19-181; Rev ; 11/ 5th-Order, Lowpass, General Description The MAX718 MAX75 5th-order, low-pass, switchedcapacitor filters (SCFs) operate from a single +5 (MAX718 MAX71) or +3 (MAX7 MAX75) supply. These devices draw only 3mA of supply current and allow corner frequencies from 1Hz to 5kHz, making them ideal for low-power post-dac filtering and antialiasing applications. They feature a shutdown mode that reduces supply current to.µa. 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 MAX718/MAX7 deliver 53 of stopband rejection and a sharp rolloff with a 1.6 transition ratio. The MAX71/MAX75 achieve a sharper rolloff with a 1.5 transition ratio while still providing 37 of stopband rejection. The MAX719/MAX73 Bessel filters provide low overshoot and fast settling, and the MAX7/MAX7 Butterworth filters provide a maximally flat passband response. Their fixed response simplifies the design task of selecting a clock frequency. ADC Anti-Aliasing DAC Postfiltering PART FILTER RESPONSE Applications CT Base Stations Speech Processing Selector Guide continued at end of data sheet. Selector Guide OPERATING OLTAGE () MAX718 r = 1.6 +5 MAX719 Bessel +5 MAX7 Butterworth +5 MAX71 r = 1.5 +5 Pin Configuration Features 5th-Order, Lowpass Filters Elliptic Response (MAX718/MAX71/ MAX7/MAX75) Bessel Response (MAX719/MAX73) Butterworth Response (MAX7/MAX7) Clock-Turnable Corner Frequency (1Hz to 5kHz) Single-Supply Operation +5 (MAX718 MAX71) +3 (MAX7 MAX75) Low Power 3mA (Operating Mode).µA (Shutdown Mode) Available in 8-Pin µmax Package Low Output Offset: ±m PART MAX718EUA.1µF SUPPLY Ordering Information TEMP. RANGE MAX718CUA C to +7 C 8 µmax - C to +85 C PIN-PACKAGE 8 µmax MAX719CUA C to +7 C 8 µmax MAX719EUA - C to +85 C 8 µmax MAX7CUA C to +7 C 8 µmax MAX7EUA - C to +85 C 8 µmax MAX71CUA C to +7 C 8 µmax MAX71EUA - C to +85 C 8 µmax Ordering Information continued at end of data sheet. Typical Operating Circuit MAX718 MAX75 TOP IEW INPUT IN DD SHDN OUT OUTPUT COM IN GND DD 1 3 MAX718 MAX75 8 7 6 5 CLK SHDN OS OUT CLOCK CLK MAX718 MAX75 GND COM OS.1µF µmax Maxim Integrated Products 1 For free samples and the latest literature, visit www.maxim-ic.com or phone 1-8-998-88. For small orders, phone 1-8-835-8769.
MAX718 MAX75 ABSOLUTE MAXIMUM RATINGS DD to GND...-.3 to +6 IN, OUT, COM, OS, CLK, SHDN...-.3 to ( DD +.3) OUT Short-Circuit Duration...1s Continuous Power Dissipation (T A = +7 C) 8-Pin µmax (derate.1mw/ C above +7 C)...33mW 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 MAX718 MAX71 Operating Temperature Ranges MAX7 C_A... C to +7 C MAX7 E_A...- C to +85 C Junction Temperature...+15 C Storage Temperature Range...-65 C to +16 C Lead Temperature (soldering, 1s)...+3 C ( DD = +5, filter output measured at OUT, 1kΩ 5pF load to GND at OUT, OS = COM,.1µF capacitor from COM to GND, SHDN = DD, f CLK =.MHz, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +5 C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX FILTER CHARACTERISTICS Corner Frequency fc IN = p-p (Note 1).1 to 3 Clock-to-Corner Ratio f CLK /f C 1:1 Clock-to-Corner Tempco 1 Output oltage Range.5 DD -.5 Output Offset oltage OFFSET IN = COM = DD / ± ±5 DC Insertion Gain with COM = DD / MAX718/MAX71.. Output Offset Removed (Note ) MAX719/MAX7 -. +. f IN = KHz, MAX718-76 Total Harmonic Distortion MAX719-78 THD+N IN = p-p, plus Noise measurement MAX7-67 bandwidth = 8kHz MAX71-78 Offset oltage Gain A OS OS to OUT 1 COM oltage Range COM Input, COM externally driven..5 3. Output, COM unconnected.3.5.7 Input oltage Range at OS OS Input, OS externally driven COM ±.1 Input Resistance at COM R COM 1 1 Clock Feedthrough 5 Resistive Output Load Drive R L 1 1 Maximum Capacitive Output Load Drive C L 5 5 Input Leakage Current at COM SHDN = GND, COM = to DD ±.1 ±1 Input Leakage Current at OS OS = to DD ±.1 ±1 CLOCK Internal Oscillator Frequency f OSC C OSC = 1pF (Note MAX718/MAX71 68 87 16 3) MAX719/MAX7 86 11 135 Clock Output Current MAX718/MAX71 ± ±6 I (Internal Oscillator Mode) CLK CLK = or 5 MAX719/MAX7 ±5 ±75 Clock Input High IH.5 Clock Input Low IL.5 UNITS khz ppm/ C m / kω mp-p kω pf µa µa khz µa
ELECTRICAL CHARACTERISTICS MAX718 MAX71 (continued) ( DD = +5, filter output measured at OUT, 1kΩ 5pF load to GND at OUT, OS = COM,.1µF capacitor from COM to GND, SHDN = DD, f CLK =.MHz, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +5 C.) Supply oltage Supply Current PARAMETER POWER REQUIREMENTS Shutdown Current Power-Supply Rejection Ratio SHUTDOWN SHDN Input High SHDN Input Low SYMBOL DD I DD I SHDN PSRR SDH SDL Operating mode, no load SHDN = GND IN = COM (Note ) ELECTRICAL CHARACTERISTICS MAX7 MAX75 CONDITIONS MIN TYP MAX MAX718/MAX71.5 5.5 ( DD = +3, filter output measured at OUT pin, 1kΩ 5pF load to GND at OUT, OS = COM,.1µF capacitor from COM to GND, SHDN = DD, f CLK =.MHz, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +5 C.).5.9 3.6 MAX719/MAX7 3..1. 1 SHDN Input Leakage Current SHDN = to DD ±. ±1 µa PARAMETER SYMBOL CONDITIONS MIN TYP MAX FILTER CHARACTERISTICS Corner-Frequency Range f C IN =.5p-p MAX7/MAX75.1 to 5 1:1 to 5 (Note 1) MAX73/MAX7 Clock-to-Corner Ratio f CLK /f C 1:1 Clock-to-Corner Tempco 1 Output oltage Range.5 DD -.5 Output Offset oltage OFFSET IN = COM = DD / ± ±5 DC Insertion Gain with Output COM = DD / MAX7/MAX75.. Offset Removed (Note ) MAX73/MAX7 -. +. f IN = khz, MAX7-8 Total Harmonic Distortion plus MAX73-81 THD+N IN =.5p-p, Noise measurement MAX7-7 bandwidth = 8kHz MAX75-8 Offset oltage Gain A OS OS to OUT 1 COM oltage Range COM Input, COM externally driven 1. 1.5 1.6 Output, COM internally driven 1. 1.5 1.6 Input oltage Range at OS OS Measured with respect to COM COM ±.1 Input Resistance at COM R COM 1 1 Clock Feedthrough 3 Resistive Output Load Drive R L 1 1 Maximum Capacitive Load at OUT C L 5 5 Input Leakage Current at COM SHDN = GND, COM = to DD ±.1 ±1 Input Leakage Current at OS OS = to DD ±.1 ±1 7.5 UNITS ma µa UNITS khz ppm/ C m / kω mp-p kω pf µa µa MAX718 MAX75 3
MAX718 MAX75 ELECTRICAL CHARACTERISTICS MAX7 MAX75 (continued) ( DD = +3, filter output measured at OUT pin, 1kΩ 5pF load to GND at OUT, OS = COM,.1µF capacitor from COM to GND, SHDN = DD, f CLK =.MHz, T A = T MIN to T MAX, unless otherwise noted. Typical values are at T A = +5 C.) PARAMETER CLOCK Internal Oscillator Frequency Clock Output Current (Internal Oscillator Mode) Clock Input High Clock Input Low POWER REQUIREMENTS Supply oltage Shutdown Current Power-Supply Rejection Ratio SHUTDOWN SHDN Input High SYMBOL f OSC I CLK IH IL DD I SHDN PSRR SDH C OSC = 1pF (Note 3) SHDN = GND Measured at DC CONDITIONS MIN TYP MAX MAX7/MAX75 68 87 16 MAX73/MAX7 86 11 135 MAX7/MAX75 68 87 16 MAX73/MAX7 86 11 135.5.5.7 3.6 Operating mode, MAX7/MAX75.6 3. Supply Current I DD ma no load MAX73/MAX7 3. 3.8 SHDN Input Low SDL.5 SHDN Input Leakage Current SHDN = to DD ±. ±1 µa.5. 1 7 UNITS khz khz µa
