19-1416; Rev 0; 1/99 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter 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 0.2µA. 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 MAX7480 Butterworth filter provides a maximally flat passband response. The fixed response simplifies the design task to selecting a clock frequency. Features ♦ 8th-Order, Lowpass Butterworth Filter ♦ Low Noise and Distortion: -73dB THD + Noise ♦ Clock-Tunable Corner Frequency (1Hz to 2kHz) ♦ 100:1 Clock-to-Corner Ratio ♦ +5V Single-Supply Operation ♦ Low Power 2.9mA (Operating Mode) 0.2µA (Shutdown Mode) ♦ Available in 8-Pin SO/DIP Package ♦ Low Output Offset: ±5mV Ordering Information Applications ADC Anti-Aliasing PART MAX7480ESA MAX7480EPA Post-DAC Filtering Pin Configuration TEMP. RANGE PIN-PACKAGE -40°C to +85°C -40°C to +85°C 8 SO 8 Plastic DIP Typical Operating Circuit TOP VIEW VSUPPLY COM 1 8 CLK 7 SHDN 3 6 OS VDD 4 5 OUT IN 2 0.1µF VDD SHDN MAX7480 GND SO/DIP INPUT IN OUT OUTPUT MAX7480 CLOCK COM CLK GND OS 0.1µF ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX7480 General Description MAX7480 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter 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 = +70°C) 8-Pin SO (derate 5.88mW/°C above +70°C)................471mW 8-Pin DIP (derate 9.09mW/°C above +70°C) ...............727mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10sec) .............................+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 (VDD = +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF from COM to GND, SHDN = VDD, fCLK = 100kHz, 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 (Note 1) 0.001 to 2 fCLK / fC Clock-to-Corner Tempco 10 Output Voltage Range Output Offset Voltage 0.25 VOFFSET DC Insertion Gain with Output Offset Removed THD+N OS Voltage Gain to OUT Input Voltage Range at OS Input Resistance at COM ppm/°C VDD - 0.25 VIN = VCOM = VDD / 2 VCOM = VDD / 2 (Note 2) Total Harmonic Distortion plus Noise COM Voltage Range kHz 100:1 -0.1 fIN = 200Hz, VIN = 4Vp-p, measurement bandwidth = 22kHz V ±5 ±25 mV 0.15 0.3 dB -73 dB AOS 1 V/ V VOS VCOM ±0.1 V Input, COM externally driven VDD / 2 - 0.5 VDD / 2 VDD / 2 + 0.5 Output, COM internally biased VDD / 2 - 0.2 VDD / 2 VDD / 2 + 0.2 VCOM V RCOM 75 Resistive Output Load Drive RL 10 Maximum Capacitive Load at OUT CL 50 Clock Feedthrough 125 kΩ 10 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 - 1V) (Note 3) ±0.1 ±10 µA 53 67 kHz ±24 ±40 µA CLOCK Internal Oscillator Frequency fOSC COSC = 1000pF (Note 4) Clock Input Current ICLK VCLK = 0 or 5V Clock Input High VIH Clock Input Low VIL 2 40 VDD - 0.5 _______________________________________________________________________________________ V 0.5 V 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter (VDD = +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF from COM to GND, SHDN = VDD, fCLK = 100kHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 5.5 V mA POWER REQUIREMENTS Supply Voltage VDD Supply Current IDD 4.5 Operating mode, no load, IN = OS = COM 2.9 3.5 Shutdown Current I SHDN SHDN = GND, CLK driven from 0 to VDD 0.2 1 Power-Supply Rejection Ratio PSRR Measured at DC 60 µA dB SHUTDOWN SHDN Input High VSDH SHDN Input Low VSDL SHDN Input Leakage Current VDD - 0.5 V ±0.1 V SHDN = 0 to VDD 0.5 V ±10 µA FILTER CHARACTERISTICS (VDD = +5V, filter output measured at OUT, 10kΩ || 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 = +25°C.) PARAMETER Insertion Gain Relative to DC Gain CONDITIONS MIN TYP fIN = 0.5fC -0.1 0.0 MAX fIN = fC -3.5 -3.0 -2.5 fIN = 2fC -48 -43 fIN = 3fC -76 -70 UNITS dB 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 75µA typical input leakage current. Note 4: fOSC (kHz) ≅ 53 · 103 / COSC (pF). _______________________________________________________________________________________ 3 MAX7480 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VDD = +5V, fCLK = 100kHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25°C, unless otherwise noted.) -0.5 -1.5 -2.0 -80 -2.5 -100 606 808 SUPPLY CURRENT vs. TEMPERATURE MAX7480 toc04 2.80 NO LOAD 2.95 2.90 2.85 2.80 2.75 2.75 2.70 2.70 1200 1600 2000 2.0 VIN = VCOM 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -40 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 800 DC OFFSET VOLTAGE vs. SUPPLY VOLTAGE DC OFFSET VOLTAGE (mV) 2.85 400 INPUT FREQUENCY (Hz) 3.00 SUPPLY CURRENT (mA) 2.90 0 1010 SUPPLY CURRENT vs. SUPPLY VOLTAGE 2.95 -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) SUPPLY VOLTAGE (V) OFFSET VOLTAGE