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1 ltc1562-2 15622fa very low noise, low distortion active rc quad universal filter the ltc ? 1562-2 is a low noise, low distortion continuous time filter with rail-to-rail inputs and outputs, optimized for a center frequency (f o ) of 20khz to 300khz. unlike most monolithic filters, no clock is needed. four independent 2nd order filter blocks can be cascaded in any combination, such as one 8th order or two 4th order filters. each blocks response is programmed with three external resistors for center frequency, q and gain, using simple design formulas. each 2nd order block provides lowpass and bandpass out- puts. highpass response is available if an external capacitor replaces one of the resistors. allpass, notch and elliptic responses can also be realized. the ltc1562-2 is designed for applications where dynamic range is important. for example, by cascading 2nd order sections in pairs, the user can configure the ic as a dual 4th order butterworth lowpass filter with 90db signal-to-noise ratio from a single 5v power supply. low level signals can exploit the built-in gain capability of the ltc1562-2. varying the gain of a section can achieve a dynamic range as high as 114db with a 5v supply. other cutoff frequency ranges can be provided upon request. please contact ltc marketing. , ltc and lt are registered trademarks of linear technology corporation. n high resolution systems (14 bits to 18 bits) n antialiasing/reconstruction filters n data communications, equalizers n dual or i-and-q channels (two matched 4th order filters in one package) n linear phase filtering n replacing lc filter modules n continuous timeno clock n four 2nd order filter sections, 20khz to 300khz center frequency n butterworth, chebyshev, elliptic or equiripple delay response n lowpass, bandpass, highpass responses n 99db typical s/n, 5v supply (q = 1) n 93db typical s/n, single 5v supply (q = 1) n rail-to-rail input and output voltages n dc accurate to 3mv (typ) n 0.5% typical center frequency accuracy n zero-power shutdown mode n single or dual supply, 5v to 10v total n resistor-programmable f o , q, gain dual 4th order 200khz butterworth lowpass filter, snr 96db r in2 7.87k r q2 10.2k r22 7.87k r24 7.87k r q4 10.2k r in4 7.87k r q3 4.22k r23 7.87k r in3 7.87k r q1 4.22k r21 7.87k r in1 7.87k v in2 v in1 5v 0.1 f 0.1 f 5v* v out1 v out2 1562-2 ta01 *v also at pins 4, 7, 14 & 17 all resistors 1% metal film 20 19 18 16 15 13 12 11 1 2 3 5 6 8 9 10 inv c v1 c v2 c v agnd v2 d v1 d inv d inv b v1 b v2 b v + shdn v2 a v1 a inv a ltc1562-2 gain (db) 10 0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 50k 1.5m 1m 1562-2 ta02 frequency (hz) 100k amplitude response descriptio u features applicatio s u typical applicatio u
2 ltc1562-2 15622fa order part number ltc1562cg-2 ltc1562ig-2 (note 1) total supply voltage (v + to v C ) .............................. 11v maximum input voltage at any pin .................... (v C C 0.3v) v (v + + 0.3v) storage temperature range ................. C 65 c to 150 c operating temperature range ltc1562c-2 ............................................ 0 c to 70 c ltc1562i-2 ........................................ C 40 c to 85 c lead temperature (soldering, 10 sec).................. 300 c t jmax = 150 c, q ja = 136 c/w top view g package 20-lead plastic ssop *g package pins 4, 7, 14, 17 are substrate/shield connections and must be tied to v 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 inv b v1 b v2 b v ? v + shdn v ? v2 a v1 a inv a inv c v1 c v2 c v ? v agnd v ? v2 d v1 d inv d electrical characteristics symbol parameter conditions min typ max units v s total supply voltage 4.75 10.5 v i s supply current v s = 2.375v, r l = 5k, c l = 30pf, outputs at 0v 21 23.5 ma v s = 5v, r l = 5k, c l = 30pf, outputs at 0v 22.5 25 ma v s = 2.375v, r l = 5k, c l = 30pf, outputs at 0v l 28 ma v s = 5v, r l = 5k, c l = 30pf, outputs at 0v l 30 ma output voltage swing, v2 outputs v s = 2.375v, r l = 5k, c l = 30pf l 4.2 4.6 v p-p v s = 5v, r l = 5k, c l = 30pf l 9.3 9.8 v p-p output voltage swing, v1 outputs v s = 2.375v, r l = 5k, c l = 30pf, f = 250khz 4.5 v p-p v s = 5v, r l = 5k, c l = 30pf, f = 250khz 8.4 9.7 v p-p v os dc offset magnitude, v2 outputs v s = 2.375v, input at agnd voltage 3 17 mv v s = 5v, input at agnd voltage 3 17 mv dc agnd reference point v s = single 5v supply 2.5 v center frequency (f o ) error (notes 2, 3) v s = 5v, v2 output has r l = 5k, c l = 30pf 0.5 1.7 % h l lowpass passband gain at v2 output v s = 2.375v, f in = 10khz, l 0 + 0.05 + 0.1 db v2 output has r l = 5k, c l = 30pf q accuracy v s = 2.375v, v2 output has r l = 5k, c l = 30pf + 2 % wideband output noise v s = 2.375v, bw = 400khz, input ac gnd 39 m v rms v s = 5v, bw = 400khz, input ac gnd 39 m v rms input-referred noise, gain = 100 bw = 400khz, f o = 200khz, q = 1, input ac gnd 7.3 m v rms the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v s = 5v, outputs unloaded, shdn pin to logic low, unless otherwise noted. ac specs are for a single 2nd order section, r in = r2 = 10.4k 0.1%, r q = 9.09k 0.1%, f o = 175khz. absolute axi u rati gs w ww u package/order i for atio uu w consult ltc marketing for parts specified with wider operating temperature ranges.