FILTER CHARACTERISTICS ( DD = +5 for MAX718-MAX7, DD = +3 for MAX7-MAX75 filter output measured at OUT, 1kΩ 5pF load to GND at OUT, SHDN = DD, f CLK =.MHz, T A = T MIN to T MAX, unless otherwise noted.) -1 PARAMETER CONDITIONS MIN TYP MAX UNITS ELLIPTIC, R = 1. MAX71/MAX75 f IN =.38f C -. -.. f IN =.68f C -. -.. f IN =.87f C -. -.. Insertion Gain with DC Gain f IN =.97f C -. -.. Error Removed (Note ) f IN = f C -.7 -.. f IN = 1.5f C -36-33 f IN = 1.3f C -37. -35 f IN = 3.5f C -37. -35 BESSEL FILTERS MAX719/MAX73 f IN =.5f C -.7 Insertion Gain Relative to f IN = f C -3.6-3. -. DC Gain f IN = f C -1. -35 f IN = 7f C -67-6 BESSEL FILTERS MAX719/MAX73 f IN =.5f C -1. -.7 Insertion Gain Relative to f IN = f C -3.6-3. -. DC Gain f IN = 7f C -1. -35 f IN = 7f C -67-6 BESSEL BUTTERWORTH FILTERS MAX79/MAX713 FILTERS MAX7/MAX7 f IN =.5f C -.3 Insertion Gain Relative to f IN = f C -3.6-3. -. DC Gain f IN = 3f C -7.5-3 f IN = 5f C -7-65 MAX718 MAX75 Note 1: The maximum f C is defined as the clock frequency f CLK = 1 x f C at which the peak S / (THD+N) drops to 68 with a sinusoidal input at.f C. Maximum f C increases as IN signal amplitude decreases. Note : DC insertion gain is defined as Δ OUT / Δ IN. Note 3: MAX718/MAX71/MAX7/MAX75: f OSC (khz) 87x1 3 / C OSC (pf). MAX719/MAX7/MAX73/MAX7: f OSC (khz) 11x1 3 / C OSC (pf). Note : PSRR is the change in output voltage from a DD of.5 and a DD of 5.5. 5
MAX718 MAX75 Typical Operating Characteristics ( DD = +5 for MAX718 MAX71, DD = +3 for MAX7 MAX75; f CLK =.MHz; SHDN = DD ; COM = OS = DD / ; T A = +5 C; unless otherwise noted.) GAIN () GAIN () GAIN () MAX718/MAX7 FREQUENCY RESPONSE (ELLIPTIC, R = 1.6) 1-1 - -3 - -5-6 -7-8 -9 6 8 1 MAX71/MAX75 FREQUENCY RESPONSE (ELLIPTIC, R = 1.5) 1-1 - -3 - -5-6 -7-8 -9 6 8 1.5 -.5-1. -1.5 -. -.5-3. -3.5 -. MAX7/MAX7 PASSBAND FREQUENCY RESPONSE (BUTTERWORTH).5 9. 13.5 18..5 MAX718 toc1 MAX718 toc MAX718 toc7 GAIN () GAIN () GAIN () 1-1 - -3 - -5-6.. -. -. -.6 -.8-1... -. -. -.6 -.8-1. MAX719/MAX73 FREQUENCY RESPONSE (BESSEL) 6 8 1 MAX718/MAX7 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, R = 1.6).5 9. 13.5 18..5 MAX71/MAX75 PASSBAND FREQUENCY RESPONSE (ELLIPTIC, R = 1.5).5 9. 13.5 18..5 MAX718 toc MAX718 toc5 MAX718 toc8 GAIN () GAIN () PHASE SHIFT (DEGREES) 1-1 - -3 - -5-6 -7-8.5 -.5-1. -1.5 -. -.5-3. -3.5 -. -5-1 -15 - -5-3 -35 - -5 MAX7/MAX7 FREQUENCY RESPONSE (BUTTERWORTH) 6 8 1 MAX719/MAX73 PASSBAND FREQUENCY RESPONSE (BESSEL).5 9. 13.5 18..5 MAX718/MAX7 PHASE RESPONSE (ELLIPTIC, R = 1.6) 8 1 16 8 MAX718 toc3 MAX718 toc6 MAX718 toc9 6
