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| LTC1562-2 Very Low Noise, Low Distortion Active RC Quad Universal Filter FEATURES s s DESCRIPTIO s s s s s s s s s s Continuous Time--No Clock Four 2nd Order Filter Sections, 20kHz to 300kHz Center Frequency Butterworth, Chebyshev, Elliptic or Equiripple Delay Response Lowpass, Bandpass, Highpass Responses 99dB Typical S/N, 5V Supply (Q = 1) 93dB Typical S/N, Single 5V Supply (Q = 1) Rail-to-Rail Input and Output Voltages DC Accurate to 3mV (Typ) 0.5% Typical Center Frequency Accuracy "Zero-Power" Shutdown Mode Single or Dual Supply, 5V to 10V Total Resistor-Programmable fO, Q, Gain APPLICATIO S s s s s s s High Resolution Systems (14 Bits to 18 Bits) Antialiasing/Reconstruction Filters Data Communications, Equalizers Dual or I-and-Q Channels (Two Matched 4th Order Filters in One Package) Linear Phase Filtering Replacing LC Filter Modules The LTC(R)1562-2 is a low noise, low distortion continuous time filter with rail-to-rail inputs and outputs, optimized for a center frequency (fO) 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 block's 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 outputs. 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. TYPICAL APPLICATIO Dual 4th Order 200kHz Butterworth Lowpass Filter, SNR 96dB RIN2 7.87k RIN1 7.87k VIN1 RQ1 4.22k R21 7.87k 5V 0.1F R23 7.87k RQ3 4.22k RIN3 7.87k VIN2 *V - ALSO AT PINS 4, 7, 14 & 17 ALL RESISTORS 1% METAL FILM 1 2 3 5 6 8 9 10 20 19 18 RQ2 10.2k R22 7.87k - 5V* 0.1F R24 7.87k RQ4 10.2k GAIN (dB) 10 VOUT1 0 -10 -20 -30 -40 -50 -60 -70 -80 50k 100k FREQUENCY (Hz) 1562-2 TA02 INV B V1 B V2 B INV C V1 C V2 C 16 V+ LTC1562-2 V - 15 SHDN AGND 13 V2 A V2 D 12 V1 A V1 D 11 INV A INV D RIN4 7.87k 1562-2 TA01 VOUT2 U Amplitude Response 1M 1.5M U U 1 LTC1562-2 ABSOLUTE MAXIMUM RATINGS (Note 1) PACKAGE/ORDER INFORMATION TOP VIEW INV B V1 B V2 B V -* V+ SHDN V -* V2 A V1 A INV A 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 INV C V1 C V2 C V -* V- AGND V -* V2 D V1 D INV D Total Supply Voltage (V + to V -) .............................. 11V Maximum Input Voltage at Any Pin ....................(V - - 0.3V) V (V + + 0.3V) Storage Temperature Range ................. - 65C to 150C Operating Temperature Range LTC1562C-2 ............................................ 0C to 70C LTC1562I-2 ........................................ - 40C to 85C Lead Temperature (Soldering, 10 sec).................. 300C ORDER PART NUMBER LTC1562CG-2 LTC1562IG-2 G PACKAGE 20-LEAD PLASTIC SSOP *G PACKAGE PINS 4, 7, 14, 17 ARE SUBSTRATE/SHIELD CONNECTIONS AND MUST BE TIED TO V - TJMAX = 150C, JA = 136C/W Consult factory for Military grade parts. ELECTRICAL CHARACTERISTICS SYMBOL VS IS PARAMETER Total Supply Voltage Supply Current The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 5V, outputs unloaded, SHDN pin to logic "low", unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = 10.4k 0.1%, RQ = 9.09k 0.1%, fO = 175kHz. CONDITIONS VS = 2.375V, RL = 5k, CL = 30pF, Outputs at 0V VS = 5V, RL = 5k, CL = 30pF, Outputs at 0V VS = 2.375V, RL = 5k, CL = 30pF, Outputs at 0V VS = 5V, RL = 5k, CL = 30pF, Outputs at 0V Output Voltage Swing, V2 Outputs Output