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19-1416; Rev 0; 1/99
8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
General Description
The MAX7480 8th-order, lowpass, Butterworth, switched-capacitor filter (SCF) operates from a single +5V supply. The device draws only 2.9mA of supply current and allows corner frequencies from 1Hz to 2kHz, making it ideal for low-power post-DAC filtering and anti-aliasing applications. The MAX7480 features a shutdown mode, which reduces the 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 MAX7480 Butterworth filter provides a maximally flat passband response. The fixed response simplifies the design task to selecting a clock frequency.
Features
o 8th-Order, Lowpass Butterworth Filter o Low Noise and Distortion: -73dB THD + Noise o Clock-Tunable Corner Frequency (1Hz to 2kHz) o 100:1 Clock-to-Corner Ratio o +5V Single-Supply Operation o Low Power 2.9mA (Operating Mode) 0.2A (Shutdown Mode) o Available in 8-Pin SO/DIP Package o Low Output Offset: 5mV
MAX7480
Applications
ADC Anti-Aliasing Post-DAC Filtering
PART MAX7480ESA MAX7480EPA
Ordering Information
TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 8 SO 8 Plastic DIP
Pin Configuration
TOP VIEW
VSUPPLY COM 1 IN 2 8 7 CLK SHDN OS OUT INPUT 0.1F
Typical Operating Circuit
VDD IN
SHDN OUT OUTPUT
MAX7480
GND 3 6 5 VDD 4
MAX7480
SO/DIP
CLOCK
CLK GND
COM OS 0.1F
________________________________________________________________ Maxim Integrated Products
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
ABSOLUTE MAXIMUM RATINGS
VDD to GND ..............................................................-0.3V to +6V IN, OUT, COM, OS, CLK ............................-0.3V to (VDD + 0.3V) SHDN........................................................................-0.3V to +6V OUT Short-Circuit Duration...................................................1sec Continuous Power Dissipation (TA = +70C) 8-Pin SO (derate 5.88mW/C above +70C)................471mW 8-Pin DIP (derate 9.09mW/C above +70C) ...............727mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+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
(VDD = +5V, filter output measured at OUT, 10k || 50pF load to GND at OUT, OS = COM, 0.1F from COM to GND, SHDN = VDD, fCLK = 100kHz, 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 Total Harmonic Distortion plus Noise OS Voltage Gain to OUT Input Voltage Range at OS THD+N AOS VOS Input, COM externally driven COM Voltage Range VCOM Output, COM internally biased 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 CLOCK Internal Oscillator Frequency Clock Input Current Clock Input High Clock Input Low fOSC ICLK VIH VIL COSC = 1000pF (Note 4) VCLK = 0 or 5V VDD - 0.5 0.5 40 53 24 67 40 kHz A V V RL CL SHDN = GND, VCOM = 0 to VDD VOS = 0 to (VDD - 1V) (Note 3) 10 50 RCOM VDD / 2 - 0.5 VDD / 2 - 0.2 75 VOFFSET VIN = VCOM = VDD / 2 VCOM = VDD / 2 (Note 2) fIN = 200Hz, VIN = 4Vp-p, measurement bandwidth = 22kHz -0.1 0.25 5 0.15 -73 1 VCOM 0.1 VDD / 2 VDD / 2 + 0.5 V VDD / 2 VDD / 2 + 0.2 125 10 1 500 0.1 0.1 10 10 k mVp-p k pF A A fC fCLK / fC (Note 1) 0.001 to 2 100:1 10 VDD - 0.25 25 0.3 ppm/C V mV dB dB V/ V V kHz SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
ELECTRICAL CHARACTERISTICS (continued)
(VDD = +5V, filter output measured at OUT, 10k || 50pF load to GND at OUT, OS = COM, 0.1F from COM to GND, SHDN = VDD, fCLK = 100kHz, 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 VSDH VSDL V SHDN = 0 to VDD 0.1 VDD - 0.5 0.5 10 V V A VDD IDD I SHDN PSRR Operating mode, no load, IN = OS = COM SHDN = GND, CLK driven from 0 to VDD Measured at DC 4.5 2.9 0.2 60 5.5 3.5 1 V mA A dB SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX7480
FILTER CHARACTERISTICS
(VDD = +5V, filter output measured at OUT, 10k || 50pF load to GND at OUT, SHDN = VDD, VCOM = VOS = VDD /2, fCLK = 100kHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER fIN = 0.5fC Insertion Gain Relative to DC Gain fIN = fC fIN = 2fC fIN = 3fC CONDITIONS MIN -0.1 -3.5 TYP 0.0 -3.0 -48 -76 -2.5 -43 -70 dB MAX UNITS
Note 1: The maximum fC is defined as the clock frequency fCLK = 100 * fC at which the peak SINAD drops to 68dB with a sinusoidal input at 0.2fC. Note 2: DC insertion gain is defined as VOUT / VIN. Note 3: OS voltages above VDD - 1V saturate the input and result in a 75A typical input leakage current. Note 4: fOSC (kHz) 53 * 103 / COSC (pF).
