterworth filter provides maximally flat passband response. fixed response simplifies design task selecting clock frequency.
Features
8th-Order, Lowpass Butterworth Filter Noise Distortion: -73dB Noise Clock-Tunable Corner Frequency (1Hz 2kHz) 100:1 Clock-to-Corner Ratio Single-Supply Operation Power 2.9mA (Operating Mode) 0.2µA (Shutdown Mode) Available 8-Pin SO/DIP Package Output Offset: ±5mV
MAX7480
Applications
Anti-Aliasing Post-DAC Filtering
PART MAX7480ESA MAX7480EPA
Ordering Information
TEMP. RANGE -40°C +85°C -40°C +85°C PIN-PACKAGE Plastic
Configuration
VIEW
VSUPPLY SHDN INPUT 0.1µF
Typical Operating Circuit
SHDN OUTPUT
MAX7480
MAX7480
SO/DIP
CLOCK
0.1µF
Maxim Integrated Products
free samples latest literature: http://www.maxim-ic.com, phone 1-800-998-8800. small orders, phone 1-800-835-8769.
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
ABSOLUTE MAXIMUM RATINGS
.-0.3V OUT, COM, .-0.3V (VDD 0.3V) SHDN.-0.3V Short-Circuit Duration.1sec Continuous Power Dissipation +70°C) 8-Pin (derate 5.88mW/°C above +70°C).471mW 8-Pin (derate 9.09mW/°C above +70°C) .727mW Operating Temperature Range .-40°C +85°C Storage Temperature Range .-65°C +150°C Lead Temperature (soldering, 10sec) .+300°C
Stresses beyond those listed under "Absolute Maximum Ratings" cause permanent damage device. These stress ratings only, functional operation device these other conditions beyond those indicated operational sections specifications implied. Exposure absolute maximum rating conditions extended periods affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDD +5V, filter output measured OUT, 50pF load OUT, COM, 0.1µF from GND, SHDN VDD, fCLK 100kHz, TMIN TMAX, unless otherwise noted. Typical values +25°C.) PARAMETER FILTER CHARACTERISTICS Corner Frequency Clock-to-Corner Ratio Clock-to-Corner Tempco Output Voltage Range Output Offset Voltage Insertion Gain with Output Offset Removed Total Harmonic Distortion plus Noise Voltage Gain Input Voltage Range THD+N Input, externally driven Voltage Range VCOM Output, internally biased Input Resistance Clock Feedthrough Resistive Output Load Drive Maximum Capacitive Load Input Leakage Current Input Leakage Current CLOCK Internal Oscillator Frequency Clock Input Current Clock Input High Clock Input fOSC ICLK COSC 1000pF (Note VCLK SHDN GND, VCOM (VDD (Note RCOM VOFFSET VCOM VCOM (Note 200Hz, 4Vp-p, measurement bandwidth 22kHz -0.1 0.25 0.15 VCOM ±0.1 ±0.1 ±0.1 mVp-p fCLK (Note 0.001 100:1 0.25 ppm/°C SYMBOL CONDITIONS UNITS
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
ELECTRICAL CHARACTERISTICS (continued)
(VDD +5V, filter output measured OUT, 50pF load OUT, COM, 0.1µF from GND, SHDN VDD, fCLK 100kHz, TMIN TMAX, unless otherwise noted. Typical values +25°C.) PARAMETER POWER REQUIREMENTS Supply Voltage Supply Current Shutdown Current Power-Supply Rejection Ratio SHUTDOWN SHDN Input High SHDN Input SHDN Input Leakage Current VSDH VSDL SHDN ±0.1 SHDN PSRR Operating mode, load, SHDN GND, driven from Measured SYMBOL CONDITIONS UNITS
MAX7480
FILTER CHARACTERISTICS
(VDD +5V, filter output measured OUT, 50pF load OUT, SHDN VDD, VCOM fCLK 100kHz, TMIN TMAX, unless otherwise noted. Typical values +25°C.) PARAMETER 0.5fC Insertion Gain Relative Gain CONDITIONS -0.1 -3.5 -3.0 -2.5 UNITS
Note maximum defined clock frequency fCLK which peak SINAD drops 68dB with sinusoidal input 0.2fC. Note insertion gain defined VOUT VIN. Note voltages above saturate input result 75µA typical input leakage current. Note fOSC (kHz) COSC (pF).
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Typical Operating Characteristics
(VDD +5V, fCLK 100kHz, SHDN VDD, VCOM +25°C, unless otherwise noted.)