Voltage Swing, V1 Outputs VOS DC Offset Magnitude, V2 Outputs DC AGND Reference Point Center Frequency (f O) Error (Notes 2, 3) HL Lowpass Passband Gain at V2 Output Q Accuracy Wideband Output Noise Input-Referred Noise, Gain = 100 VS = 2.375V, RL = 5k, CL = 30pF VS = 5V, RL = 5k, CL = 30pF VS = 2.375V, RL = 5k, CL = 30pF, f = 250kHz VS = 5V, RL = 5k, CL = 30pF, f = 250kHz VS = 2.375V, Input at AGND Voltage VS = 5V, Input at AGND Voltage VS = Single 5V Supply VS = 5V, V2 Output Has RL = 5k, CL = 30pF VS = 2.375V, fIN = 10kHz, V2 Output Has RL = 5k, CL = 30pF VS = 2.375V, V2 Output Has RL = 5k, CL = 30pF VS = 2.375V, BW = 400kHz, Input AC GND VS = 5V, BW = 400kHz, Input AC GND BW = 400kHz, f O = 200kHz, Q = 1, Input AC GND q q q q q MIN 4.75 TYP 21 22.5 MAX 10.5 23.5 25 28 30 UNITS V mA mA mA mA VP-P VP-P VP-P VP-P 4.2 9.3 8.4 4.6 9.8 4.5 9.7 3 3 2.5 0.5 1.7 + 0.1 17 17 0 + 0.05 +2 39 39 7.3 VRMS VRMS VRMS 2 U W U U WW W mV mV V % dB % LTC1562-2 The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VS = 5V, outputs unloaded, SHDN pin to logic "low", unless otherwise noted. AC specs are for a single 2nd order section, RIN = R2 = 10.4k 0.1%, RQ = 9.09k 0.1%, fO = 175kHz. SYMBOL THD PARAMETER Total Harmonic Distortion, V2 Output CONDITIONS fIN = 20kHz, 2.8VP-P, V1 and V2 Outputs Have RL = 5k, CL = 30pF fIN = 20kHz, 9VP-P, V1 and V2 Outputs Have RL = 5k, CL = 30pF Shutdown Supply Current Shutdown-Input Logic Threshold Shutdown-Input Bias Current Shutdown Delay Shutdown Recovery Delay Inverting Input Bias Current, Each Biquad Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: fO change from 5V to 2.375 supplies is - 0.2% typical, fO temperature coefficient magnitude, 25C to 85C, is 50ppm/C typical. As with the LTC1562, fO decreases with increasing temperature. SHDN Pin to 0V SHDN Pin Steps from 0V to V + SHDN Pin Steps from V + to 0V SHDN Pin to V + SHDN Pin to V +, VS = 2.375V MIN TYP - 100 - 82 1.5 1.0 2.5 - 10 20 100 5 Note 3: Tighter frequency tolerance is available, consult factory. - 20 15 MAX UNITS dB dB A A V A s s pA ELECTRICAL CHARACTERISTICS TYPICAL PERFOR A CE CHARACTERISTICS fO Error vs Nominal fO (VS = 5V) 3.0 3.0 T = 25C 2.5 RA = R IN Q 2.0 1.5 fO ERROR (%) fO ERROR (%) Q ERROR (%) 1.0 0.5 0 - 0.5 -1.0 -1.5 -2.0 -2.5 -3.0 120 140 160 180 200 220 240 260 280 NOMINAL fO (kHz) 1562-2 G01 Q=5 Q = 2.5 UW Q=1 fO Error vs Nominal fO (VS = 2.5V) 45 40 35 30 Q=5 25 20 15 10 5 Q=1 0 T = 25C 2.5 RA = R IN Q 2.0 1.5 1.0 0.5 0 - 0.5 -1.0 -1.5 -2.0 -2.5 Q = 2.5 Q Error vs Nominal fO (VS = 5V) TA = 70C TA = 25C RIN = RQ Q=5 Q = 2.5 Q=1 -3.0 120 140 160 180 200 220 240 260 280 NOMINAL fO (kHz) 1562-2 G02 -5 100 120 140 160 180 200 220 240 260 280 300 NOMINAL fO (kHz) 1562-2 G03 3 LTC1562-2 TYPICAL PERFOR A CE CHARACTERISTICS Q Error vs Nominal fO (VS = 2.5V) 55 50 45 40 Q ERROR (%) 3.00 TA = 70C TA = 25C RIN = RQ Q=5 PEAK BP GAIN (dB) 35 30 25 20 15 10 5 0 -5 100 120 140 160 180 200 220 240 260 280 300 NOMINAL fO (kHz) 1562-2 G04 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 100 120 140 160 180 200 220 240 260 280 300 NOMINAL fO (kHz) 1562-2 G5 Q = 2.5 Q=1 Q = 2.5 Q=1 PEAK BP GAIN (dB) LP Noise vs Nominal fO (VS = 5V, 25C) (Figure 3, V2 