3 ltc1562-2 15622fa symbol parameter conditions min typ max units thd total harmonic distortion, v2 output f in = 20khz, 2.8v p-p , v1 and v2 outputs have C 100 db r l = 5k, c l = 30pf f in = 20khz, 9v p-p , v1 and v2 outputs have C 82 db r l = 5k, c l = 30pf shutdown supply current shdn pin to v + 1.5 15 m a shdn pin to v + , v s = 2.375v 1.0 m a shutdown-input logic threshold 2.5 v shutdown-input bias current shdn pin to 0v C 10 C 20 m a shutdown delay shdn pin steps from 0v to v + 20 m s shutdown recovery delay shdn pin steps from v + to 0v 100 m s inverting input bias current, each biquad 5 pa electrical characteristics note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: f o change from 5v to 2.375 supplies is C 0.2% typical, f o temperature coefficient magnitude, 25 c to 85 c, is 50ppm/ c typical. as with the ltc1562, f o decreases with increasing temperature. note 3: tighter frequency tolerance is available, consult factory. the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v s = 5v, outputs unloaded, shdn pin to logic low, unless otherwise noted. ac specs are for a single 2nd order section, r in = r2 = 10.4k 0.1%, r q = 9.09k 0.1%, f o = 175khz. typical perfor a ce characteristics uw f o error vs nominal f o (v s = 5v) f o error vs nominal f o (v s = 2.5v) nominal f o (khz) 100 q error (%) 35 40 45 30 25 20 15 10 5 0 ? 260 1562-2 g03 140 180 220 300 240 120 160 200 280 t a = 70 c t a = 25 c r in = r q q = 5 q = 2.5 q = 1 q error vs nominal f o (v s = 5v) nominal f o (khz) 120 f o error (%) 0 1.0 2.0 1.5 3.0 2.5 1562-2 g01 ?.0 2.0 0.5 0.5 ?.5 2.5 3.0 160 200 240 280 260 140 180 220 q = 5 q = 2.5 q = 1 t a = 25 c r in = r q nominal f o (khz) 120 f o error (%) 0 1.0 2.0 1.5 3.0 2.5 1562-2 g02 ?.0 2.0 0.5 0.5 ?.5 2.5 3.0 160 200 240 280 260 140 180 220 q = 5 q = 2.5 q = 1 t a = 25 c r in = r q
4 ltc1562-2 15622fa typical perfor a ce characteristics uw peak bp gain vs nominal f o (v s = 5v) (figure 3, v1 output) q error vs nominal f o (v s = 2.5v) peak bp gain vs nominal f o (v s = 2.5v) (figure 3, v1 output) nominal f o (khz) 100 q error (%) 55 50 45 40 35 30 25 20 15 10 5 0 ? 260 1562-2 g04 140 180 220 300 240 120 160 200 280 q = 5 q = 2.5 q = 1 t a = 70 c t a = 25 c r in = r q nominal f o (khz) 100 peak bp gain (db) 0.25 0 0.50 1.00 3.00 2.00 140 180 200 280 2.50 1.50 0.75 1.25 2.25 2.75 1.75 120 160 220 240 260 300 1562-2 g5 q = 5 q = 2.5 q = 1 t a = 70 c t a = 25 c r in = r q nominal f o (khz) 100 peak bp gain (db) 0.25 0 0.50 1.00 3.00 2.00 140 180 200 280 2.50 1.50 0.75 1.25 2.25 2.75 4.00 3.50 3.25 3.75 1.75 120 160 220 240 260 300 1562-2 g6 q = 5 q = 2.5 q = 1 t a = 70 c t a = 25 c r in = r q lp noise vs nominal f o (v s = 5v, 25 c) (figure 3, v2 output) (r in = r2) nominal f o (khz) 120 10 lp noise ( v rms ) 30 60 70 80 90 100 160 200 220 1562-2 g07 20 50 40 140 180 240 260 280 q = 5 q = 2.5 q = 1 bp noise vs nominal f o (v s = 5v, 25 c) (figure 3, v1 output) (r in = r q ) nominal f o (khz) 120 10 bp noise ( v rms ) 30 60 70 80 90 100 160 200 220 1562-2 g08 20 50 40 140 180 240 260 280 q = 5 q = 2.5 q = 1 pi n fu n ctio n s uuu distortion vs external load resistance and frequency (v s = 5v, 25 c) (figure 8) external load resistance ( ) 10k ?00 thd (amplitude below fundamental) (db) thd (amplitude below fundamental) (%) ?0 ?0 ?0 ?0 ?0 ?0 5k 2k 1562-2 g09 ?0 ?0 0 0.001 0.01 0.1 1 10 100 ?0 1k f in = 20khz f in = 50khz 2nd order lowpass f o = 200khz q = 0.7 output level 1v rms (2.83v p-p ) 5v supplies f in = 100khz power supply pins: the v + and v C pins should be bypassed with 0.1 m f capacitors to an adequate analog ground or ground plane. these capacitors should be connected as closely as possible to the supply pins. pins 4, 7, 14 and 17 are internally connected to v C (pin 16) and should also be tied to the same point as pin 16 for best shielding. low noise linear supplies are recommended. switching supplies are not recommended as they will lower the filter dynamic range. analog ground (agnd): the agnd pin is the midpoint of a resistive voltage divider, developing a potential halfway between the v + and v C pins, with an equivalent series resistance nominally 7k. this serves as an internal ground reference. filter performance will reflect the quality of the analog signal ground and an analog ground plane surrounding the package is recommended. the analog ground plane should be connected to any digital ground at a single point. for dual supply operation, the agnd pin