Typical Operating Characteristics (continued) ( DD = +5 for MAX718 MAX71, DD = +3 for MAX7 MAX75; f CLK =.MHz; SHDN = DD ; COM = OS = DD / ; T A = +5 C; unless otherwise noted.) PHASE SHIFT (DEGREES) THD + N () THD + N () -5-1 -15 - MAX719/MAX73 PHASE RESPONSE (BESSEL) -5 8 1 16 8 MAX718 (ELLIPTIC, R = 1.6) -1 - -3 - -5-6 -7-8 -9 - -5-6 -7-8 -9 1 3 5 AMPLITUDE (p-p) MAX71 (ELLIPTIC, R = 1.5) -1 - -3 E D 1 3 5 AMPLITUDE (p-p) D E MAX718 toc1 MAX718 toc13 MAX718 toc16 PHASE SHIFT (DEGREES) THD + N () THD + N () -5-1 -15 - -5-3 -35 - -5-6 -7-8 -9 MAX7/MAX7 PHASE RESPONSE (BUTTERWORTH) 8 1 16 8 MAX719 (BESSEL) -1 - -3 E D 1 3 5 AMPLITUDE (p-p) MAX7 (ELLIPTIC, R = 1.6) -1 - -3 - -5-6 -7-8 -9 C.5 1. 1.5..5 3. AMPLITUDE (p-p) A B MAX718 toc11 MAX718 toc1 MAX718 toc17 PHASE SHIFT (DEGREES) THD + N () THD + N () -5-1 -15 - -5-3 -35 - -5 - -5-6 -7-8 -9 MAX7/MAX75 PHASE RESPONSE (ELLIPTIC, R = 1.5) 8 1 16 8 MAX7 (BUTTERWORTH) -1 - -3 - -5-6 -7-8 -9 E D 1 3 5 AMPLITUDE (p-p) MAX73 (BESSEL) -1 - -3 C A.5 1. 1.5..5 3. AMPLITUDE (p-p) B MAX718 toc1 MAX718 toc15 MAX718 toc18 MAX718 MAX75 7
MAX718 MAX75 Typical Operating Characteristics (continued) ( DD = +5 for MAX718 MAX71, DD = +3 for MAX7 MAX75; f CLK =.MHz; SHDN = DD ; COM = OS = DD / ; T A = +5 C; unless otherwise noted.) THD + N () MAX7 (BUTTERWORTH) -1 - -3 - -5-6 -7-8 -9 C A.5 1. 1.5..5 3. AMPLITUDE (p-p) B MAX718 toc19 THD + N () MAX75 (ELLIPTIC, R = 1.5) -1 - -3 - -5-6 -7-8 -9 C A.5 1. 1.5..5 3. AMPLITUDE (p-p) B MAX718 toc OSCILLATOR FREQUENCY (khz) 7 6 5 3 1 INTERNAL OSCILLATOR FREQUENCY vs. SMALL CAPACITANCE (pf) ELLIPTIC BESSEL/BUTTERWORTH 1 1 1 1 1 CAPACITANCE ( pf) MAX718 toc1 OSCILLATOR REQUENCY (Hz) SUPPLY CURRENT (μa) 6 5 3 1 INTERNAL OSCILLATOR FREQUENCY vs. LARGE CAPACITANCE (nf) ELLIPTIC BESSEL/BUTTERWORTH 1 1 1 CAPACITANCE (nf) 3.3 3.1.9.7.5 ELLIPTIC SUPPLY CURRENT vs. SUPPLY OLTAGE MAX718 toc MAX718 toc5 OSCILLATOR FREQUENCY (khz) SUPPLY CURRENT (ma) 87. 86.5 86. 85.5 85. 8.5 8. 3..9.8.7.6 ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. SUPPLY OLTAGE C OSC = 1PF.5 3. 3.5..5 5. 5.5 SUPPLY OLTAGE () ELLIPTIC SUPPLY CURRENT vs. TEMPERATURE DD = 5 DD = 3 MAX718 toc3 MAX718 toc6 OSCILLATOR FREQUENCY (khz) 87. 86.5 86. 85.5 85. 8.5 8. ELLIPTIC INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE C OSC = 1PF DD = 3 DD = 5 - -15 1 35 6 85 TEMPERATURE ( C) Table A. LABEL A B C f IN (khz) 1 D E 1 f C (khz) 3 1 1 f CLK (khz) 3 1 1 MAX718 toc BW (khz) 8 8 8.3.5 3. 3.5..5 5. 5.5 SUPPLY OLTAGE ().5 - -15 1 35 6 85 TEMPERATURE ( C) 8
Typical Operating Characteristics (continued) ( DD = +5 for MAX718 MAX71, DD = +3 for MAX7 MAX75; f CLK =.MHz; SHDN = DD ; COM = OS = DD / ; T A = +5 C; unless otherwise noted.) DC OFFSET OLTAGE (m) 3..5. 1.5 1..5 DC OFFSET OLTAGE vs. TEMPERATURE DD = 3 DD = 5 - -15 1 35 6 85 TEMPERATURE ( C) MAX718 toc7 DC OFFSET OLTAGE (m).5. 1.5 1..5 DC OFFSET OLTAGE vs. SUPPLY OLTAGE.5 3. 3.5..5 5. 5.5 SUPPLY OLTAGE () MAX718 toc8 MAX718 MAX75 Pin Description PIN NAME FUNCTION 1 COM Common Input Pin. Biased internally at midsupply. Bypass COM externally to GND with a.1µf capacitor. To override internal biasing, drive COM with an external supply. IN Filter Input 3 GND Ground DD Positive Supply Input: +5 for MAX718 MAX71, +3 for MAX7 MAX75. Bypass DD to GND with a.1µf capacitor. 5 OUT Filter Output 6 OS Offset Adjust Input. To adjust output offset, connect OS to an external supply through a resistive voltagedivider (Figure ). Connect OS to COM if no offset adjustment is needed. The Offset and Common-Mode Input Adjustment section. 