Output) (RIN = R2) 100 90 80 100 90 80 BP NOISE (VRMS) THD (AMPLITUDE BELOW FUNDAMENTAL) (dB) LP NOISE (VRMS) 70 60 50 40 30 20 Q=5 Q = 2.5 Q=1 10 120 140 160 180 200 220 240 260 280 NOMINAL fO (kHz) 1562-2 G07 PIN FUNCTIONS Power Supply Pins: The V + and V - pins should be bypassed with 0.1F 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 - (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 - 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 4 UW Peak BP Gain vs Nominal fO (VS = 5V) (Figure 3, V1 Output) 2.75 2.50 2.25 TA = 70C TA = 25C RIN = RQ Q=5 Peak BP Gain vs Nominal fO (VS = 2.5V) (Figure 3, V1 Output) 4.00 TA = 70C 3.75 TA = 25C 3.50 3.25 RIN = RQ 3.00 Q=5 2.75 2.50 2.25 2.00 1.75 1.50 Q = 2.5 1.25 1.00 0.75 Q=1 0.50 0.25 0 100 120 140 160 180 200 220 240 260 280 300 NOMINAL fO (kHz) 1562-2 G6 BP Noise vs Nominal fO (VS = 5V, 25C) (Figure 3, V1 Output) (RIN = RQ) 0 -10 -20 -30 - 40 - 50 - 60 -70 - 80 - 90 Distortion vs External Load Resistance and Frequency (VS = 5V, 25C) (Figure 8) 2nd ORDER LOWPASS fO = 200kHz Q = 0.7 OUTPUT LEVEL 1VRMS (2.83VP-P) 5V SUPPLIES 100 THD (AMPLITUDE BELOW FUNDAMENTAL) (%) 10 70 60 50 40 30 20 10 120 140 160 180 200 220 240 260 280 NOMINAL fO (kHz) 1562-2 G08 Q=5 Q = 2.5 Q=1 1 0.1 fIN = 100kHz fIN = 50kHz fIN = 20kHz 2k 5k EXTERNAL LOAD RESISTANCE () 1k 0.01 -100 10k 0.001 1562-2 G09 U U U LTC1562-2 PIN FUNCTIONS 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 0.1F capacitor (at least 1F for best AC performance) (Figure 2). ANALOG GROUND PLANE 1 2 3 4 V+ 0.1F 5 6 7 8 9 10 SINGLE-POINT SYSTEM GROUND Figure 1. Dual Supply Ground Plane Connection (Including Substrate Pins 4, 7, 14, 17) ANALOG GROUND PLANE 1 2 3 4 20 18 17 LTC1562-2 16 15 14 13 12 11 V +/2 REFERENCE RESPONSE RESPONSE ZIN TYPE AT V1 AT V2 R BANDPASS LOWPASS C HIGHPASS BANDPASS V2 R2 INV RQ V1 V+ 0.1F 5 6 7 8 9 10 1F ZIN + - VIN IN EACH CASE, fO = (200kHz) SINGLE-POINT SYSTEM GROUND DIGITAL GROUND PLANE (IF ANY) 1562-2 F01 Q = RQ 200kHz R2 fO Figure 3. Equivalent Circuit of a Single 2nd Order Section (Inside Dashed Line) Shown in Typical Connection. Form of ZIN Determines Response Types at the Two Outputs (See Table) Figure 2. Single Supply Ground Plane Connection (Including Substrate Pins 4, 7, 14, 17) + 19 - U U U 20 19 18 17 LTC1562-2 16 15 14 13 12 11 V- 0.1F 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 effectively 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 - 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 - for logic "low," do not connect SHDN to the AGND pin. DIGITAL GROUND PLANE (IF ANY) 1562-2 F01 1/4 LTC1562-2 1 sR1C* *R1 AND C ARE PRECISION INTERNAL COMPONENTS C () ( 7958 R2 ) 1562-2 F03 5 LTC1562-2 PIN FUNCTIONS INV A, INV B, INV C, INV D: Each of the INV pins is a virtualground summing point for the corresponding 2nd order section. For each section, all three external components ZIN, R2, RQ 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 circuitry (see Applications Information section). Each V1 pin also connects to the RQ resistor of the corresponding 2nd order filter section (see Figure 3 and Applications Information). Each output is designed to drive a nominal net load of 4k and 30pF, which includes the loading due to the external RQ. 