5 ltc1562-2 15622fa pi n fu n ctio n s uuu shutdown (shdn): when the shdn input goes high or is open-circuited, the ltc1562-2 enters a zero-power shutdown state and only junction leakage currents flow. the agnd pin and the amplifier outputs (see figure 3) assume a high impedance state and the amplifiers effec- tively disappear from the circuit. (if an input signal is applied to a complete filter circuit while the ltc1562-2 is in shutdown, some signal will normally flow to the output through passive components around the inactive op amps.) a small pull-up current source at the shdn input defaults the ltc1562-2 to the shutdown state if the shdn pin is left floating . therefore, the user must connect the shdn pin to a logic low (0v for 5v supplies, v C for 5v total supply) for normal operation of the ltc1562-2. (this convention permits true zero-power shutdown since not even the driving logic must deliver current while the part is in shutdown.) with a single supply voltage, use v C for logic low, do not connect shdn to the agnd pin. should be connected to the ground plane (figure 1). for single supply operation, the agnd pin should be bypassed to the ground plane with at least a 0.1 m f capacitor (at least 1 m f for best ac performance) (figure 2). 0.1 m f v 1562-2 f01 digital ground plane (if any) v + ltc1562-2 0.1 m f analog ground plane 20 19 18 17 16 15 14 13 12 11 1 2 3 4 5 6 7 8 9 10 single-point system ground figure 1. dual supply ground plane connection (including substrate pins 4, 7, 14, 17) 1 m f 1562-2 f01 digital ground plane (if any) v + ltc1562-2 v + /2 reference 0.1 m f analog ground plane 20 19 18 17 16 15 14 13 12 11 1 2 3 4 5 6 7 8 9 10 single-point system ground figure 2. single supply ground plane connection (including substrate pins 4, 7, 14, 17) + + r2 r q v in inv *r1 and c are precision internal components v2 v1 1/4 ltc1562-2 1562-2 f03 c 1 sr1c* z in z in type r c response at v1 bandpass highpass response at v2 lowpass bandpass 7958 w r2 in each case, q = f o = (200khz) rq r2 () 200khz f o () figure 3. equivalent circuit of a single 2nd order section (inside dashed line) shown in typical connection. form of z in determines response types at the two outputs (see table)
6 ltc1562-2 15622fa v + v shdn 1562-2 bd 2nd order sections r r inv v1 v2 c shutdown switch shutdown switch agnd v + v + inv v1 v2 inv v1 v2 inv v1 v2 c cc ab dc + + + block diagra w pi n fu n ctio n s uuu inv a, inv b, inv c, inv d: each of the inv pins is a virtual- ground summing point for the corresponding 2nd order section. for each section, all three external components z in , r2, r q connect to the inv pin as shown in figure 3 and described further in the applications information. note that the inv pins are sensitive internal nodes of the filter and will readily receive any unintended signals that are capacitively coupled into them. capacitance to the inv nodes will also affect the frequency response of the filter sections. for these reasons, printed circuit connections to the inv pins must be kept as short as possible, less than one inch (2.5cm) total and surrounded by a ground plane. v1 a, v1 b, v1 c, v1 d: output pins. provide a bandpass, highpass or other response depending on external cir- cuitry (see applications information section). each v1 pin also connects to the r q resistor of the corresponding 2nd order filter section (see figure 3 and applications informa- tion). each output is designed to drive a nominal net load of 4k w and 30pf, which includes the loading due to the external r q . distortion performance improves when the outputs are loaded as lightly as possible. v2 a, v2 b, v2 c, v2 d: output pins. provide a lowpass, bandpass or other response depending on external cir- cuitry (see applications information section). each v2 pin also connects to the r2 resistor of the corresponding 2nd order filter section (see figure 3 and applications informa- tion). each output is designed to drive a nominal net load of 4k w and 30pf, which includes the loading due to the external r2. distortion performance improves when the outputs are loaded as lightly as possible. overall block diagram showing four 3-terminal 2nd order sections
