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general description the max7490/max7491 consist of two identical low- power, low-voltage, wide dynamic range, rail-to-rail , 2nd-order switched-capacitor building blocks. each of the two filter sections, together with two to four external resistors, can generate all standard 2nd-order func- tions: bandpass, lowpass, highpass, and notch (band reject). three of these functions are simultaneously available. fourth-order filters can be obtained by cas- cading the two 2nd-order filter sections. similarly, high- er order filters can easily be created by cascading multiple max7490/max7491s. two clocking options are available: self-clocking (through the use of an external capacitor) or external clocking for tighter cutoff frequency control. the clock- to-center frequency ratio is 100:1. sampling is done at twice the clock frequency, further separating the cutoff frequency and nyquist frequency. the max7490/max7491 have an internal rail splitter that establishes a precise common voltage needed for single-supply operation. the max7490 operates from a single +5v supply and the max7491 operates from a single +3v supply. both devices feature a low-power shutdown mode and come in a 16-pin qsop package. ________________________applications tunable active filters multipole filters adc anti-aliasing post-dac filtering adaptive filtering phase-locked loops (plls) set-top boxes features ? dual 2nd-order filter in a 16-pin qsop package ? high accuracy q accuracy: ?.2% clock-to-center frequency error: ?.2% ? rail-to-rail input and output operation ? single-supply operation: +5v (max7490) or +3v (max7491) ? internal or external clock ? highpass, lowpass, bandpass, and notch filters ? clock-to-center frequency ratio of 100:1 ? internal sampling-to-center frequency ratio of 200:1 ? center frequency up to 40khz ? easily cascaded for multipole filters ? low-power shutdown: <1? supply current max7490/max7491 dual universal switched-capacitor filters ________________________________________________________________ maxim integrated products 1 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 lpa lpb bpb nb/hpb invb sb com extclk clk top view max7490 max7491 qsop bpa na/hpa shdn inva sa gnd v dd pin configuration 19-1768; rev 0; 7/00 for free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800. for small orders, phone 1-800-835-8769. ordering information rail-to-rail is a registered trademark of nippon motorola, ltd. part temp. range pin- package supply voltage (+v) max7490 cee 0 c to +70 c 16 qsop 5 max7490eee -40 c to +85 c 16 qsop 5 max7491 cee 0 c to +70 c 16 qsop 3 max7491eee -40 c to +85 c 16 qsop 3 typical application circuit appears at end of data sheet.
max7490/max7491 dual universal switched-capacitor filters 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics?ax7490 (v dd = extclk = +5v, f clk = 625khz, t a = t min to t max , 10k ? || 50pf load to v dd /2 at lp_, bp_, and n_/hp_, shdn = v dd , 0.1? from com to gnd, 50% duty-cycle clock input, com = v dd /2. typical values are at t a = +25?, unless otherwise noted.) (note 1) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. v dd to gnd ..............................................................-0.3v to +6v extclk, shdn to gnd ...........................................-0.3v to +6v inv_, lp_, bp_, n_/hp_, s_, com, clk to gnd............................................-0.3v to (v dd + 0.3v) maximum current into any pin ...........................................50ma continuous power dissipation (t a = +70?) 16-pin qsop (derate 8.30mw/? above +70?).........667mw operating temperature range max749_cee ....................................................0? to +70? max749_eee ..................................................-40? to +85? die temperature ..............................................................+150? storage temperature.........................................-65? to +150? lead temperature (soldering, 10s) .................................