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 circuitry (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 Information). Each output is designed to drive a nominal net load of 4k and 30pF, which includes the loading due to the external R2. Distortion performance improves when the outputs are loaded as lightly as possible. BLOCK DIAGRA V+ SHUTDOWN SWITCH R V- 2ND ORDER SECTIONS R SHUTDOWN SWITCH V- AGND D C SHDN + - C - INV V1 V2 INV 6 + + V+ - - W U U U Overall Block Diagram Showing Four 3-Terminal 2nd Order Sections INV V1 V2 INV V1 V2 A C B C + C 1562-2 BD V1 V2 LTC1562-2 APPLICATIONS INFORMATION 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 ZIN). The three external resistors program fO, Q and gain. A combination of internal precision components and external resistor R2 sets the center frequency fO of each 2nd order block. The LTC1562-2 is trimmed at manufacture so that fO will be 200kHz 0.5% if the external resistor R2 is exactly 7958. The LTC15622 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 ZIN in Figure 3 becomes a capacitor CIN (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). Moreover, the virtual-ground input gives each 2nd order section the built-in capability for analog operations such as gain (preamplification), summing and weighting of multiple inputs, or accepting current or charge signals directly. These Operational FilterTM frequency-selective building blocks are nearly as versatile as operational amplifiers. 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 designing custom filters, is the general-purpose analog filter design software FilterCADTM for Windows(R). This software includes tools for finding the necessary f0, Q and gain parameters to meet target filter specifications such as frequency response. Setting fO 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 ZIN. These transfer functions all have the same denominator, a complex pole pair with center frequency O = 2fO and quality parameter Q. (The numerators depend on the response type as described below.) External resistors R2 and RQ set fO and Q as follows: fO = Q= 7958 1 = (200kHz) R2 2C (R1)R2 RQ = (R1)R2 U W U U (7958)R2 RQ = RQ 200kHz R2 fO R1 = 7958 and C = 100pF are internal to the LTC1562-2 while R2 and RQ are external. A typical design procedure proceeds from the desired fO and Q as follows, using finite-tolerance fixed resistors. First find the ideal R2 value for the desired fO: 200kHz R2(Ideal) = (7958) fO Then select a practical R2 value from the available finitetolerance resistors. Use the actual R2 value to find the desired RQ, which also will be approximated with finite tolerance: RQ = Q (7958)R2 The fO 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 fO. This relationship desensitizes fO to R2's tolerance (by a factor of 2 incrementally), but it also implies that R2 has a wider range than fO. (RQ and RIN also Operational Filter and FilterCAD are trademarks of Linear Technology Corporation. Windows is a registered trademark of Microsoft Corporation. 