19-0020; Rev 2; 06/08 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters General Description The MAX293/MAX294/MAX297 are easy-to-use, 8th- order, lowpass, elliptic, switched-capacitor filters that can be set up with corner frequencies from 0.1Hz to 25kHz (MAX293/MAX294) or from 0.1Hz to SOkHz (MAX297). The MAX293/MAX297's 1.5 transition ratio provides sharp rolloff and -80dB of stopband rejection. The MAX294s 1.2 transition ratio provides the steepest rolloff and -58dB of stopband rejection. All three filters have fixed responses, so the design task is limited to select- ing the clock frequency that controls the filter's corner frequency. An external capacitor is used to generate a clock using the internal oscillator, or an external clock signal can be used. An uncommitted op amp (noninverting input grounded) is provided for building a continuous-time lowpass filter for post-filtering or anti-aliasing. Steep rolloff and high order make these filters ideal for anti- aliasing applications that require maximum bandwidth, and for communication applications that require filtering signals in close proximity within the frequency domain. The MAX293/MAX294/MAX297 are available in 8-pin DIP and 16-pin wide SO packages, delivering aggressive performance from a tiny area. Applications Data-Acquisition Systems Anti-Aliasing DAC Post-Filtering Voice/Data Signal Filtering Typical Operating Circuit +5V 7 V. INPUT 8 IN . OUT 5 OUTPUT MAAXIAA OPOUT 3 MAX293 MAX294 cock ck mare? a GND V- ri? -5V PIN CONFIGURATION IS 8-PIN DIP. MA MALSVI Features @ 8th-Order Lowpass Elliptic Filters @ Clock-Tunable Corner-Frequency Range: 0.1Hz to 25kHz (MAX293/MAX294) 0.1Hz to 50kHz (MAX297) @ No External Resistors or Capacitors Required @ Internal or External Clock @ Clock to Corner Frequency Ratio: 100:1 (MAX293/294) 50:1 (MAX297) @ Operate with a Single +5V Supply or Dual +5V Supplies @ Uncommitted Op Amp for Anti-Aliasing or Clock-Noise Filtering @ 8-Pin DIP and 16-Pin Wide SO Packages Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX293CPA OC to +70C 8 Plastic DIP MAX293CWE OC to +70C 16 Wide SO MAX293C/D O'C to +70C Dice* MAX293EPA -40C to +85C 8 Plastic DIP MAX293EWE -40C to +85C 16 Wide SO MAX293MJA -55C to +125C 8 CERDIP** MAX294CPA 0C to +70C 8 Plastic DIP MAX294CWE OC to +70C 16 Wide SO MAX294C/D 0C to +70C Dice* _| Ordering information continued on last page. * Contact factory for dice specifications. ** Contact factory for availability and processing to MIL-STD-883. Pin Configurations TOP VIEW CLK rl ra] WN MAXIAA \- 24 axa |Z) OP OUT [3 MAX294 6 | GND OP IN- [4 MAX297 5] OUT DIP Wide SO on last page MAKIN Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxims website at www.maxim-ic.com. L6ZXVW/P6CXVW/E6CXVUNMAX293/MAX294/MAX297 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters ABSOLUTE MAXIMUM RATINGS Supply Voltage (V+ toV-) 2... cece 12v Input Voitage at Any Pin...... Continuous Power Dissipation 8-Pin Plastic DIP (derate 9.09mW/"C above +70C) .. .727mW 16-Pin Wide SO (derate 9.52mW/"C above +70C) ...762mW 8-Pin CERDIP (derate 8.00mW/"C above +70C) vee 640mW Operating Temperature Ranges: ee eee eee OC to +70C eee eee -40C to +85C eee ete eee -65C to +125C MAX29_C__ MAX29_E__ MAX29_MJA Storage Temperature Range Lead Temperature (soldering, 10sec) .... ... 