+300? parameter symbol conditions min typ max units filter center frequency range f o mode 1 0.001 to 40 khz clock-to-center frequency accuracy f clk /f o mode 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5, deviation from 100:1 0.2 0.7 % q accuracy m od e 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5 0.2 2% f o temperature coefficient 1 ppm/ c q temperature coefficient 5 ppm/ c dc lowpass gain accuracy mode 1, r1 = r2 = 10k ? 0.1 0.5 % v os1 dc offset of input inverter 3 12.5 v os2 dc offset of 1st integrator 4 15 dc offset voltage (figure 8) v os3 dc offset of 2nd integrator 4 30 mv crosstalk (note 2) f in = 10khz -60 db input: com externally driven v dd /2 - 0.5 v dd /2 v dd /2 + 0.5 com voltage range v com output: com internally driven v dd /2 - 0.2 v dd /2 v dd /2 + 0.2 v input resistance at com r com 140 250 325 k ? clock feedthrough up to 5th harmonic of f clk 200 v rms noise (note 3) mode 1, r1 = r2 = r3 =10k ? , lp output, q = 1 60 v rms output voltage swing 0.2 v d d - 0.2 v input leakage current at com shdn = gnd, v com = 0 to v dd 0.1 10 a clock maximum clock frequency f clk 4 mhz extclk = gnd, c osc = 1000pf 95 135 175 khz internal oscillator frequency (note 4) f osc extclk = gnd, c osc = 100pf 1.35 mhz clock input high v dd - 0.5 v
max7490/max7491 dual universal switched-capacitor filters _______________________________________________________________________________________ 3 parameter symbol conditions min typ max units clock input low 0.5 v clock duty cycle 50 5 % shdn and extclk input high v ih v dd - 0.5 v input low v il 0.5 v input leakage current v input = 0 to v dd 0.4 10 a power requirements supply voltage v dd 4.5 5.5 v power-supply current i dd no external load, mode 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5 3.5 4.0 ma shutdown current i shdn shdn = gnd 1 a internal op amps characteristics output short-circuit current 18 ma dc open-loop gain r l 10k ? , c l 50pf 130 db gain bandwidth product gbw r l 10k ? , c l 50pf 7 mhz slew rate sr r l 10k ? , c l 50pf 6.4 v/ s electrical characteristics?ax7490 (continued) (v dd = extclk = +5v, f clk = 625khz, t a = t min to t max , 10k ? || 50pf load to v dd /2 at lp_, bp_, and n_/hp_, shdn = v dd , 0.1? from com to gnd, 50% duty-cycle clock input, com = v dd /2. typical values are at t a = +25?, unless otherwise noted.) (note 1)
max7490/max7491 dual universal switched-capacitor filters 4 _______________________________________________________________________________________ parameter symbol conditions min typ max units filter center frequency range f o mode 1 0.001 to 40 khz clock-to-center frequency accuracy f clk /f o mode 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5, deviation from 100:1 0.2 0.7 % q accuracy mode 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5 0.2 2% f o temperature coefficient 1 ppm/ c q temperature coefficient 5 ppm/ c dc lowpass gain accuracy mode 1, r1 = r2 = 10k ? 0.1 0.5 % v os1 dc offset of input inverter 3 12.5 v os2 dc offset of 1st integrator 4 15 dc offset voltage (figure 8) v os3 dc offset of 2nd integrator 4 25 mv crosstalk (note 2) f in = 10khz -60 db input: com externally driven v dd /2 - 0.1 v dd /2 v dd /2 + 0.1 com voltage range v com output: com internally driven v dd /2 - 0.1 v dd /2 v dd /2 + 0.1 v input resistance at com r com 60 80 120 k ? clock feedthrough up to 5th harmonic of f clk 200 v rms noise (note 3) mode 1, r1= r2 = r3 = 10k ? , lp output, q = 1 60 v rms output voltage swing 0.2 v d d - 0.2 v input leakage current at com shdn = gnd, v com = 0 to v dd 0.1 10 a clock maximum clock frequency f clk 4 mhz extclk = gnd, c osc = 1000pf 95 135 175 khz internal oscillator frequency (note 4) f osc extclk = gnd, c osc = 100pf 1.35 mhz clock input high v dd - 0.5 v clock input low 0.5 v clock duty cycle 50 5 % shdn and extclk input high v ih v dd - 0.5 v input low v il 0.5 v input leakage current v input = 0 to v dd 0.4 10 a electrical characteristics?ax7491 (v dd = extclk = +3v, f clk = 625khz, t a = t min to t max , 10k ? || 50pf load to v dd /2 at lp_, bp_, and n_/hp_, shdn = v dd , 0.1? from com to gnd, 50% duty-cycle clock input, com = v dd /2. typical values are at t a = +25?, unless otherwise noted.) (note 1)