2 7 LTC1562-2 APPLICATIONS INFORMATION tend to scale with R2.) At high fO 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 fO limit of 20kHz reflects an arbitrary upper resistor limit of 1M. The LTC1562-2's MOS input circuitry can accommodate higher resistor values than this, but junction leakage current from the input protection circuitry may cause DC errors. The 2nd order transfer functions HLP(s), HBP(s) and HHP(s) (below) are all inverting so that, for example, at DC the lowpass gain is - HL. If two such sections are cascaded, 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 ZIN of Figure 3 is a resistor of value RIN, a standard 2nd order lowpass transfer function results from VIN to V2 (Figure 5): V2(s) - HL O = HLP (s) = 2 VIN(s) s2 + O / Q s + O 2 BANDPASS RESPONSE GAIN (V/V) HB 0.707 HB GAIN (V/V) GAIN (V/V) HP HL 0.707 HL fL fO fH f (LOG SCALE) Q= fO ; fO = fL fH fH - fL 2 1 -1 fL = fO + + 1 2Q 2Q 2 1 1 fH = fO + + 1 2Q 2Q Figure 4. Characteristics of Standard 2nd Order Filter Responses 8 U W U U ( ) HL = R2/RIN is the DC gain magnitude. (Note that the transfer function includes a sign inversion.) Parameters RIN VIN RQ INV V1 2nd ORDER 1/4 LTC1562-2 1562 F05 R2 VOUT V2 Figure 5. Basic Lowpass Configuration LOWPASS RESPONSE HP HH 0.707 HH HIGHPASS RESPONSE fP fC fC fP f (LOG SCALE) 1562-2 F04 f (LOG SCALE) 1 1 1- 2 + 1- 2 + 1 2Q 2Q 1 2Q2 2 fC = fO fP = fO 1 - 2 1 1 fC = fO 1 - + 1 - 2 + 1 2 2Q 2Q 1 fP = fO 1 - 2Q2 1 HP = HH 1 1 Q 1- 2 4Q -1 -1 1 HP = HL 1 1 Q 1- 2 4Q LTC1562-2 APPLICATIONS INFORMATION O (= 2fO) and Q are set by R2 and RQ as above. For a 2nd order lowpass response the gain magnitude becomes QHL at frequency fO, and for Q > 0.707, a gain peak occurs at a frequency below fO, 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: HBP (s) = - HB O / Q s s2 + O / Q s + O ( ( ) ) 2 The value of the gain parameter HB depends on the circuit configuration as follows. When ZIN is a resistor of value RIN, a bandpass response results at the V1 output (Figure 6a) with a gain parameter HB = RQ/RIN. Alternatively, a capacitor of value CIN gives a bandpass response at the V2 output (Figure 6b), with the same HBP(s) expression, and the gain parameter now HB = (RQ/7958)(CIN/100pF). This transfer function has a gain magnitude of HB (its peak value) when the frequency equals fO and has a phase shift of 180 at that frequency. Q measures the sharpness of the peak (the ratio of fO to - 3dB bandwidth) in a 2nd order bandpass function, as illustrated in Figure 4. O = 2fO and Q are set by R2 and RQ as described previously in Setting fO and Q. RIN VIN RQ INV V1 2nd ORDER 1/4 LTC1562-2 V2 R2 VOUT INV V1 2nd ORDER 1/4 LTC1562-2 1562-2 F06 CIN VIN RQ V2 R2 VOUT (a) Resistive Input (b) Capacitive Input Figure 6. Basic Bandpass Configurations Basic Highpass When ZIN of Figure 3 is a capacitor of value CIN, a highpass response appears at the V1 output (Figure 7). V1(s) - HHs2 = HHP (s) = 2 VIN(s) s2 + O / Q s + O ( ) U W U U HH = CIN/100pF is the highpass gain parameter. Parameters O = 2fO and Q are set by R2 and RQ as above. For a 2nd order highpass response the gain magnitude at frequency fO is QHH, and approaches HH at high frequencies (f >> fO). For Q > 0.707, a gain peak occurs at a frequency above fO as shown in Figure 4. The transfer function includes a sign inversion. CIN VIN RQ INV V1 2nd ORDER 1/4 LTC1562-2 1562-2 F07 R2 VOUT V2 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 section's 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 = 2f, have the ratio, | V2( j )| (200kHz) = | V1( j )| f regardless of the details of ZIN. Therefore, an input frequency 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 frequency response with an LTC1562-2 section. 