65C to +160C eee nee +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 (V+ = 5V, V- = -5V, filter output measured at OUT pin, 20kQ load resistor to ground at OUT, foLk = 100kHz (MAX293/MAX294) or fCLK = SOKHz (MAX297) Ta = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS | _MIN TYP MAX | __UNITS FILTER CHARACTERISTICS Corner-Frequency Range MAX293/MAX294 0.1-25k Hz MAX297 0.1-50k Clock to Corner MAX293/MAX294 100:1 Frequency Ratio MAX297 50-4 Clock to Corner MAX293 8 0 Frequency Tempco MAX294 7 pem/'C MAX297 4 fIN = 0.381F5 0.12 -0.10 -0.17 fin = 0.594Fo 0.12 0.02 -0.17 fin = 0.759Fo 0.12 -0.11 -0.17 fin = 0.866Fo 0.12 -0.03 -0.17 fin = 0.939F 5 0.12 -0.11 -0.17 MAX298 fin = 0.993Fo 0.22 0.04 0.17 fin = 1.000F5 0.22 0.01 -0.17 fin = 1.500F 5 -73 -78 Insertion Gain Relative to fin = 1.610Fo 80 87 DC Gain (Note 1) fin = 2.020Fo -80 -84 fin =4.020Fo -80 -84 aB fin = 0.425Fo 0.10 -0.11 -0.17 fin = 0.644Fo 0.10 0.02 -0.17 fin = 0.802Fo 0.10 -0.10 -0.17 fin = 0.895F5 0.10 -0.03 -0.17 fin = 0.946Fo 0.10 -0.07 -0.17 MAX294 fin = 0.994Fo 0.36 0.16 0.17 fin = 1.000Fo 0.36 0.13 -0.17 fiIN = 1.200Fo -49 -54 fin = 1.270F 9 -57 -62 fIN = 1.530F9 -57 -60 fin = 2.840Fo -57 -60 MAXIMA8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters ELECTRICAL CHARACTERISTICS (continued) (V+ = 5V, V- = -5V, filter output measured at OUT pin, 20kQ load resistor to ground at OUT, fcLK = 100kHz (MAX293/MAX294) or fCLK = 50kHz (MAX297) Ta = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS fin = 0.377Fo | 0.10 -0.11 -0.17 fin = 0.591Fo 0.10 0.03 -0.17 fin = 0.754Fo 0.10 -0.12 -0.17 / / / fin = 0.873F5 0.10 0.02 -0.17 De Gain foes to oar fIN = 0.944Fo 0.10 -0.07 0.17 - (continued) fin = 0.996F5 0.30 0.11 -0.17 fin = 1.000F5 0.30 0.10 -0.17 fin = 1.500Fo -73 -79 fin = 1.610Fo -80 -87 fin = 2.020Fo -80 -84 fin = 4.000Fo -80 -85 MAX293 0.15 Passband Ripple MAX294 0.27 dB MAX297 0.23 Output DC Swing +4 Vv Output Offset Voltage IN = GND +150 +400 mV Outs Oteet Remove F 0.15 +0.01 0.15 dB . . MAX293 -71 pea piaemonic Distortion Ta = +25C [wean 69 dB MAX297 -77 Clock Feedthrough Ta =+25C 5.0 mVp-p Output Drive Capability 20 10 kQ CLOCK Fesuaney Cosc = 1000pF 29 35 43 kHz Current Source/Sink Vouk = OV or SV 470 120 BA Clock Input (Note 2) 40 High V Low 1.0 UNCOMMITTED OP AMP Input Offset Voltage +10 +50 mV Output Drive Capability 20 10 kQ Output DC Swing +4 Vv | Gain-Bandwidth Product 4 MHz POWER REQUIREMENTS Sippy gavage 42.375 $5.5 y Single Supply V- = OV, GND = V+/2 4.75 11.0 V+ = 5V, V- = -5V, VeLk = OV to 5V 15.0 22.0 Supply Current V+ = 2.375V, V- = -2.375V, 70 120 mA VOLK = -2V to 2V Note 1: Test frequencies selected at ripple peaks and troughs. Note 2: Guaranteed by design. MAXIM 3 L6SXVW/VECXVW/E6CXVNMAX293/MAX294/MAX297 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters Typical Operating Characteristics (V+ = 5V, V- = -5V, foLK = 100kHz (MAX293/MAX294) or fCLK = 5OkHz (MAX297), Ta = +25C, unless otherwise noted.) INTERNAL OSCILLATOR PERIOD vs. CAPACITANCE VALUE 3 a 3S oc = a 2 = 3 & 02 4 6 8 10 12 14 16 CAPACITANCE (nF) MAX293 FREQUENCY RESPONSE o= kHz s z =< Oo 0 1 2 3 4 INPUT FREQUENCY (kHz} MAX293 PASSBAND FREQUENCY RESPONSE 0.12 0.08 Fo= tkHz G.04 0 gS ze 0.04 & -0.08 0.12 0.16 -0.20 816 408 612 INPUT FREQUENCY (Hz) 0 204 18 1.02k GAIN (dB) NORMALIZED OSCILLATOR FREQUENCY GAIN (dB) NORMALIZED INTERNAL OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE TTT 1nF EXTERNAL CAPACITOR ON CLK 1.030 T 1.020 ~L_ \ 1.010 Ny 1.000 T 0005 | | | 20 25 30 35 40 45 50 55 SUPPLY VOLTAGE (V) MAX294 FREQUENCY RESPONSE 20 mT | | Fo = 1ktlz 0 [ob -20 + | 40} | 60 ut 7 = L L 100 | 0 4 2 3 4 5 INPUT FREQUENCY (kHz} MAX284 PASSBAND FREQUENCY RESPONSE 024 ote Fo = 1kHz 0.08 0 -0.08 0.16 0.24 032 0,40 0 204 408 612 4.02k INPUT FREQUENCY (Hz) 816 GAIN (dB) NORMALIZED OSCILLATOR FREQUENCY