max7490/max7491 dual universal switched-capacitor filters _______________________________________________________________________________________ 5 parameter symbol conditions min typ max units power requirements supply voltage v dd 2.7 3.6 v power-supply current i dd no load, mode 1, r1 = r3 = 50k ? , r2 = 10k ? , q = 5 3.5 4.0 ma shutdown current i shdn shdn = gnd 1 a internal op amps characteristics output short-circuit current 11 ma dc open-loop gain r l 10k ? , c l 50pf 130 db gain bandwidth product gbw r l 10k ? , c l 50pf 7 mhz slew rate sr r l 10k ? , c l 50pf 6 v/ s note 1: resistive loading of the n_/hp_, lp_, bp_ outputs includes the resistors used for the filter implementation. note 2: crosstalk between internal filter sections is measured by applying a 1v rms 10khz signal to one bandpass filter section input and grounding the input of the other bandpass filter section. the crosstalk is the ratio between the output of the grounded filter section and the 1v rms input signal of the other section. note 3: bandwidth of noise measurement is 80khz. note 4: f osc (khz) = 135 x 10 3 / c osc (c osc in pf) electrical characteristics?ax7491 (continued) (v dd = extclk = +3v, f clk = 625khz, t a = t min to t max , 10k ? || 50pf load to v dd /2 at lp_, bp_, and n_/hp_, shdn = v dd , 0.1? from com to gnd, 50% duty-cycle clock input, com = v dd /2. typical values are at t a = +25?, unless otherwise noted.) (note 1) typical operating characteristics (v dd = +5v for max7490, v dd = +3v for max7491, f clk = 625khz, shdn = extclk = v dd , com = v dd /2, mode 1, r3 = r1 = 50k ? , r2 = 10k ? , q = 5, t a = +25?, unless otherwise noted.) 10 -60 1 10 100 2nd-order bandpass filter frequency response -50 max7490-01 frequency (khz) gain (db) -30 -40 -10 -20 0 300 0 110100 2nd-order bandpass filter phase response 50 max7490-02 frequency (khz) phase (%) 100 200 150 250 v dd = +5v f clk = 625khz q = 5 0 -0.8 100 1000 10,000 f clk /f o deviation vs. f clk -0.7 max7490-03 f clk (khz) f clk /f o deviation (%) -0.6 -0.2 -0.3 -0.4 -0.5 -0.1 v dd = 5v v dd = 3v
max7490/max7491 dual universal switched-capacitor filters 6 _______________________________________________________________________________________ -0.6 -0.4 -0.5 -0.3 -0.2 -0.1 0 0.1 0.2 040 20 60 80 100 f clk /f o deviation vs. q max7490-04 q f clk /f o deviation (%) v dd = 5v v dd = 3v -0.7 -0.3 -0.4 -0.6 -0.2 -0.5 -0.1 0 0.1 0.3 0.2 0.4 0.5 0.6 0.7 -40 10 -15 356085 f clk /f o deviation vs. temperature max7490-05 temperature (?) f clk /f o deviation (%) 1 -6 100 1000 10,000 q deviation vs. f clk -5 max7490-06 f clk (khz) q deviation (%) -4 -1 -2 -3 0 v dd = 3v v dd = 5v -2.0 -1.0 -1.5 -0.5 0 0.5 1.0 1.5 2.0 -40 10 -15 356085 q deviation vs. temperature max7490-07 temperature (?) q deviation (%) 0 150 100 50 200 250 300 350 400 450 500 040 20 60 80 100 noise vs. q max7490-08 q noise (? rms ) 3.0 3.2 3.1 3.4 3.3 3.6 3.5 3.7 -40 10 -15 356085 supply current vs. temperature max7490-09 temperature (?) i dd (ma) v dd = 5v v dd = 3v 3.0 3.3 3.2 3.1 3.4 3.5 3.6 3.7 3.8 3.9 4.0 3.0 4.0 3.5 4.5 5.0 5.5 supply current vs. supply voltage max7490-10 v dd (v) i dd (ma) f clk = 3mhz f clk = 625khz f clk = 2khz 3.32 3.34 3.33 3.37 3.36 3.35 3.40 3.39 3.38 3.41 3.0 4.0 3.5 4.5 5.0 5.5 supply current vs. supply voltage max7490-11 v dd (v) i dd (ma) -40? +25? +85? -20 -120 1k 10k max7491 thd + noise vs. frequency max7490-12 input frequency (hz) thd + noise (db) -30 -40 -50 -60 -70 -80 -90 -100 -110 a b a = mode 1 b = mode 3 typical operating characteristics (continued) (v dd = +5v for max7490, v dd = +3v for max7491, f clk = 625khz, shdn = extclk = v dd , com = v dd /2, mode 1, r3 = r1 = 50k ? , r2 = 10k ? , q = 5, t a = +25?, unless otherwise noted.)