7 ltc1562-2 15622fa applicatio n s i n for m atio n wu u u the ltc1562-2 contains four matched, 2nd order, 3-terminal universal continuous-time filter blocks, each with a virtual-ground input node (inv) and two rail-to-rail outputs (v1, v2). in the most basic application, one such block and three external resistors provide 2nd order lowpass and bandpass responses simultaneously (figure 3, with a resistor for z in ). the three external resistors program f o , q and gain. a combination of internal preci- sion components and external resistor r2 sets the center frequency f o of each 2nd order block. the ltc1562-2 is trimmed at manufacture so that f o will be 200khz 0.5% if the external resistor r2 is exactly 7958 w . the ltc1562- 2 is a higher frequency, pin compatible variant of the ltc1562, with different internal r and c values and higher speed amplifiers. however, lowpass/bandpass filtering is only one specific application for the 2nd order building blocks in the ltc1562-2. highpass response results if the external impedance z in in figure 3 becomes a capacitor c in (whose value sets only gain, not critical frequencies) as described below. responses with zeroes (e.g, elliptic or notch responses) are available by feedforward connections with multiple 2nd order blocks (see typical applicatons). more- over, the virtual-ground input gives each 2nd order sec- tion the built-in capability for analog operations such as gain (preamplification), summing and weighting of mul- tiple inputs, or accepting current or charge signals di- rectly. these operational filter tm frequency-selective building blocks are nearly as versatile as op amps. the user who is not copying exactly one of the typical applications schematics shown later in this data sheet is urged to read carefully the next few sections through at least signal swings, for orientation about the ltc1562-2, before attempting to design custom application circuits. also available free from ltc, and recommended for de- signing custom filters, is the general-purpose analog filter design software filtercad tm for windows ? . this software includes tools for finding the necessary f 0 , q and gain parameters to meet target filter specifications such as frequency response. setting f o and q each of the four 2nd order sections in the ltc1562-2 can be programmed for a standard filter function (lowpass, bandpass or highpass) when configured as in figure 3 with a resistor or capacitor for z in . these transfer func- tions all have the same denominator, a complex pole pair with center frequency w o = 2 p f o and quality parameter q. (the numerators depend on the response type as de- scribed below.) external resistors r2 and r q set f o and q as follows: f crr r khz q r rr r r r r khz f o qqq o == w ? ? ? ? () == w () = ? ? ? ? 1 212 7958 2 200 12 7958 2 2 200 p () () r1 = 7958 w and c = 100pf are internal to the ltc1562-2 while r2 and r q are external. a typical design procedure proceeds from the desired f o and q as follows, using finite-tolerance fixed resistors. first find the ideal r2 value for the desired f o : r ideal khz f o 2 200 7958 2 () = ? ? ? ? () w then select a practical r2 value from the available finite- tolerance resistors. use the actual r2 value to find the desired r q , which also will be approximated with finite tolerance: rq r q = () 7958 2 w the f o range is approximately 20khz to 300khz, limited mainly by the magnitudes of the external resistors required. as shown above, r2 varies with the inverse square of f o . this relationship desensitizes f o to r2s tolerance (by a factor of 2 incrementally), but it also implies that r2 has a wider range than f o . (r q and r in also operational filter and filtercad are trademarks of linear technology corporation. windows is a registered trademark of microsoft corporation.