9 LTC1562-2 APPLICATIONS INFORMATION Because 2nd order sections with Q 1 have response peaks near fO, the gain ratio above implies some rules of thumb: fO < 200kHz V2 tends to have the larger swing fO > 200kHz V1 tends to have the larger swing. The following situations are convenient because the relative swing issue does not arise. The unused output's swing is naturally the smaller of the two in these cases: Lowpass response (resistor input, V2 output, Figure 5) with fO < 200kHz Bandpass response (capacitor input, V2 output, Figure 6b) with fO < 200kHz Bandpass response (resistor input, V1 output, Figure 6a) with fO > 200kHz Highpass response (capacitor input, V1 output, Figure 7) with fO > 200kHz The LTC1562, a lower frequency variant of the LTC1562 -2, has a design center fO 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 fO above 100kHz automatically satisfies the desirable condition of the unused output carrying the smaller signal swing. RIN 7.87k VIN RQ 5.49k INV V1 2nd ORDER 1/4 LTC1562-2 V2 R2 7.87k CL 30pF VOUT RL (EXTERNAL LOAD RESISTANCE) 1562-2 F08 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 ZIN impedance in each 2nd order section controls the block's 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 10 U W U U require further dynamic range, reducing the value of ZIN boosts the signal gain while reducing the input referred noise. This feature can increase the SNR for low level signals. Varying or switching ZIN is also an efficient way to effect automatic gain control (AGC). From a system viewpoint, this technique boosts the ratio of maximum signal to minimum noise, for a typical 2nd order lowpass response (Q = 1, fO = 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 VIN appears only across the external impedance ZIN in Figure 3. To accept beyond-the-supply input voltages, it is important 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 impedance of ZIN 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 (VIN/ZIN in Figure 3) should be limited by design to less than 1mA for good distortion performance. On the other hand, the input voltage VIN appears across the LTC1562-2 APPLICATIONS INFORMATION external component ZIN, 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 ZIN 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: fP = 1 2RPCT A practical limitation of this technique is that the CT capacitor values that tend to be required (hundreds or thousands of pF) can destabilize the op amp in Figure 3 if RINB is too small, leading to AC errors such as Q enhancement. For this reason, when RINA and RINB are unequal, preferably the larger of the two should be placed in the RINB 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 CIN into two series parts CINA and CINB, with a resistor RT 