max7490/max7491 dual universal switched-capacitor filters _______________________________________________________________________________________ 7 -20 -120 1k 10k max7490 thd + noise vs. frequency max7490-13 input frequency (hz) thd + noise (db) -30 -40 -50 -60 -70 -80 -90 -100 -110 b a a = mode 1 b = mode 3 -90 -80 -70 -60 -50 -40 -30 -20 -10 01.0 0.5 1.5 2.0 2.5 3.0 max7491 thd + noise vs. input voltage max7490-14 input voltage (vp-p) thd + noise (db) b a a = mode 1 b = mode 3 -90 -80 -70 -60 -50 -40 -30 -20 -10 012345 max7490 thd + noise vs. input voltage max7490-15 input voltage (vp-p) thd + noise (db) b a a = mode 1 b = mode 3 2.0 3.0 2.5 3.5 4.0 4.5 5.0 08 4 121620 output voltage swing vs. r load max7490-16 r load (k ? ) to com output swing (vp-p) v dd = 5v v dd = 3v 0 500 1500 1000 2000 2500 0 400 200 600 800 1000 internal oscillator period vs. small capacitance max7490-17 capacitance (pf) internal oscillator frequency (khz) v dd = 3v v dd = 5v 0 20 40 60 80 100 120 140 160 13 2 4567 internal oscillator period vs. large capacitance max7490-18 capacitance (nf) internal oscillator frequency (khz) v dd = 5v v dd = 3v 126 128 127 130 129 132 131 133 3.0 4.0 3.5 4.5 5.0 5.5 internal oscillator frequency vs. supply voltage max7490-19 v dd (v) internal oscillator frequency (khz) c osc = 1000pf 124 130 128 126 132 134 136 138 140 142 144 -40 10 -15 35 60 85 internal oscillator frequency vs. temperature max7490-20 temperature ( c) internal oscillator frequency (khz) v dd = 3v v dd = 5v c osc = 1000pf typical operating characteristics (continued) (v dd = +5v for max7490, v dd = +3v for max7491, f clk = 625khz, shdn = extclk = v dd , com = v dd /2, mode 1, r3 = r1 = 50k ? , r2 = 10k ? , q = 5, t a = +25?, unless otherwise noted.)
max7490/max7491 dual universal switched-capacitor filters 8 _______________________________________________________________________________________ _______________detailed description the max7490/max7491 are universal switched-capaci- tor filters designed with a fixed internal f clk /f o ratio of 100:1. operating modes use external resistors connect- ed in different arrangements to realize different filter functions (highpass, lowpass, bandpass, notch) in all of the classical filter topologies (butterworth, bessel, ellip- tic, chebyshev). figure 1 shows a block diagram. clock signal external clock the max7490/max7491 switched-capacitor filters are designed for use with external clocks that have a 50% ?% duty cycle. when using an external clock, drive the extclk pin high or connect to v dd . drive clk with cmos logic levels (gnd and v dd ). varying the rate of the external clock adjusts the center frequency of the filter: f o = f clk /100 internal clock when using the internal oscillator, drive the extclk pin low or connect to gnd and connect a capacitor (c osc ) between clk and gnd. the value of the capacitor (c osc ) determines the oscillator frequency as follows: f osc (khz) = 135 x 10 3 / c osc (pf) since c osc is in the low picofarads, minimize the stray capacitance at clk so that it does not affect the inter- nal oscillator frequency. varying the frequency of the internal oscillator adjusts the filter? center frequency by a 100:1 clock-to-center frequency ratio. for example, an internal oscillator frequency of 135khz produces a nominal center frequency of 1.35khz. name pin filter a filter b function lp_ 1 16 2nd-order lowpass filter output bp_ 2 15 2nd-order bandpass filter output n_/hp_ 3 14 2nd-order notch/highpass filter output inv_ 4 13 inverting input of filter summing op amp s_ 5 12 summing input. the connection of the summing input, along with the other resistor connections, determine the circuit topology (mode) of each 2nd- order section. s_ must never be left floating. shdn 6 shutdown input. drive shdn low to enable shutdown mode; drive high or connect to v dd for normal operation. gnd 7 ground pin v dd 8 positive supply. v dd should be bypassed with a 0.1 f capacitor to gnd. a low-noise supply is recommended. input +5v for max7490 or +3v for max7491. clk 9 clock input. connect to an external capacitor (c osc ) between clk and ground to set the internal oscillator frequency. for external clock operation, drive with a cmos-level clock. the duty cycle of the external clock should be between 45% and 55% for best performance. extclk 10 external/internal clock select input. connect extclk to v dd when driving clk externally. connect to gnd when using the internal oscillator. com 11 common pin. biased internally at v dd /2. bypass externally to gnd with 0.1 f capacitor. to override the internal biasing, drive with an external low- impedance source. pin description