8 ltc1562-2 15622fa tend to scale with r2.) at high f o these resistors fall below 4k, heavily loading the outputs of the ltc1562-2 and leading to increased thd and other effects. at the other extreme, a lower f o limit of 20khz reflects an arbitrary upper resistor limit of 1m w . the ltc1562-2s mos input circuitry can accommodate higher resistor values than this, but junc tion leakage current from the input protection circuitry may cause dc errors. the 2nd order transfer functions h lp (s), h bp (s) and h hp (s) (below) are all inverting so that, for example, at dc the lowpass gain is C h l . if two such sections are cas- caded, these phase inversions cancel. thus, the filter in the application schematic on the first page of this data sheet is a dual dc preserving, noninverting, rail-to-rail lowpass filter, approximating two straight wires with frequency selectivity. figure 4 shows further details of 2nd order lowpass, bandpass and highpass responses. configurations to obtain these responses appear in the next three sections. basic lowpass when z in of figure 3 is a resistor of value r in , a standard 2nd order lowpass transfer function results from v in to v2 (figure 5): vs vs hs h sqs in lp l o o o 2 2 22 () () () / == + () + w ww h l = r2/r in is the dc gain magnitude. (note that the transfer function includes a sign inversion.) parameters applicatio n s i n for m atio n wu u u f l gain (v/v) 0.707 h b h b f o f (log scale) bandpass response f h gain (v/v) 0.707 h l h p h l h h f p f (log scale) lowpass response f c f c 1562-2 f04 gain (v/v) 0.707 h h h p f p f (log scale) highpass response q f ff fff ff qq ff qq o hl olh l o ho == = + ? ? ? ? + ? ? ? ? ? ? =+ ? ? ? ? + ? ? ? ? ? ? ; 1 2 1 2 1 1 2 1 2 1 2 2 ff qq ff q hh q q co po pl = ? ? ? ? + ? ? ? ? + = = ? ? ? ? ? ? ? ? 1 1 2 1 1 2 1 1 1 2 1 1 1 1 4 22 2 2 2 ff qq ff q hh q q co po ph = ? ? ? ? + ? ? ? ? + ? ? ? ? ? ? = ? ? ? ? = ? ? ? ? ? ? ? ? 1 1 2 1 1 2 1 1 1 2 1 1 1 1 4 22 2 1 2 1 2 figure 4. characteristics of standard 2nd order filter responses inv v1 2nd order 1/4 ltc1562-2 v2 1562 f05 r2 r q r in v in v out figure 5. basic lowpass configuration
9 ltc1562-2 15622fa applicatio n s i n for m atio n wu u u w o (= 2 p f o ) and q are set by r2 and r q as above. for a 2nd order lowpass response the gain magnitude becomes qh l at frequency f o , and for q > 0.707, a gain peak occurs at a frequency below f o , as shown in figure 4. basic bandpass there are two different ways to obtain a bandpass function in figure 3, both of which give the following transfer function form: hs hqs sqs bp bo o o () / / = () + () + w ww 22 the value of the gain parameter h b depends on the circuit configuration as follows. when z in is a resistor of value r in , a bandpass response results at the v1 output (figure 6a) with a gain parameter h b = r q /r in . alternatively, a capacitor of value c in gives a bandpass response at the v2 output (figure 6b), with the same h bp (s) expression, and the gain parameter now h b = (r q /7958 w )(c in /100pf). this transfer function has a gain magnitude of h b (its peak value) when the frequency equals f o and has a phase shift of 180 at that frequency. q measures the sharpness of the peak (the ratio of f o to C 3db bandwidth) in a 2nd order bandpass function, as illustrated in figure 4. w o = 2 p f o and q are set by r2 and r q as described previously in setting f o and q. h h = c in /100pf is the highpass gain parameter. param- eters w o = 2 p f o and q are set by r2 and r q as above. for a 2nd order highpass response the gain magnitude at frequency f o is qh h , and approaches h h at high frequen- cies (f >> f o ). for q > 0.707, a gain peak occurs at a frequency above f o as shown in figure 4. the transfer function includes a sign inversion. inv v1 2nd order 1/4 ltc1562-2 (b) capacitive input (a) resistive input v2 1562-2 f06 r2 r q c in v in v out inv v1 2nd order 1/4 ltc1562-2 v2 r2 r q r in v in v out figure 6. basic bandpass configurations basic highpass when z in of figure 3 is a capacitor of value c in , a highpass response appears at the v1 output (figure 7). vs vs hs hs sqs in hp h o o 1 2 22 () () () / == + () + ww inv v1 2nd order 1/4 ltc1562-2 v2 1562-2 f07 r2 r q c in v in v out figure 7. basic highpass configuration signal swings the v1 and v2 outputs are capable of swinging to within roughly 100mv of each power supply rail. as with any analog filter, the signal swings in each 2nd order section must be scaled so that no output overloads (saturates), even if it is not used as a signal output. (filter literature often calls this the dynamics issue.) when an unused output has a larger swing than the output of interest, the sections gain or input amplitude must be scaled down to avoid overdriving the unused output. the ltc1562-2 can still be used with high performance in such situations as long as this constraint is followed. for an ltc1562-2 section as in figure 3, the magnitudes of the two outputs v2 and v1, at a frequency w = 2 p f, have the ratio, |()| |()| () vj vj khz f 2 1 200 w w = regardless of the details of z in . therefore, an input fre- quency above or below 200khz produces larger output amplitude at v1 or v2, respectively. this relationship can guide the choice of filter design for maximum dynamic range in situations (such as bandpass responses) where there is more than one way to achieve the desired fre- quency response with an ltc1562-2 section.