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: fP = 1 2RTCP where CT is the new external capacitor and RP is the parallel combination of the two input resistors RINA and RINB. This pair of resistors must normally have a prescribed series total value RIN to set the filter's gain as described above. The parallel value RP can however be set arbitrarily (to RIN/4 or less) which allows choosing a convenient standard capacitor value for CT and fine tuning the new pole with RP. RINA VIN CT INV V1 2nd ORDER 1/4 LTC1562-2 1562-2 F09 RINB RQ V2 R2 Figure 9. Lowpass "T" Input Circuit The procedure therefore is to begin with the target extra pole frequency fP. Determine the series value RIN from the gain requirement. Select a capacitor value CT such that RP = 1/(2fPCT) is no greater than RIN/4, and then choose RINA and RINB that will simultaneously have the parallel value RP and the series value RIN. Such RINA and RINB can be found directly from the expression: 2 1 1 RIN RIN - 4RINRP 2 2 ( ) U W U U where CP = CINA + CINB is the parallel combination of the two capacitors. At the same time, the total series capacitance CIN will control the filter's gain parameter (HH in Basic Highpass). For a given series value CIN, the parallel value CP can still be set arbitrarily (to 4CIN or greater). CINA VIN RT INV V1 2nd ORDER 1/4 LTC1562-2 1562-2 F10 CINB RQ V2 R2 Figure 10. Highpass "T" Input Circuit The procedure then is to begin with the target corner (pole) frequency fP. Determine the series value CIN from the gain requirement (for example, CIN = HH(100pF) for a highpass). Select a resistor value RT such that CP = 1/(2RTfP) is at least 4CIN, and select CINA and CINB that will simultaneously have the parallel value CP and the series value CIN. Such CINA and CINB can be found directly from the expression: 2 1 1 CP CP - 4CINCP 2 2 ( ) 11 LTC1562-2 This procedure can be iterated, adjusting the value of RT, to find convenient values for CINA and CINB 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 applicationspecific filters. Also provided are terminals for inputs, outputs and power supplies. Notches and Elliptic Responses Further circuit techniques appear in the LTC1562 final 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 and C is 100pF. TYPICAL APPLICATIONS 175kHz 8th Order Elliptic Highpass Filter CIN2 82pF RIN2 20.5k CIN3 47pF CIN1 220pF VIN 1 RQ1 9.09k R21 7.15k 5V 0.1F R23 11.3k RQ3 59k 2 3 5 6 8 9 10 INV B V1 B V2 B V + LTC1562-2 V - 5V* 0.1F R24 4.02k RQ4 3.24k SHDN V2 A V1 A INV A AGND V2 D V1 D INV D RIN4 40.2k 15 13 12 11 GAIN (dB) *V - ALSO AT PINS 4, 7, 14 & 17 ALL RESISTORS 1% METAL FILM ALL CAPACITORS 5% STANDARD VALUES 12 U Amplitude Response INV C V1 C V2 C 20 19 18 RQ2 26.7k R22 10k RIN3 45.3k 10 0 -10 -20 -30 -40 -50 -60 -70 VOUT - 16 -80 -90 50k 200k FREQUENCY (Hz) 900k 1562-2 TA03b CIN4 100pF 1562-2 TA03a LTC1562-2 TYPICAL APPLICATIONS Dual 5th Order 170kHz Elliptic Highpass Filter, Single 5V Supply CIN2 220pF RIN2 15k CI1 CIN1 100pF 82pF VIN1 RI1 2k 5V 0.1F R23 11.5k CI3 CIN3 100pF 82pF VIN2 RI3 2k RQ3 43.2k 1 RQ1 43.2k R21 11.5k 2 3 5 6 8 9 10 INV B V1 B V2 B V+ INV C V1 C V2 C LTC1562-2 V - AGND V2 D V1 D INV D RIN4 15k CIN4 