7 SHDN Shutdown Input. Drive low to enable shutdown mode; drive high or connect to DD for normal operation. 8 CLK Clock Input. Connect an external capacitor (C OSC ) from CLK to ground. To override the internal oscillator, connect CLK to an external clock: f C = f CLK /1. Detailed Description The MAX718/MAX71/MAX7/MAX75 elliptic lowpass filters provide sharp rolloff with good stopband rejection. The MAX719/MAX73 Bessel filters provide low overshoot and fast settling responses, and the MAX7/MAX7 Butterworth filters provide a maximally flat passband response. All parts operate with a 1:1 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 MAX718 MAX75 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
MAX718 MAX75 + - R S IN C1 Figure 1. 5th-Order Ladder Filter Network L C3 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. Elliptic Characteristics Lowpass elliptic filters such as the MAX718/MAX71/ MAX7/MAX75 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 ). Beyond this zero, the response rises as the frequency increases until the next transmission zero. The stopband begins at the stopband frequency, f S. At frequencies above f S, the filter s gain does not exceed the gain at f S. The corner frequency, f C, 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 = f S / f C The MAX718/MAX7 have a transition ratio of 1.6 and typically 53 of stopband rejection. The MAX71/MAX75 have a transition ratio of 1.5 (providing a steeper rolloff) and typically 37 of stopband rejection. Bessel Characteristics Lowpass Bessel filters such as the MAX719/MAX73 L C5 R L 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 Lowpass Butterworth filters such as the MAX7/ MAX7 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. Clock Signal External Clock These SCFs are designed for use with external clocks that have a % to 6% duty cycle. When using an external clock, drive the CLK pin with a CMOS gate powered from to DD. arying the rate of the external clock adjusts the corner frequency of the filter: GAIN () PASSBAND fc Figure. Elliptic Filter Response f C f C f = CLK 1 f S RIPPLE TRANSITION RATIO = f S fc f S STOPBAND FREQUENCY 1
A B C Internal Clock When using the internal oscillator, the capacitance (C OSC ) on CLK determines the oscillator frequency: fosc(khz) μs/div where k=87x1 3 for the MAX718/MAX71/MAX7/MAX75 and k=11x1 3 for the MAX719/MAX7/MAX73/ MAX7. = k COSC(pF) /div /div /div A: 1kHz INPUT SIGNAL B: MAX719 BESSEL FILTER RESPONSE; f C = 5kHz C: MAX7 BUTTERWORTH FILTER RESPONSE; f C = 5kHz Figure 3. Bessel vs. Butterworth Filter Response Since C OSC is in the low picofarads, minimize the stray capacitance at CLK so that it does not affect the internal oscillator frequency. arying the rate of the internal oscillator adjusts the filter s corner frequency by a 1:1 clock-to-corner frequency ratio. For example, an internal oscillator frequency of.khz produces a nominal corner frequency of.mhz. Input Impedance vs. Clock Frequencies The MAX718 MAX75s input impedance is effectively that of a switched-capacitor resistor (see the following equation), and is inversely proportional to frequency. The input impedance values determined by the equation represent the average input impedance, since the input current is not