max7490/max7491 dual universal switched-capacitor filters _______________________________________________________________________________________ 9 2nd-order filter stage the max7490/max7491 are dual biquad filters. the biquad topology allows the use of standard filter tables and equations to implement simultaneous lowpass, bandpass, and notch or highpass filters. topologies such as butterworth, chebyshev, bessel, elliptic, as well as custom algorithms are possible. internal common voltage the com pin sets the common-mode input voltage and is internally biased to v dd /2 with a resistor-divider. the resistors used are typically 250k ? for the max7490, and typically 80k ? for the max7491. the common- mode voltage is easily overdriven by an external volt- age supply if desired. the com pin should be bypassed to the analog ground with at least a 0.1? capacitor. inverting inputs locate resistors that are connected to inv_ as close as possible to inv_ to reduce stray capacitance and noise pickup. inv_ are inverting inputs to continuous-time op amps, and behave like a virtual ground. there is no sampling energy present on these inputs. outputs each switched-capacitor section, together with two to four external resistors, can generate all standard 2nd- order functions: bandpass, lowpass, highpass, and notch (band-reject) functions. three of these functions are simultaneously available. the maximum signal swing is limited by the power-supply voltages used. the amplifiers?outputs in the max7490/max7491 are able to swing to within approximately 0.2v of either supply. driving coaxial cable, large capacitive loads, or total resistive loads less than 10k ? will degrade the total harmonic distortion (thd) performance. note that the effective resistive load at the output must include both the feedback resistors and any external load resistors. low-power shutdown mode the max7490/max7491 have a shutdown mode that is activated by driving shdn low. in shutdown mode, the filter supply current reduces to <1? (max), and the fil- ter outputs become high impedance. the com input also becomes high impedance during shutdown. for normal operation, drive shdn high or connect to v dd . __________applications information designing with the max7490/max7491 begins by selecting the mode that best fits the desired circuit requirements. table 1 lists the available modes and their relative advantages and disadvantages. table 2 lists the different nomenclature used in the explanations that follow. mode 1 figure 2 shows the max7490/max7491s?configuration of mode 1. this mode provides 2nd-order notch, low- pass, and bandpass filter functions. the gain at all three outputs is inversely proportional to the value of r1. the center frequency, f o , is fixed at f clk /100. high- q bandpass filters can be built without exceeding the bandpass amplifier? output swing (i.e., h obp does not na/hpa (3) shdn v dd (8) gnd (7) clk (9) extclk (10) invb (13) com (11) r r inva (4) (6) sa (5) bpa (2) lpa (1) + - nb/hpb (14) sb (12) bpb (15) lpb (16) + - figure 1. block diagram
max7490/max7491 dual universal switched-capacitor filters 10 ______________________________________________________________________________________ table 1. filter operating modes mode lp hp bp n l p- n * h p- n * comments 1 ??? f clk /f o ratio is the nominal value. good for bandpass filters with identical sections cascaded, higher order butterworth filters, high-q bandpass, low-q notches. 1b ??? same as mode 1 with f clk /f o ratios greater than the nominal value. 2 ??? combination of mode 1 and mode 3; f clk /f o ratios always less than the nominal value. less sensitivity to resistor tolerances than mode 3. 2n ? extension of mode 2 that allows higher frequencies. highpass and lowpass outputs are summed with external op amp and two resistors. good for lowpass elliptic filters. 3 ??? adjustable f o above and below the nominal frequency. commonly used for multiple-pole chebyshev filters, all-pole higher order bandpass, lowpass, and highpass filters. 3a ??? ?? extension of mode 3 that needs an external op amp and two additional resistors. commonly used for lowpass or higher elliptic or cauer filters. * lp-n = lowpass notch, hp-n = highpass notch. both require an external op amp. see definition of terms (table 2). table 2. definition of terms term definition f clk the clock frequency applied to the switched-capacitor filter. f o the center frequency of the 2nd-order complex pole pair, f o , is determined by measuring the peak response frequency at the bandpass output. f notch the frequency of minimum amplitude response at the notch output. q quality factor, or q, is the ratio of f o to the -3db bandwidth of the 2nd-order bandpass filter. q also determines the amount of amplitude peaking at the lowpass and highpass outputs, but is not measured at these outputs. h obp the gain in v/v of the bandpass output at f = f o . h olp the gain in v/v of the lowpass output at f 0hz. h ohp the gain in v/v of the highpass output at f f clk /2. h on1 the notch output gain as f 0hz. h on2 the notch output gain at f = f clk /2. lp-n a notch output with h on1 > h on2. hp-n a notch output with h on1 < h on2.