10 ltc1562-2 15622fa because 2nd order sections with q 3 1 have response peaks near f o , the gain ratio above implies some rules of thumb: f o < 200khz t v2 tends to have the larger swing f o > 200khz t v1 tends to have the larger swing. the following situations are convenient because the relative swing issue does not arise. the unused outputs swing is naturally the smaller of the two in these cases: lowpass response (resistor input, v2 output, figure 5) with f o < 200khz bandpass response (capacitor input, v2 output, figure 6b) with f o < 200khz bandpass response (resistor input, v1 output, figure 6a) with f o > 200khz highpass response (capacitor input, v1 output, figure 7) with f o > 200khz the ltc1562, a lower frequency variant of the ltc1562 -2, has a design center f o of 100khz compared to 200khz in the ltc1562-2. the rules summarized above apply to the ltc1562 but with 100khz replacing the 200khz limits. thus, an ltc1562 highpass filter section with f o above 100khz automatically satisfies the desirable condition of the unused output carrying the smaller signal swing. require further dynamic range, reducing the value of z in boosts the signal gain while reducing the input referred noise. this feature can increase the snr for low level signals. varying or switching z in is also an efficient way to effect automatic gain control (agc). from a system view- point, this technique boosts the ratio of maximum signal to minimum noise, for a typical 2nd order lowpass re- sponse (q = 1, f o = 200khz), to 114db. input voltages beyond the power supplies properly used, the ltc1562-2 can accommodate input voltage excursions well beyond its supply voltage. this requires care in design but can be useful, for example, when large out-of-band interference is to be removed from a smaller desired signal. the flexibility for different input voltages arises because the inv inputs are at virtual ground potential, like the inverting input of an op amp with negative feedback. the ltc1562-2 fundamentally responds to input current and the external voltage v in appears only across the external impedance z in in figure 3. to accept beyond-the-supply input voltages, it is impor- tant to keep the ltc1562-2 powered on, not in shutdown mode, and to avoid saturating the v1 or v2 output of the 2nd order section that receives the input. if any of these conditions is violated, the inv input will depart from a virtual ground, leading to an overload condition whose recovery timing depends on circuit details. in the event that this overload drives the inv input beyond the supply voltages, the ltc1562-2 could be damaged. the most subtle part of preventing overload is to consider the possible input signals or spectra and take care that none of them can drive either v1 or v2 to the supply limits. note that neither output can be allowed to saturate, even if it is not used as the signal output. if necessary the passband gain can be reduced (by increasing the imped- ance of z in in figure 3) to reduce output swings. the final issue to be addressed with beyond-the-supply inputs is current and voltage limits. current entering the virtual ground inv input flows eventually through the output circuitry that drives v1 and v2. the input current magnitude ( ? v in ? / ? z in ? in figure 3) should be limited by design to less than 1ma for good distortion performance. on the other hand, the input voltage v in appears across the applicatio n s i n for m atio n wu u u inv v1 2nd order 1/4 ltc1562-2 v2 1562-2 f08 r2 7.87k c l 30pf r l (external load resistance) r q 5.49k r in 7.87k v in v out figure 8. 200khz, q = 0.7 lowpass circuit for distortion vs loading test low level or wide range input signals the ltc1562-2 contains a built-in capability for low noise amplification of low level signals. the z in impedance in each 2nd order section controls the blocks gain. when set for unity passband gain, a 2nd order section can deliver an output signal 99db above the noise level. if low level inputs