220pF *GROUND ALSO AT PINS 4, 7, 14 & 17 20 19 18 16 RQ2 7.68k R22 6.34k * 1F R24 6.34k RQ4 7.68k VOUT1 SHDN V2 A V1 A INV A 13 12 11 Amplitude Response 10 0 - 10 - 20 GAIN (dB) - 30 - 40 - 50 - 60 - 70 - 80 - 90 10k 100k FREQUENCY (Hz) 1M 1562-2 TA05b + U 15 VOUT2 1562-2 TA05a 13 LTC1562-2 TYPICAL APPLICATIONS 100kHz 8th Order Bandpass Linear Phase, - 3dB BW = fCENTER/10 CIN1 10pF 1 VIN RQ1 78.7k R21 31.6k 5V 0.1F R23 35.7k RQ3 142k 2 3 5 6 8 9 10 CIN3 10pF INV B V1 B V2 B V+ INV C V1 C V2 C LTC1562-2 V - AGND V2 D V1 D INV D RIN4 221k 20 19 18 16 15 13 12 11 AMPLITUDE RESPONSE (dB) 14 U RIN2 178k RQ2 76.8k R22 30.1k - 5V* 0.1F R24 28.7k RQ4 118k SHDN V2 A V1 A INV A VOUT 1562-2 TA6a *V - ALSO AT PINS 4, 7, 14 & 17 Frequency Response 10 0 -10 - 20 -30 - 40 - 50 - 60 -70 60k 80k 100k FREQUENCY (Hz) 1562-2 TA06b AMPLITUDE RESPONSE 60 GROUP DELAY (s) GROUP DELAY 120k 0 140k LTC1562-2 TYPICAL APPLICATIONS LTC1562-2 9th Order 200kHz Lowpass Elliptic Filter RIN2 7.32k CIN2 27pF RIN1A 4.02k VIN 180pF RQ1 6.04k R21 8.06k 5V 0.1F RIN1B 4.02k 1 2 3 5 6 8 R23 12.4k 9 10 INVB V1B V2B V+ LTC1562-2 SHDN V2A V1A INVA AGND V2D V1D INVD CIN4 22pF 15 13 12 0.1F GAIN (dB) RIN3 10.2k RQ3 5.36k PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V - ALL RESISTORS ARE 1%, ALL CAPACITORS ARE 5% PACKAGE DESCRIPTION 0.205 - 0.212** (5.20 - 5.38) 0.005 - 0.009 (0.13 - 0.22) 0.022 - 0.037 (0.55 - 0.95) *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U U Amplitude Response INVC V1C V2C V- 20 19 RQ2 13k 10 0 -10 -20 16 - 5V -30 -40 -50 -60 R24 6.04k -70 -80 -90 -100 VOUT 10 100 FREQUENCY (kHz) 1000 1562-2 TA07b 1562-2 TA07a 18 R22 6.04k 11 RQ4 14.3k RIN4 6.04k Dimensions in inches (millimeters) unless otherwise noted. G Package 20-Lead Plastic SSOP (0.209) (LTC DWG # 05-08-1640) 0.278 - 0.289* (7.07 - 7.33) 20 19 18 17 16 15 14 13 12 11 0.301 - 0.311 (7.65 - 7.90) 1 2 3 4 5 6 7 8 9 10 0.068 - 0.078 (1.73 - 1.99) 0 - 8 0.0256 (0.65) BSC 0.010 - 0.015 (0.25 - 0.38) 0.002 - 0.008 (0.05 - 0.21) G20 SSOP 0595 15 LTC1562-2 TYPICAL APPLICATIONS 256kHz Linear Phase 6th Order Lowpass Filter with a 2nd Order Allpass Phase Equalizer, Single Supply RFF1 6.19k VIN RIN1 7.5k 1 RQ1 3.24k R21 6.81k 5V 0.1F R23 4.12k RQ3 7.32k 2 3 5 6 8 9 10 RIN3 4.12k INV B V1 B V2 B V+ LTC1562-2 INV C V1 C V2 C V- 20 19 18 16 * 13 12 11 VOUT RIN4 4.12k CIN4 22pF 5% R24 4.12k RQ4 7.32k RQ2 4.12k R22 6.19k RB1 1.54k SHDN V2 A V1 A INV A AGND V2 D V1 D INV D *GROUND ALSO AT PINS 4, 7, 14 & 17 ALL RESISTORS 1% METAL FILM Amplitude Response 10 0 -10 -20 -30 -40 -50 -60 -70 -80 10k 100k FREQUENCY (Hz) 1M 1562-2 TA04b 8 7 6 DELAY (s) GAIN (dB) 5 4 3 2 1 0 50 100 150 200 250 300 FREQUENCY (kHz) 350 400 RELATED PARTS PART NUMBER LTC1068-X LTC1560-1 LTC1562 DESCRIPTION Quad 2-Pole Switched Capacitor Building Block 5-Pole Elliptic Lowpass, fC = 1MHz/0.5MHz Quad 2-Pole Active RC, 10kHz to 150kHz COMMENTS Clock Tuned No External Components, SO8 Same Pinout as LTC1562-2 16 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com + U 15 1F 1562-2 TA04a Group Delay Response 1562-2 TA04c 15622f LT/TP 0599 4K * PRINTED IN USA (c) LINEAR TECHNOLOGY CORPORATION 1998 |
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