max7490/max7491 dual universal switched-capacitor filters ______________________________________________________________________________________ 11 have to track q). the notch and bandpass center fre- quencies are identical. the notch output gain is the same above and below the notch center frequency. mode 1 can also be used to make high-order butter- worth lowpass filters, low q notches, and multiple-order bandpass filters obtained by cascading identical switched-capacitor sections. mode 1 design equations mode 1b figure 3 shows the configuration of mode 1b. r5 and r6 are added to lower the feedback voltage from the lowpass output to the summing input. this allows the clock-to-center frequency to be adjusted beyond the nominal value. this mode essentially has the same functions and speed as mode 1 while providing a high- q with f clk /f o ratios greater than the nominal value. mode 1b design equations mode 2 figure 4 shows the configuration of mode 2. mode 2 is a combination of mode 1 and mode 3. in this mode, f clk /f o is always less than the part? nominal ratio. however, it provides less sensitivity to resistor toler- ances than does mode 3. it has a highpass notch out- put where the notch frequency depends solely on the clock frequency. f f r rr ff q r r r rr h r r rr r h r r hasfhz r r hatff r r o clk no olp obp on on clk = + = = + = + = = == ? ? ? ? 100 6 65 3 2 6 65 2 1 65 6 3 1 0 2 1 2 2 1 1 2 () (/) f f ff q r r h r r h r r hasfhz r r hatff r r o clk notch o olp obp on on clk = = = = = = == ? ? ? ? 100 3 2 2 1 3 1 0 2 1 2 2 1 1 2 () (/) lp bp s n r3 r2 r1 v in c c com + - figure 2. mode 1, 2nd-order filter providing notch, bandpass, and lowpass outputs lp bp s n r3 r6 r5 r2 r1 v in c c com com + - figure 3. mode 1b, 2nd-order filter providing notch, bandpass, and lowpass outputs
max7490/max7491 dual universal switched-capacitor filters 12 ______________________________________________________________________________________ mode 2 design equations mode 2n figure 5 shows the configuration of mode 2n. this mode extends the topology of mode 3a to mode 2, where the highpass and lowpass outputs are summed through two external resistors, r h and r l , to create a lowpass notch filter that has higher frequency than the one in mode 2. mode 2 is most useful in lowpass elliptic designs. when cascading the sections of the max7490/max7491, the highpass and lowpass outputs can be summed directly into the inverting input of the next section. only one external op amp is needed. mode 2n design equations f f r r f fr r q r r r r hfhz r r r r r r r rr o clk n clk h l on g h g l =+ =+ =+ = + ? ? ? ? ? ? ? ? ? ? ? ? + ? ? ? ? ? ? 100 1 2 4 100 1 3 2 1 2 4 0 2 1 4 42 1 () f f r r f f q r r r r h r r r rr h r r hfhz r r r rr hatff r r o clk n clk olp obp on on clk =+ = =+ = + ? ? ? ? ? ? = = + ? ? ? ? ? ? == ? ? ? ? 100 1 2 4 100 3 2 1 2 4 2 1 4 42 3 1 0 2 1 4 42 2 2 1 1 2 () (/) lp bp s hp/n r3 r4 r2 r1 v in c c com com lowpass notch output + - r h r l r g figure 5. mode 2n, 2nd-order filter providing a lowpass notch output lp bp s hp/n r3 r4 r2 r1 v in c c com + - figure 4. mode 2, 2nd-order filter providing a highpass notch, bandpass, and lowpass outputs
max7490/max7491 dual universal switched-capacitor filters ______________________________________________________________________________________ 13 mode 3 figure 6 shows the configuration of mode 3. this mode is a sampled time (z transform) equivalent of the classi- cal 2nd-order state variable filter. in this versatile mode, the ratio of resistors r2 and r4 can move the center frequency both above and below the nominal ratio. mode 3 is commonly used to make multiple-pole chebyshev filters with a single clock frequency. this mode can also be used to make high-order all-pole bandpass, lowpass, and highpass filters. mode 3 design equations mode 3a figure 7 shows the configuration of mode 3a. similar to mode 2, this mode adds an external op amp. see table 3 for op amp selection ideas. this op amp cre- ates a highpass notch and lowpass notch by summing the highpass and lowpass outputs through two external resistors, r h and r l . the ratio of resistors r h and r l adjusts the notch frequency, while r2 and r4 adjust the bandpass center frequency, since the notch (zero pair) frequency can be adjusted to both above and below f o . mode 3a is suitable for both lowpass and highpass elliptic or cauer filters. in multipole elliptic fil- ters, only one external op amp is needed. use the inverting input of the internal op amp as the summing node for all but the final section of the filter. f o f clk r r q r r r r h ohp r r h olp r r h obp r r = = = = = ? ? ? 