continuous. As a rule, use a driver with an output resistance less than 1% of the filter s input impedance..1μf INPUT CLOCK SUPPLY IN CLK Estimate the input impedance of the filter by using the following formula: Z IN = 1 (f CLK C IN) where f CLK = clock frequency and C IN = 1pF. DD MAX718 MAX75 GND Figure. Offset Adjustment Circuit SHDN OUT COM.1μF.1μF OUTPUT Low-Power Shutdown Mode The MAX718 MAX75 have a shutdown mode that is activated by driving SHDN low. In shutdown mode, the filter supply current reduces to.µa, and the output of the filter becomes high impedance. For normal operation, drive SHDN high or connect to DD. 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. 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: OUT = ( IN COM) + OS DD COM = ( typical) where ( IN - COM ) is lowpass filtered by the SCF and OS is added at the output stage. See the Electrical OS 5k 5k 5k MAX718 MAX75 11
MAX718 MAX75 Characteristics table for the input voltage range of COM and OS. Changing the voltage on COM or OS significantly from midsupply reduces the dynamic range. Power Supplies The MAX718 MAX71 operate from a single +5 supply and the MAX7 MAX75 operate from a single +3 supply. Bypass DD to GND with a.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 (- in dual supply operation) to DD. Use the MAX718 MAX71 for ±.5, and use the MAX7 MAX75 for ±1.5. For ±5 dual-supply applications, refer to the MAX91/ MAX9/MAX95/MAX96 and MAX93/MAX9/ MAX97 data sheets. 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 When using the MAX718 MAX75 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. + - INPUT CLOCK IN CLK + DD MAX718 MAX75 GND *CONNECT SHDN TO - FOR LOW-POWER SHUTDOWN MODE. Figure 5. Dual-Supply Operation - * SHDN OUT COM 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,, and 3 list typical harmonic distortion values with a 1kΩ load at T A = +5 C. OS OUTPUT.1μF.1μF Table 1. MAX718/MAX71/MAX7/MAX75 Typical Harmonic Distortion FILTER f CLK (MHz) f IN (khz) IN (p-p) TYPICAL HARMONIC DISTORTION () nd 3rd th 5th MAX718. 1.5 MAX71. 1.5 MAX7.. MAX75.. 1
Table. MAX7/MAX7 Typical Harmonic Distortion FILTER MAX7 MAX7 Table 3. MAX719/MAX73 Typical Harmonic Distortion FILTER MAX719 f CLK (MHz). 1.5 3.5. f CLK (MHz). 1.5 f IN (khz) 3 f IN (khz) IN (p-p) IN (p-p) TYPICAL HARMONIC DISTORTION () nd -77 < -8 < -8 < -8 3rd th -67 < -8-76 -7-7 -77 < -8 < -8 < -8 5th < -8 < -8 < -8 TYPICAL HARMONIC DISTORTION () nd < -8 < -8 3rd -77 < -8 < -8-8 th < -8 5th < -8 MAX718 MAX75 MAX73 3.5. 3 < -8 < -8-75 < -8 < -8 < -8 < -8 < -8 Ordering Information (continued) PART TEMP. RANGE PIN-PACKAGE MAX7CUA C to +7 C 8 µmax MAX7EUA - C to +85 C 8 µmax MAX73CUA C to +7 C 8 µmax MAX73EUA - C to +85 C 8 µmax MAX7CUA C to +7 C 8 µmax MAX7EUA - C to +85 C 8 µmax MAX75CUA C to +7 C 8 µmax MAX75EUA - C to +85 C 8 µmax Selector Guide (continued) PART FILTER RESPONSE OPERATING OLTAGE () MAX7 r = 1.6 +3 MAX73 Bessel +3 MAX7 Butterworth +3 MAX75 r = 1.5 +3 TRANSISTOR COUNT: 157 PROCESS: BiCMOS Chip Information 13
MAX718 MAX75 Package Information α α 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. 1 Maxim Integrated Products, 1 San Gabriel Drive, Sunnyvale, CA 986 8-737-76 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.