FREQUENCY RESPONSE
MAX7480 toc01
PASSBAND FREQUENCY RESPONSE
MAX7480 toc02
PHASE RESPONSE
1kHz PHASE SHIFT (DEGREES)
MAX7480 toc03 MAX7480-09 MAX7480-06
GAIN (dB) -100 -120 1kHz
-0.5 GAIN (dB) -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 1kHz
1010 1200 1600 2000 INPUT FREQUENCY (Hz) INPUT FREQUENCY (Hz)
INPUT FREQUENCY (kHz)
SUPPLY CURRENT SUPPLY VOLTAGE
MAX7480 toc04
SUPPLY CURRENT TEMPERATURE
MAX7480 toc05
OFFSET VOLTAGE SUPPLY VOLTAGE
VCOM OFFSET VOLTAGE (mV) -0.5 -1.0 -1.5 -2.0
3.00 LOAD 2.95 SUPPLY CURRENT (mA) 2.90 2.85 2.80 2.75 2.70
3.00 LOAD 2.95 SUPPLY CURRENT (mA) 2.90 2.85 2.80 2.75 2.70
SUPPLY VOLTAGE
SUPPLY VOLTAGE
TEMPERATURE (°C)
OFFSET VOLTAGE TEMPERATURE
MAX7401 toc07
INTERNAL OSCILLATOR FREQUENCY COSC CAPACITANCE
MAX7480 toc08
NORMALIZED INTERNAL OSCILLATOR FREQUENCY SUPPLY VOLTAGE
1.05 NORMALIZED OSCILLATOR FREQUENCY 1.04 1.03 1.02 1.01 1.00 0.99 0.98 0.97 0.96 0.95 SUPPLY VOLTAGE COSC 530pF
VCOM OFFSET VOLTAGE (mV)
1000
FREQUENCY (kHz)
-0.5
-1.0
-1.5 TEMPERATURE (°C)
0.01 CAPACITANCE (nF) 1000
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
Typical Operating Characteristics (continued)
(VDD +5V, fCLK 100kHz, SHDN VDD, VCOM +25°C, unless otherwise noted.)
MAX7480
NORMALIZED OSCILLATOR FREQUENCY TEMPERATURE
MAX7480 toc10
TOTAL HARMONIC DISTORTION PLUS NOISE INPUT SIGNAL AMPLITUDE
THD+N (dB) LOAD (SEE TABLE
MAX7480 toc11
1.03 NORMALIZED OSCILLATOR FREQUENCY COSC 530pF 1.02 1.01 0.99 0.98
0.97 TEMPERATURE (°C) AMPLITUDE (Vp-p)
Table THD+N Input Signal Amplitude Test Conditions
TRACE (Hz) (kHz) fCLK (kHz) MEASUREMENT BANDWIDTH (kHz)
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Description
NAME SHDN FUNCTION Common Input Pin. Biased internally mid-supply. Bypass externally with 0.1µF capacitor. override internal biasing, drive with external supply. Filter Input Ground Supply Input Filter Output Offset Adjust Input. adjust output offset, bias externally. Connect offset adjustment needed. Refer Offset Common-Mode Input Adjustment section. Shutdown Input. Drive enable shutdown mode; drive high connect normal operation. Clock Input. override internal oscillator, connect external clock; otherwise, connect external capacitor (COSC) from internal oscillator frequency.
_Detailed Description
MAX7480 Butterworth filter operates with 100:1 clock-to-corner frequency ratio 2kHz maximum corner frequency. Lowpass Butterworth filters provide maximally flat passband response, making them ideal instrumentation applications that require minimum deviation from gain throughout passband. Figure shows difference between Bessel Butterworth filter frequency responses. With filter cutoff frequencies 1kHz, trace shows Bessel filter response trace shows Butterworth filter response.
GAIN (dB) -100 FREQUENCY (kHz) BESSEL FILTER RESPONSE; 1kHz BUTTERWORTH FILTER RESPONSE; 1kHz
Background Information
Most switched-capacitor filters (SCFs) designed with biquadratic sections. Each section implements filtering poles, sections cascaded produce higher-order filters. advantage this approach ease design. However, this type design highly sensitive component variations section's high. alternative approach emulate passive network using switched-capacitor integrators with summing scaling. Figure shows basic 8th-order ladder filter structure. switched-capacitor filter such MAX7480 emulates passive ladder filter. filter's component sensitivity when compared cascaded biquad design, because each component affects entire filter shape, just pole-zero pair. other words, mismatched component biquad design will have concentrated error respective poles, while same mismatch ladder filter design results error distributed over poles.