GAIN (dB) NORMALIZED INTERNAL OSCILLATOR FREQUENCY vs. TEMPERATURE nF ON CLK 1.06 1.03 1.00 0,97 0,94 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) MAX297 FREQUENCY RESPONSE Fo = kHz -100 120 -140 0 1 2 3 4 5 INPUT FREQUENCY (kHz) MAX297 PASSBAND FREQUENCY RESPONSE 0 0.16 Fo = 1kHz 0.12 0.08 0.04 0.00 -0.04 ~0.08 0.12 816 0 204 408 612 1.02k INPUT FREQUENCY (Hz) MAMNXIAMN8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters Typical Operating Characteristics (continued) (V+ = 5V, V- = -5V, foLK = 100kHz (MAX293/MAX294) or fcLk = 50kKHz (MAX297), Ta = +25C, unless otherwise noted.) MAX293 MAX294 MAX297 PHASE RESPONSE PHASE RESPONSE PHASE RESPONSE 0 0 80 Fo = 1kHz -100 Fo = 1KHz -80 Fo = 1kHz gq 180 7200 _ 160 3 20 = -300 = 200 E320 = 400 E -320 u 5 b 2 -400 wd we -400 x << <= a =z = -480 -600 -480 -560 -700 -560 -640 -800 -640 0 602 40050 (075 6100 (125 0 802 6050 075 6100 1.25 0 025 050 O75 100 125 NORMALIZED INPUT FREQUENCY NORMALIZED INPUT FREQUENCY NORMALIZED INPUT FREQUENCY SUPPLY CURRENT vs. SUPPLY VOLTAGE EXTERNAL 2 = INPUT MEASUREMENT = LABEL | fcik (Hz) | Fo (kHz) | cREQ. (Hz) BANDWIDTH (kHz) = = A 200k 2 200 30 2 B 1M 10 1k 80 s Cc 200k 4 400 30 i D 1M 20 2k 80 a a 20 25 30 35 40 45 50 58 SUPPLY VOLTAGE, V+ AND | V- | V} (V+ = 5V, V- = -5V, RLOAD = 20kQ, Ta = +25C, unless otherwise noted.) MAX293 MAX294 MAX297 THD + NOISE vs. THD + NOISE vs. THD + NOISE vs. INPUT SIGNAL AMPLITUDE INPUT SIGNAL AMPLITUDE INPUT SIGNAL AMPLITUDE s 8 s 8 # # S 2 2 i - F D v 1 2 3 4 5 6 7 8 9 10 1 2 3 4 6 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 AMPLITUDE (Vp-p} AMPLITUDE (Vp-p) AMPLITUDE (Vp-p) MAXIMA 5 Z6CXVW/VECXVW/E6CXUNMAX293/MAX294/MAX297 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters Pin Description PIN NAME 8-PIN | 16-PIN FUNCTION DIP so 1.2,7,8,9, | c, | No Connectnot internally 10,15,16 | connected Clock Inputuse internal or 1 3 CLK external clock. Negative Supply pin. 2 4 V- Dual supplies: -2.375V to -5.5V. Single supply: V- = OV. 3 5 Sor Uncommitted Op-Amp Output ; Inverting Input to the uncom- 4 6 OP mitted op amp. The noninvert- IN- | ing op amp is internally tied to GND. 5 11 QUT | Filter Output f Ground. In single-supply 6 12 GND | operation, GND must be biased to the mid-supply voltage level. Positive Supply pin. 7 13 V+ Dual supplies: +2.375V to +5.5V. Single supply: +4.75V to +11.0V. 8 14 IN Filter Input _| Detailed Description The MAX293/MAX294/MAX297 8th-order (eight-pole), el- liptic, switched-capacitor, lowpass filters provide the steepest possible rolloff with frequency of the four com- mon filter types (Butterworth, Bessel, Chebyshev, ellip- tic). The high Q value of the poles near the passband edge combined with stopband zeros allows for the sharp attenuation characteristic of elliptic filters. The MAX293/MAX297 have a 1.5 transition ratio and typically -78d0B and -79cB of stopband rejection, respectively; the MAX294 has a 1.2 transition ratio (providing the steepest rolloff) and typically -58dB of stopband rejection. Passband Ripple and Corner Frequency The MAX293/MAX294 operate with a 100:1 clock to corner frequency ratio and a 25kHz maximum corner frequency, with corner frequency defined as the point where the fifter output attenuation falls just below the passband ripple (Figure 1). The passband ripple is typi- cally 0.15dB (MAX293) and 0.27dB (MAX294). The MAX297 operates with a 50:1 clock to corner frequency ratio and a 50kHz maximum corner frequency. Its passband ripple is typically 0.23qB. Transition Ratio and Stopband Response In the frequency domain, the first transmission zero causes the filter's amplitude to drop to a minimum level. 