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Typical Operating Characteristics
(VDD = +5V, fCLK = 100kHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.)
FREQUENCY RESPONSE
MAX7480 toc01
PASSBAND FREQUENCY RESPONSE
MAX7480 toc02
PHASE RESPONSE
fC = 1kHz 80 PHASE SHIFT (DEGREES) 160 240 320 400 480 560
MAX7480 toc03 MAX7480-09 MAX7480-06
20 0 -20 GAIN (dB) -40 -60 -80 -100 -120 0 fC = 1kHz
0.5 0 -0.5 GAIN (dB) -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 fC = 1kHz
0
640 0 202 404 606 808 1010 0 400 800 1200 1600 2000 INPUT FREQUENCY (Hz) INPUT FREQUENCY (Hz)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT FREQUENCY (kHz)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX7480 toc04
SUPPLY CURRENT vs. TEMPERATURE
MAX7480 toc05
DC OFFSET VOLTAGE vs. SUPPLY VOLTAGE
2.0 VIN = VCOM 1.5 DC OFFSET VOLTAGE (mV) 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0
3.00 NO LOAD 2.95 SUPPLY CURRENT (mA) 2.90 2.85 2.80 2.75 2.70
3.00 NO LOAD 2.95 SUPPLY CURRENT (mA) 2.90 2.85 2.80 2.75 2.70
4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 SUPPLY VOLTAGE (V)
-40
-20
0
20
40
60
80
100
4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 SUPPLY VOLTAGE (V)
TEMPERATURE (C)
OFFSET VOLTAGE vs. TEMPERATURE
MAX7401 toc07
INTERNAL OSCILLATOR FREQUENCY vs. COSC CAPACITANCE
MAX7480 toc08
NORMALIZED INTERNAL OSCILLATOR FREQUENCY vs. 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 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 SUPPLY VOLTAGE (V) COSC = 530pF
1.0 VIN = VCOM = VDD / 2 0.5 OFFSET VOLTAGE (mV)
1000
100 FREQUENCY (kHz)
0
10
-0.5
1
-1.0
0.1
-1.5 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0.01 0.1 1 10 CAPACITANCE (nF) 100 1000
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter
Typical Operating Characteristics (continued)
(VDD = +5V, fCLK = 100kHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25C, unless otherwise noted.)
MAX7480
NORMALIZED OSCILLATOR FREQUENCY vs. TEMPERATURE
MAX7480 toc10
TOTAL HARMONIC DISTORTION PLUS NOISE vs. INPUT SIGNAL AMPLITUDE
-10 -20 -30 THD+N (dB) -40 -50 -60 -70 A B NO LOAD (SEE TABLE A)
MAX7480 toc11
1.03 NORMALIZED OSCILLATOR FREQUENCY COSC = 530pF 1.02 1.01 1 0.99 0.98
0
-80 0.97 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) -90 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 AMPLITUDE (Vp-p)
Table A. THD+N vs. Input Signal Amplitude Test Conditions
TRACE A B fIN (Hz) 400 200 fC (kHz) 2 1 fCLK (kHz) 200 100 MEASUREMENT BANDWIDTH (kHz) 22 22
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
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 mid-supply. Bypass externally to GND with a 0.1F capacitor. To override internal biasing, drive with an external supply. Filter Input Ground +5V Supply Input Filter Output Offset Adjust Input. To adjust output offset, bias OS externally. Connect OS to COM if no offset adjustment is needed. Refer to 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. To override the internal oscillator, connect to an external clock; otherwise, connect an external capacitor (COSC) from CLK to GND to set the internal oscillator frequency.
_______________Detailed Description
The MAX7480 Butterworth filter operates with a 100:1 clock-to-corner frequency ratio and a 2kHz maximum corner frequency. Lowpass Butterworth filters provide a maximally flat passband response, making them ideal for instrumentation applications that require minimum deviation from the DC gain throughout the passband. Figure 1 shows the difference between Bessel and Butterworth filter frequency responses. With the filter cutoff frequencies set at 1kHz, trace A shows the Bessel filter response and trace B shows the Butterworth filter response.
20 0 -20 GAIN (dB) A -40 B -60 -80 -100 0.1 0.2 0.5 1 2 5 10 FREQUENCY (kHz) A: BESSEL FILTER RESPONSE; fC = 1kHz B: BUTTERWORTH FILTER RESPONSE; fC = 1kHz
Background Information
Most switched-capacitor filters (SCFs) are designed with biquadratic sections. Each section implements two filtering poles, and the sections are 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. An alternative approach is to emulate a passive network using switched-capacitor integrators with summing and scaling. Figure 2 shows a basic 8th-order ladder filter structure. A switched-capacitor filter such as the MAX7480 emulates a passive ladder filter. The filter's component sensitivity is low when compared to a cascaded biquad design, because each component affects the entire filter shape, not just one pole-zero pair. In other words, a mismatched component in a biquad design will have a concentrated error on its respective poles, while the same mismatch in a ladder filter design results in an error distributed over all poles.