100 2 4 3 2 2 4 2 1 4 1 3 1 lp bp hp r3 r4 r2 r1 v in c c com + - s com figure 6. mode 3, 2nd-order section providing highpass, bandpass, and lowpass outputs lp bp n/hp r3 r4 r2 r1 v in c c com com lowpass notch output + - r h r l r g s com figure 7. mode 3a, 2nd-order filter providing highpass notch or lowpass notch outputs
max7490/max7491 dual universal switched-capacitor filters 14 ______________________________________________________________________________________ mode 3a design equations note : when the passband gain error exceeds 1db, the use of capacitor c c between the lowpass output and the inverting input will reduce the gain error. the value can best be determined experimentally. typically, it should be about 5pf/db (c c-max = 15pf). offset voltage switched-capacitor integrators generally exhibit higher input offsets than discrete rc integrators. the larger offset is mainly due to the charge injection of the cmos switches into the integrating capacitors. the internal op amp offset also adds to the overall offset value. figure 8 shows the input offsets from a single 2nd-order section. table 4 lists the formula for the out- put offset voltage for various modes and output pins. power supplies the max7490 operates from a single +5v supply, and the max7491 operates from a single +3v supply. bypass v dd to gnd with at least a 0.1? capacitor. v dd should be isolated from other digital or high-volt- age analog supplies. if dual supplies are required, connect the com pin to the system ground and the gnd pin to the negative supply. figure 9 shows an example of dual-supply operation. single-supply and dual-supply performances are equivalent. for dual- supply operation, drive clk, shdn , and extclk from gnd (which is now v-) to v dd . if using the internal oscillator in dual-supply mode, c osc can be returned to either gnd or the actual ground voltage. use the max7490 for ?.5v and use the max7491 for ?.5v. for most applications, a 0.1? bypass capacitor from com to gnd is sufficient. if the v dd supply has signifi- cant 60hz energy, increase this capacitor to 1f or greater to provide better power-supply rejection. f f r r f fr r q r r r r h r r h r r h r r hfhz r r r r hatff r r r r o clk n clk h l ohp olp obp on g l on clk g h = = = = = = = ? ? ? ? ? ? == ? ? ? ? ? ? ? ? ? 100 2 4 100 3 2 2 4 2 1 4 1 3 1 0 4 1 2 2 1 1 2 () (/) figure 8. block diagram of a 2nd-order section showing the input offsets n/hp s inv bp lp com v os1 v os2 v os3 + - part gbw (mhz) slew rate (v/ s) i supply/amp (ma) pin-package max4281 2 0.7 0.5 5 sot23 max4322 5 2.0 1.1 5 sot23 max4130 10 4.0 1.15 5 sot23 max4490 10 10.0 2.0 5 sot23 table 3. suggested external op amps
max7490/max7491 dual universal switched-capacitor filters ______________________________________________________________________________________ 15 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 character- istics show the thd + noise response as the input sig- nal? peak-to-peak amplitude is varied. in most systems, the input signal should be kept as large as possible to maximize the signal-to-noise ratio (snr). allow sufficient headroom to ensure no signal clipping under expected operating conditions. anti-aliasing and post-dac filtering when using the max7490/max7491 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. aliasing aliasing is an inherent phenomenon of most switched- capacitor filters. as with all sampled systems, frequen- cy components of the input signal above one half the sampling rate will be aliased. the max7490/max7491 sample at twice the clock frequency, yielding a 200:1 sampling to cutoff frequency ratio. in particular, input signal components (f in ) near the sampling rate generate a difference frequency (f sampling - f in ) that often falls within the passband of the filter. such