6 Tt RIPPLE bo TRANSITION RATIO = 0 fg A GAIN (dB) STOPBAND > FREQUENCY k- PASSBAND *] + es Fo fs Figure 1. Elliptic Filter Response Figure 2. 8th-Order Ladder Filter Network Beyond this zero, the response rises as the frequency increases until the next transmission zero. Several repeti- tions of this response create the filters stopband comb shape (Figure 1). The stopband begins at fs. At frequen- cies above fs, the filter's gain does not exceed the gain at fs. The transition ratio is defined as the ratio of the stopband frequency to the corner frequency. Background Information Most switched-capacitor filters are designed with bi- quadratic sections. Each section implements two filter- ing poles, 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 sections Q is high. An alternative approach is to emulate a passive network using switched-capacitor integrators with summing and scaling. The passive network can be synthesized using CAD programs, or can be found in many filter books. Figure 2 shows the basic ladder filter structure. Aswitched-capacitor filter that emulates a passive ladder filter retains many of its advantages. The filter's com- ponent sensitivity is low when compared to a cascaded biquad design because each component affects the entire filter shape, not just one pole pair. That is, a mismatched component in a biquad design will have a concentrated error on its respective poles, while the MAMXILAN8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters Applications Information +5V 7 +5V 1 V+ 5 su a CLK OUT OUTPUT ov OP OUT { 10k , MNAAXIAA * suk 4 uF +1V to +4V OP IN- MAN 6 | INPUT SIGNAL Bly RANGE Ve 10k as O.1yF 2 ov PIN CONFIGURATION IS 8-PIN DIP. Figure 3. +5V Single-Supply Operation 22k R2 OP IN- INPUT OUTPUT c2 . 7 OP OUT : MA AXIAA MAX29_ PIN CONFIGURATION {S 8-PIN DIP. Figure 4. Uncommitted Op Amp Configured as a 2nd-Order Butterworth Lowpass Filter (Fp = 10kHz) same mismatch in a ladder filter design will spread its error over all poles. Clock-Signal Requirements The MAX293/MAX294/MAX297 maximum recom- mended clock frequency is 2.5MHz, producing a cutoff frequency of 25kHz for the MAX293/MAX294 and 50kHz for the MAX297. The CLK pin can be driven by an external clock or by the internal oscillator with an external capacitor. For external clock applications, the clock circuitry has been designed to interface with +5V CMOS logic. Drive the CLK pin with a CMOS gate powered from OV and +5V when using either a single supply or dual +5V supplies. Varying the rate of an external clock will dynami- cally adjust the filters corner frequency. When using the internal oscillator, the capacitance (Cosc) on the CLK pin determines the oscillator frequency: 10 3CoOsc(PF) The stray capacitance at CLK should be minimized, since it will affect the internal oscillator frequency. fOSC(kKHz) = Power Supplies The MAX293/MAX294/MAX297 operate from either dual or single power supplies. The dual-supply voltage range is +2.375V to +5.5V (0.1pF bypass capacitors from each supply to GND are recommended). When using a single supply, tie the V- pin to ground and bias the GND pin to the mid-supply point using a resistor-divider network, as shown in Figure 3. input-Signal Amplitude Range The ideal input-signal range is determined by observing at what voltage level the signal-to-noise plus distortion (SINAD) ratio is maximized for a given corner frequency. The Typical Operating Characteristics show the MAX293/MAX294/MAX297 THD + Noise response