11 ltc1562-2 15622fa applicatio n s i n for m atio n wu u u external component z in , usually a resistor or capacitor. this component must of course be rated to sustain the magnitude of voltage imposed on it. lowpass t input circuit the virtual ground inv input in the operational filter block provides a means for adding an extra lowpass pole to any resistor-input application (such as the basic lowpass, figure 5, or bandpass, figure 6a). the resistor that would otherwise form z in is split into two parts and a capacitor to ground added, forming an r-c-r t network (figure 9). this adds an extra, independent real pole at a frequency: f rc p pt = p 1 2 where c t is the new external capacitor and r p is the parallel combination of the two input resistors r ina and r inb . this pair of resistors must normally have a pre- scribed series total value r in to set the filters gain as described above. the parallel value r p can however be set arbitrarily (to r in /4 or less) which allows choosing a convenient standard capacitor value for c t and fine tuning the new pole with r p . inv v1 2nd order 1/4 ltc1562-2 v2 1562-2 f10 r2 r q c inb r t v in c ina figure 10. highpass t input circuit a practical limitation of this technique is that the c t capaci- tor values that tend to be required (hundreds or thousands of pf) can destabilize the op amp in figure 3 if r inb is too small, leading to ac errors such as q enhancement. for this reason, when r ina and r in b are unequal, preferably the larger of the two should be placed in the r inb position. highpass t input circuit a method similar to the preceding technique adds an extra highpass pole to any capacitor-input application (such as the bandpass of figure 6b or the highpass of figure 7). this method splits the input capacitance c in into two series parts c ina and c inb , with a resistor r t to ground between them (figure 10). this adds an extra 1st order highpass corner with a zero at dc and a pole at the frequency: f rc p tp = p 1 2 where c p = c ina + c inb is the parallel combination of the two capacitors. at the same time, the total series capaci- tance c in will control the filters gain parameter (h h in basic highpass). for a given series value c in , the parallel value c p can still be set arbitrarily (to 4c in or greater). inv v1 2nd order 1/4 ltc1562-2 v2 1562-2 f09 r2 r q r inb r ina c t v in figure 9. lowpass t input circuit the procedure therefore is to begin with the target extra pole frequency f p . determine the series value r in from the gain requirement. select a capacitor value c t such that r p = 1/(2 p f p c t ) is no greater than r in /4, and then choose r ina and r inb that will simultaneously have the parallel value r p and the series value r in . such r ina and r inb can be found directly from the expression: 1 2 1 2 4 2 rrrr in in in p () the procedure then is to begin with the target corner (pole) frequency f p . determine the series value c in from the gain requirement (for example, c in = h h (100pf) for a high- pass). select a resistor value r t such that c p = 1/(2 p r t f p ) is at least 4c in , and select c ina and c inb that will simulta- neously have the parallel value c p and the series value c in . such c ina and c inb can be found directly from the expression: 1 2 1 2 4 2 cccc p p in p ()
12 ltc1562-2 15622fa typical applicatio n s u r in2 20.5k r q2 26.7k r22 10k r24 4.02k r q4 3.24k r in4 40.2k r q3 59k r23 11.3k r q1 9.09k r21 7.15k v in 5v 0.1 f 0.1 f 5v* v out *v also at pins 4, 7, 14 & 17 all resistors 1% metal film all capacitors 5% standard values c in1 220pf c in2 82pf c in3 47pf r in3 45.3k c in4 100pf 1562-2 ta03a 20 19 18 16 15 13 12 11 1 2 3 5 6 8 9 10 inv c v1 c v2 c v agnd v2 d v1 d inv d inv b v1 b v2 b v + shdn v2 a v1 a inv a ltc1562-2 175khz 8th order elliptic highpass filter gain (db) 10 0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 50k 900k frequency (hz) 200k 1562-2 ta03b amplitude response this procedure can be iterated, adjusting the value of r t , to find convenient values for c ina and c inb since resistor values are generally available in finer increments than capacitor values. ltc1562/ltc1562-2 demo board the ltc demonstration board dc266 is assembled with an ltc1562 or ltc1562-2 in a 20-pin ssop package and power supply decoupling capacitors. jumpers on the board configure the filter chip for dual or single supply operation and power shutdown. pads for surface mount resistors and capacitors are provided to build application- specific filters. also provided are terminals for inputs, outputs and power supplies. notches and elliptic responses further circuit techniques appear in the ltc1562 data sheet under the heading notches and elliptic responses. these techniques are directly applicable to the ltc1562-2 with the substitution of the different values for the internal components r1 and c. in the ltc1562-2, r1 is 7958 w and c is 100pf.