Figure Bessel Butterworth Filter Frequency Response
Figure 8th-Order Ladder Filter Network
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Clock Signal
External Clock MAX7480 designed with external clocks that have duty cycle. When using external clock with these devices, drive with CMOS gate powered from VDD. Varying rate external clock adjusts corner frequency filter follows: fCLK Internal Clock When using internal oscillator, connect capacitor (COSC) between ground. value capacitor determines oscillator frequency follows:
fOSC (kHz) COSC COSC
connect COM. applications requiring offset adjustment level shifting, apply external bias voltage through resistor-divider network shown Figure (Note: leave unconnected.) output voltage represented this equation: VOUT (VIN VCOM) with VCOM (typical), where (VIN VCOM) lowpass-filtered added output stage. Electrical Characteristics voltage range Changing voltage significantly from mid-supply reduces filter's dynamic range.
Power Supplies
MAX7480 operates from single supply. Bypass with 0.1µF capacitor. dual supplies (±2.5V) required, connect system ground connect negative supply. Figure shows example dual-supply operation. Single- dual-supply performances equivalent. either single- dual-supply operation, drive SHDN from dual-supply operation) dual-supply applications, MAX291-MAX297.
Minimize stray capacitance that does affect internal oscillator frequency. Vary rate internal oscillator adjust filter's corner frequency 100:1 clock corner-frequency ratio. example, internal oscillator frequency 100kHz produces nominal corner frequency 1kHz.
Input Impedance Clock Frequencies
MAX7480's input impedance effectively that switched-capacitor resistor, inversely proportional frequency. input impedance values determined below represent average input impedance, since input current continuous. rule, driver with output impedance less than filter's input impedance. Estimate input impedance filter using following formula:
Input Signal Amplitude Range
optimal input signal range determined observing voltage level which total harmonic distortion plus noise (THD+N) minimized given corner frequency. Typical Operating Characteristics shows graph device's THD+N response input signal's peak-to-peak amplitude varied. This measurement made with biased midsupply.
VSUPPLY
fCLK
where fCLK clock frequency 2.31pF.
0.1µF INPUT SHDN 0.1µF
Low-Power Shutdown Mode
This device features shutdown mode that activated driving SHDN low. shutdown mode, filter's supply current reduces 0.2µA (typ) output becomes high impedance. normal operation, drive SHDN high connect VDD.
OUTPUT
MAX7480
CLOCK 0.1µF
_Applications Information
Offset Common-Mode Input Adjustment
voltage sets common-mode input voltage biased mid-supply with internal resistor-divider. Bypass with 0.1µF capacitor
Figure Offset Adjustment Circuit
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Anti-Aliasing Post-DAC Filtering
When using MAX7480 anti-aliasing post-DAC filtering, synchronize filter clocks. clocks synchronized, beat frequencies alias into passband. high clock-to-corner frequency ratio (100:1) also eases requirements pre- post-SCF filtering. input, lowpass filter prevents aliasing frequencies around clock frequency into passband. output, lowpass filter attenuates clock feedthrough. high clock corner-frequency ratio allows simple lowpass filter, with cutoff frequency above corner frequency provide input anti-aliasing reasonable output clock attenuation.
+2.5V
INPUT
SHDN
OUTPUT
MAX7480
CLOCK 0.1µF 0.1µF
Harmonic Distortion
Harmonic distortion arises from nonlinearities within filter. These nonlinearities generate harmonics when pure sine wave applied filter input. Table lists MAX7480's typical harmonic-distortion values with load +25°C.
-2.5V *DRIVE SHDN LOW-POWER SHUTDOWN MODE.
Figure Dual-Supply Operation
Table Typical Harmonic Distortion
FILTER fCLK (kHz) MAX7480 (kHz) (Hz) (Vp-p) TYPICAL HARMONIC DISTORTION (dB)
Chip Information
TRANSISTOR COUNT: 1116
Maxim cannot assume responsibility circuitry other than circuitry entirely embodied Maxim product. circuit patent licenses implied. Maxim reserves right change circuitry specifications without notice time.
_Maxim Integrated Products, Gabriel Drive, Sunnyvale, 94086 408-737-7600 1999 Maxim Integrated Products Printed registered trademark Maxim Integrated Products.

8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX748

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