as the input signal's peak-to-peak amplitude is varied. Uncommitted Op Amp The uncommitted op amp has its noninverting input connected to the GND pin, and can be used to build a 1st- or 2nd-order continuous-time lowpass filter. This filter is intended for anti-aliasing applications preceding the switched-capacitor filter, but it can be used as a post-filter to reduce clock noise. Figure 4 shows one of many filters that can be built with this op amp: a 2nd-order Butterworth filter with a 10kHz corner frequency and an input impedance greater than 22kQ. Table 1 gives alter- native component values for different corner frequencies of the same Butterworth filter. Table 1. Component Values for Figure 4s Filter Corner Freq. R1 R2 R3 C1 c2 (Hz) (kQ) | (ka) | (KO) | (FA (F) 100k 10 10 10 68p | 330p | _50k 20 | 20 20 68p_| 330p | 25k 20 20 20 | 150p | 680p 10k 22 22 22 | 3380p | 1.5n 1k 22 22 22 | 3.an | 16n 100 22 22 22 33n__| 150n_| 10 =| 22 22 | 330n | 1.5n | NOTE: Some approximations have been made in selecting preferred component values. The passband error caused by a 2nd-order Butterworth can be calculated using the formula: Gain error = -10log } + Ge dB fc MAXUM ZL6CXVW/P6ECXVN/E6GCXUNMAX293/MAX294/MAX297 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters As the passband ripple of the MAX293/MAX294/MAX297 elliptic filters is of the order of +0.1dB, it is normally appropriate to keep the passband errors of any anti-aliasing filter at or below this level. This is achieved by choosing the corner frequency of Figure 4s Butterworth filter (fcB) to be higher than the corner frequency of the elliptic switched-capacitor filter (fcE) by a factor of 2.5 or more. A factor of 5 or more is recommended to avoid problems with component tolerances, i.e. fcB > (5)(fcE). When using the uncommitted op amp as a post-filter to reduce clock noise, keep the filters input impedance above 20kQ to avoid excessive loading of the switched- capacitor filter. Note that the op amp experiences some clock feedthrough, so it is generally more useful for anti-aliasing than for clock-noise attenuation. DAC Post-Fiitering When using the MAX293/MAX294/MAX297 for DAC post- filtering, synchronize the DAC and the filter clocks. If Pin Configurations (continued) clocks are not synchronized, beat frequencies will alias into the desired passband. The DACs clock should be generated by dividing down the switched-capacitor filter's clock. 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 2 lists typical harmonic distortion values for the MAX293/MAX294/MAX297 with a 1kHz 5Vp-p sine wave input signal, a 1MHz clock frequency, and a 20kQ load. Table 2. Typical Harmonic Distortion (dB) FILTER HARMONIC | 2nd 3rd 4th sth MAX293 70 90 88 92 MAX294 67 90 92 94 MAX297 84 89 | 93 99 _ Ordering Information (continued) TOP VIEW ne.{a] 6] NC. nec. [2| 15! N.C. cLk [3] AAAXLAA|4] IN v-[4] MAX293 fra} vs op out [5] Me 112] GND oPIN- [6 [v4] our NC.[7 NC. nec. [8 }9} Ne. WIDE SO PART TEMP.RANGE _PIN-PACKAGE MAX294EPA -40C 10 +85C 8 Plastic DIP MAX294EWE -40C to +85C 16 Wide SO MAX294MJA 65C to +125C 8 CERDIP* MAX297CPA O'Cto+70C 8 Plastic DIP MAX297CWE O'Cto+70C 16 Wide SO MAX297C/D OC to +70C Dice* | MAX297EPA -40C to +85C__8 Plastic DIP | MAX297EWE -40C to +85C 16 Wide SO | MAX297MJA 55C 10 +125C 8 CERDIP* * Contact factory for dice specifications. ** Contact factory for availability and processing to MIL-STD-883. 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 2008 Maxim Integrated Products MAXIM is a registered trademark of Maxim Integrated Products, Inc.