vs. TEMPERATURE INTERNAL OSCILLATOR FREQUENCY vs. COSC CAPACITANCE NORMALIZED INTERNAL OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE FREQUENCY (kHz) 0.5 100 0 -0.5 10 1 -1.0 0.1 -1.5 0.01 -40 -20 0 20 40 60 TEMPERATURE (°C) 80 100 1.05 NORMALIZED OSCILLATOR FREQUENCY VIN = VCOM = VDD / 2 1000 MAX7401 toc07 1.0 MAX7480 toc08 SUPPLY CURRENT (mA) 404 INPUT FREQUENCY (Hz) NO LOAD 4 202 INPUT FREQUENCY (kHz) 3.00 480 640 0 MAX7480 toc05 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 400 fC = 1kHz -3.5 0 320 560 -3.0 -120 240 MAX7480-06 -60 -1.0 160 COSC = 530pF 1.04 MAX7480-09 GAIN (dB) GAIN (dB) -40 fC = 1kHz 80 PHASE SHIFT (DEGREES) 0 -20 0 MAX7480 toc02 MAX7480 toc01 fC = 1kHz 0 PHASE RESPONSE PASSBAND FREQUENCY RESPONSE 0.5 MAX7480 toc03 FREQUENCY RESPONSE 20 OFFSET VOLTAGE (mV) MAX7480 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter 1.03 1.02 1.01 1.00 0.99 0.98 0.97 0.96 0.95 0.1 1 10 CAPACITANCE (nF) 100 1000 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) _______________________________________________________________________________________ 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter (VDD = +5V, fCLK = 100kHz, SHDN = VDD, VCOM = VOS = VDD / 2, TA = +25°C, unless otherwise noted.) NORMALIZED OSCILLATOR FREQUENCY vs. TEMPERATURE 1.02 0 NO LOAD (SEE TABLE A) -10 -20 MAX7480 toc11 COSC = 530pF MAX7480 toc10 -30 1.01 THD+N (dB) NORMALIZED OSCILLATOR FREQUENCY 1.03 TOTAL HARMONIC DISTORTION PLUS NOISE vs. INPUT SIGNAL AMPLITUDE 1 -40 -50 A -60 0.99 B -70 0.98 -80 -90 0.97 -40 -20 0 20 40 60 80 100 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 TEMPERATURE (°C) AMPLITUDE (Vp-p) Table A. THD+N vs. Input Signal Amplitude Test Conditions TRACE fIN (Hz) fC (kHz) fCLK (kHz) MEASUREMENT BANDWIDTH (kHz) A 400 2 200 22 B 200 1 100 22 _______________________________________________________________________________________ 5 MAX7480 Typical Operating Characteristics (continued) MAX7480 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter Pin Description PIN NAME 1 COM FUNCTION Common Input Pin. Biased internally at mid-supply. Bypass externally to GND with a 0.1µF capacitor. To override internal biasing, drive with an external supply. 2 IN 3 GND Filter Input Ground 4 VDD +5V Supply Input 5 OUT Filter Output 6 OS 7 SHDN 8 CLK 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 GAIN (dB) -20 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. 6 B -60 -80 -100 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 -40 0.1 0.2 0.5 1 2 5 10 FREQUENCY (kHz) A: BESSEL FILTER RESPONSE; fC = 1kHz B: BUTTERWORTH FILTER RESPONSE; fC = 1kHz Figure 1. Bessel vs. Butterworth Filter Frequency Response R1 + - VIN L1 L5 L3 C2 C4 L7 C6 Figure 2. 8th-Order Ladder Filter Network _______________________________________________________________________________________ C8 R2 V0 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter 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: 53 ⋅10 ; COSC in pF COSC 3 fOSC (kHz) = 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. 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 0.1µF 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. 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 = ( 1 fCLK ⋅ CIN ) where fCLK = clock frequency and CIN = 2.31pF. 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 0.1µF VDD 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 0.2µA (typ) and its output becomes high impedance. For normal operation, drive SHDN high or connect to VDD. ___________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 0.1µF capacitor and INPUT IN SHDN OUT OUTPUT COM 0.1µF 50k MAX7480 CLOCK CLK 50k OS 0.1µF GND 50k Figure 3. Offset Adjustment Circuit _______________________________________________________________________________________ 7 MAX7480 Clock Signal MAX7480 8th-Order, Lowpass, Butterworth, Switched-Capacitor Filter Anti-Aliasing and Post-DAC Filtering V+ = +2.5V 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. VDD OUT INPUT V+ V- IN * OUTPUT COM MAX7480 CLOCK CLK OS 0.1µF 0.1µF GND V- = -2.5V 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 10kΩ load at TA = +25°C. SHDN *DRIVE SHDN TO V- FOR LOW-POWER SHUTDOWN MODE. Figure 4. Dual-Supply Operation Table 1. Typical Harmonic Distortion FILTER fCLK (kHz) fC (kHz) fIN (Hz) 100 1 200 MAX7480 VIN (Vp-p) TYPICAL HARMONIC DISTORTION (dB) 2nd 3rd 4th 5th -89 -73 -91 -93 -82 -68 -85 -89 4 200 2 400 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 © 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.