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Figure 1. Bessel vs. Butterworth Filter Frequency Response
R1 + -
L1 C2
L3 C4
L5 C6
L7 R2 V0
VIN
C8
Figure 2. 8th-Order Ladder Filter Network
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Clock Signal
External Clock The MAX7480 SCF is designed for use with external clocks that have a 40% to 60% duty cycle. When using an external clock with these devices, drive CLK with a CMOS gate powered from 0 to VDD. Varying the rate of the external clock adjusts the corner frequency of the filter as follows: fC = fCLK / 100 Internal Clock When using the internal oscillator, connect a capacitor (COSC) between CLK and ground. The value of the capacitor determines the oscillator frequency as follows:
fOSC (kHz) = 53 10 ; COSC in pF COSC
3
connect OS to COM. For applications requiring offset adjustment or DC level shifting, apply an external bias voltage through a resistor-divider network to OS, as shown in Figure 3. (Note: Do not leave OS unconnected.) The output voltage is represented by this equation: VOUT = (VIN - VCOM) + VOS with VCOM = VDD / 2 (typical), where (VIN - VCOM) is lowpass-filtered by the SCF and VOS is added at the output stage. See the Electrical Characteristics for the voltage range of COM and OS. Changing the voltage on COM or OS significantly from mid-supply reduces the filter's dynamic range.
Power Supplies
The MAX7480 operates from a single +5V supply. Bypass VDD to GND with a 0.1F capacitor. If dual supplies (2.5V) are required, connect COM to system ground and connect GND to the negative supply. Figure 4 shows an example of dual-supply operation. Single- and dual-supply performances are equivalent. For either single- or dual-supply operation, drive CLK and SHDN from GND (V- in dual-supply operation) to V DD . For 5V dual-supply applications, use the MAX291-MAX297.
Minimize the stray capacitance at CLK so that it does not affect the internal oscillator frequency. Vary the rate of the internal oscillator to adjust the filter's corner frequency by a 100:1 clock to corner-frequency ratio. For example, an internal oscillator frequency of 100kHz produces a nominal corner frequency of 1kHz.
Input Impedance vs. Clock Frequencies
The MAX7480's input impedance is effectively that of a switched-capacitor resistor, and is inversely proportional to frequency. The input impedance values determined below represent the average input impedance, since the input current is not continuous. As a rule, use a driver with an output impedance less than 10% of the filter's input impedance. Estimate the input impedance of the filter using the following formula: ZIN =
Input Signal Amplitude Range
The optimal input signal range is determined by observing the voltage level at which the total harmonic distortion plus noise (THD+N) is minimized for a given corner frequency. The Typical Operating Characteristics shows a graph of the device's THD+N response as the input signal's peak-to-peak amplitude is varied. This measurement is made with OS and COM biased at midsupply.
VSUPPLY
(
1
fCLK CIN
)
where fCLK = clock frequency and CIN = 2.31pF.
0.1F VDD INPUT IN SHDN OUT COM 0.1F 50k
Low-Power Shutdown Mode
This device features a shutdown mode that is activated by driving SHDN low. In shutdown mode, the filter's supply current reduces to 0.2A (typ) and its output becomes high impedance. For normal operation, drive SHDN high or connect to VDD.
OUTPUT
MAX7480
CLOCK CLK OS 0.1F GND 50k 50k
___________Applications Information
Offset and Common-Mode Input Adjustment
The voltage at COM sets the common-mode input voltage and is biased at mid-supply with an internal resistor-divider. Bypass COM with a 0.1F capacitor and
Figure 3. Offset Adjustment Circuit
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8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter MAX7480
Anti-Aliasing and Post-DAC Filtering
When using the MAX7480 for anti-aliasing or post-DAC filtering, synchronize the DAC and the filter clocks. If the clocks are not synchronized, beat frequencies may alias into the passband. The high clock-to-corner frequency ratio (100:1) also eases the requirements of pre- and post-SCF filtering. At the input, a lowpass filter prevents the aliasing of frequencies around the clock frequency into the passband. At the output, a lowpass filter attenuates the clock feedthrough. A high clock to corner-frequency ratio allows a simple RC lowpass filter, with the cutoff frequency set above the SCF corner frequency to provide input anti-aliasing and reasonable output clock attenuation.
V+ = +2.5V
VDD INPUT IN
SHDN OUT COM
* OUTPUT
V+ V-
MAX7480
CLOCK CLK OS 0.1F GND 0.1F
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. Table 1 lists the MAX7480's typical harmonic-distortion values with a 10k load at TA = +25C.
V- = -2.5V *DRIVE SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.
Figure 4. Dual-Supply Operation
Table 1. Typical Harmonic Distortion
FILTER fCLK (kHz) 100 MAX7480 200 2 400 fC (kHz) 1 fIN (Hz) 200 4 -82 -68 -85 -89 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd -89 3rd -73 4th -91 5th -93
Chip Information
TRANSISTOR COUNT: 1116
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.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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