13 ltc1562-2 15622fa dual 5th order 170khz elliptic highpass filter, single 5v supply amplitude response typical applicatio n s u r in2 15k c in2 220pf r q2 7.68k r22 6.34k r24 6.34k r q4 7.68k r in4 15k r q3 43.2k r23 11.5k r i3 2k r q1 43.2k c in1 82pf c i1 100pf c in3 82pf c i3 100pf r i1 2k r21 11.5k v in1 v in2 5v 0.1 f 1 f v out2 * v out1 c in4 220pf 1562-2 ta05a 20 19 18 16 15 13 12 11 1 2 3 5 6 8 9 10 inv c v1 c v2 c v agnd v2 d v1 d inv d inv b v1 b v2 b v + shdn v2 a v1 a inv a ltc1562-2 *ground also at pins 4, 7, 14 & 17 + frequency (hz) 10k ?0 gain (db) ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 0 100k 1m 1562-2 ta05b 10
14 ltc1562-2 15622fa typical applicatio n s u 100khz 8th order bandpass linear phase, C 3db bw = f center /10 frequency response r in2 178k r q2 76.8k r22 30.1k r24 28.7k r q4 118k r in4 221k r q3 142k r23 35.7k r q1 78.7k r21 31.6k c in1 10pf v in 5v 0.1 f c in3 10pf 0.1 f 5v* v out 1562-2 ta6a *v also at pins 4, 7, 14 & 17 20 19 18 16 15 13 12 11 1 2 3 5 6 8 9 10 inv c v1 c v2 c v agnd v2 d v1 d inv d inv b v1 b v2 b v + shdn v2 a v1 a inv a ltc1562-2 frequency (hz) 60k amplitude response (db) group delay ( m s) ?0 ?0 60 0 0 120k 1562-2 ta06b ?0 ?0 80k 100k 140k ?0 ?0 ?0 10 amplitude response group delay
15 ltc1562-2 15622fa package descriptio n u information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. typical applicatio n s u r q1 6.04k r q3 5.36k r in3 10.2k r in1a 4.02k r in1b 4.02k v in r q2 13k r in2 7.32k r in4 6.04k c in2 27pf c in4 22pf r q4 14.3k r22 6.04k r24 6.04k 0.1 f v out ?v 5v r21 8.06k r23 12.4k ltc1562-2 invb v1b v2b v + shdn v2a v1a inva 20 19 18 16 15 13 12 11 invc v1c v2c v agnd v2d v1d invd 1 2 3 5 6 8 9 10 0.1 f 180pf 1562-2 ta07a pins 4, 7, 14, 17 (not shown) also connect to v all resistors are 1%, all capacitors are 5% frequency (khz) 10 ?00 gain (db) ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 100 1000 1562-2 ta07b 10 0 ltc1562-2 9th order 200khz lowpass elliptic filter amplitude response g package 20-lead plastic ssop (5.3mm) (reference ltc dwg # 05-08-1640) g20 ssop 0501 .13 ?.22 (.005 ?.009) 0 ?8 .55 ?.95 (.022 ?.037) 5.20 ?5.38** (.205 ?.212) 7.65 ?7.90 (.301 ?.311) 1234 5 6 7 8910 7.07 ?7.33* (.278 ?.289) 17 18 14 13 12 11 15 16 19 20 1.73 ?1.99 (.068 ?.078) .05 ?.21 (.002 ?.008) .65 (.0256) bsc .25 ?.38 (.010 ?.015) millimeters (inches) dimensions do not include mold flash. mold flash shall not exceed .152mm (.006") per side dimensions do not include interlead flash. interlead flash shall not exceed .254mm (.010") per side * ** note: 1. controlling dimension: millimeters 2. dimensions are in 3. drawing not to scale
16 ltc1562-2 15622fa part number description comments ltc1068-x quad 2-pole switched capacitor building block clock tuned ltc1560-1 5-pole elliptic lowpass, f c = 1mhz/0.5mhz no external components, so8 ltc1562 quad 2-pole active rc, 10khz to 150khz same pinout as ltc1562-2 ltc1563-2/ltc1563-3 4th order active rc lowpass filters f cutoff(max) = 256khz, resistor programmable ltc1564 10khz to 150khz digitally controlled filter and pga continuous time low noise 8th order with pga ltc1565-31 650khz continuous time, linear phase lowpass filter 7th order, differential inputs and outputs ltc1566-1 2.3mhz continuous time lowpass filter 7th order, differential inputs and outputs lt/tp 0102 1.5k rev a ? printed in usa ? linear technology corporation 1998 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com related parts delay ( s) 8 7 6 5 4 3 2 1 0 frequency (khz) 50 100 150 200 250 300 1562-2 ta04c 350 400 group delay response gain (db) 10 0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 ?0 10k 1m 1562-2 ta04b frequency (hz) 100k amplitude response r b1 1.54k r ff1 6.19k r q2 4.12k r22 6.19k r24 4.12k r q4 7.32k r in4 4.12k r q3 7.32k r23 4.12k r in3 4.12k r in1 7.5k r q1 3.24k r21 6.81k v in 5v 0.1 f 1 f * v out *ground also at pins 4, 7, 14 & 17 all resistors 1% metal film c in4 22pf 5% 1562-2 ta04a 20 19 18 16 15 13 12 11 1 2 3 5 6 8 9 10 inv c v1 c v2 c v agnd v2 d v1 d inv d inv b v1 b v2 b v + shdn v2 a v1 a inv a ltc1562-2 + 256khz linear phase 6th order lowpass filter with a 2nd order allpass phase equalizer, single supply u typical applicatio

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