aliased signals, when they appear at the output, are indistinguishable from real input informa- tion. for example, the aliased output signal generated when a 99khz waveform is applied to a filter sampling at 100khz, (f clk = 50khz) is 1khz. this waveform is an attenuated version of the output that would result from a true 1khz input. since sampling is done at twice the clock frequency, the nyquist frequency is the same as the clock frequency. a simple passive rc lowpass input filter is usually suffi- cient to remove input frequencies that can be aliased. in many cases, the input signal itself may be band limit- ed and require no special anti-alias filtering. selecting a passive filter cutoff frequency equal to f c /2 gives 12db rejection at the nyquist frequency. clock feedthrough clock feedthrough is defined as the rms value of the clock frequency and its harmonics that are present at the filter? output pins, even without input signal. the clock feedthrough can be greatly reduced by adding a simple rc lowpass network at the final filter output. choose a cutoff frequency as low as possible to pro- vide maximum noise attenuation. the attenuation and phase shift of the external filter will limit the actual fre- quency selected. v dd v+ v- clk gnd 0.1 f clock *drive shdn to v- for low-power shutdown mode. shdn com 0.1 f max7490 max7491 * v+ v- figure 9. dual-supply operation mode v osn/hp v osbp v oslp 1 v os1 [1 + (r2 / r3) + (r2 / r1)] - (v os3 ) (r2 / r3) v os3 v osn/hp - v os2 1b v os1 [1 + (r2 / r3) + (r2 / r1)] - (v os3 ) (r2 / r3) v os3 (v osn/hp - v os2 )[1 + r5 / r6)] 2 v os1 [1 + (r2 / r3) + (r2 / r1) + (r2 / r4) - (v os3 )(r2 / r3)][r4 / r2 + r4] + (v os2 )[r2 / r2 + r4] v os3 v osn/hp - v os2 3v os2 v os3 v os1 [1 + (r4 / r1) + (r4 / r2) + (r4 / r3)] - (v os2 ) (r4 / r2) - (v os3 )(r4 / r3) table 4. output dc offsets for a 2nd-order section
wideband noise the wideband noise of the filter is the total rms value of the device? noise spectral density and is used to determine the operating snr. most of its frequency contents lie within the filter? passband and cannot be reduced with postfiltering. the total noise depends mainly on the q of each filter section and the cascade sequence. therefore, in multistage filters, the section with the highest q should be placed first for lower out- put noise. multiple filter stages in some designs, such as very narrow band filters, or in modes where f o cannot be tuned with resistors, several 2nd-order sections with identical f o may be cascaded without multiple feedback. the total q of the resultant filter (q t ) is: total q t = q / (2 n - 1) 1/2 q is the q of each individual filter section, and n is the number of 2nd-order sections. in table 5, the total q and total bandwidth (bw) are listed for up to five identi- cal 2nd-order sections. b is the bandwidth of each sec- tion. chip information transistor count: 1439 technology: bicmos max7490/max7491 dual universal switched-capacitor filters 16 ______________________________________________________________________________________ total sections total bw total q 1 1.000 b 1.00 q 2 0.644 b 1.55 q 3 0.510 b 1.96 q 4 0.435 b 2.30 q 5 0.386 b 2.60 q table 5. cascading identical bandpass filter sections
max7490/max7491 dual universal switched-capacitor filters ______________________________________________________________________________________ 17 nb/hpb invb inva v in v dd f clk = 1mhz na/hpa out r1b 200k r3b 200k r2b 10k r3a 200k r2a 10k r1 200k lpa bpa lpb bpb c1 0.1 f c2 0.1 f shdn v dd gnd sa sb com extclk clk max7490 max7491 4th-order 10khz bandpass filter -40 -30 -25 -20 -15 -10 -5 0 5 8 9 10 11 12 4th-order 10khz bandpass filter frequency response frequency (khz) gain (db) -35 typical application circuit
max7490/max7491 dual universal switched-capacitor filters maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 18 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 18 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 18 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 18 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 18 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. package information qsop.eps

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