APPLICATIO S. LTC1563-2/LTC Active RC, 4th Order Lowpass Filter Family DESCRIPTIO FEATURES TYPICAL APPLICATIO

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1 FEATRES Extremely Easy to se A Single Resistor Value Sets the Cutoff Frequency (Hz < f C < khz) Extremely Flexible Different Resistor Values Allow Arbitrary Transfer Functions with or without Gain (Hz < f C < khz) Supports Cutoff Frequencies p to khz sing FilterCAD TM LTC-: nity-gain Butterworth Response ses a Single Resistor Value, Different Resistor Values Allow Other Responses with or without Gain LTC-: nity-gain Bessel Response ses a Single Resistor Value, Different Resistor Values Allow Other Responses with or without Gain Rail-to-Rail Input and Output Voltages Operates from a Single V (.V Min) to ±V Supply Low Noise: µv RMS for f C =.khz, µv RMS for f C Accuracy < ±% (Typ) DC Offset < mv Cascadable to Form th Order Lowpass Filters APPLICATIO S Replaces Discrete RC Active Filters and Modules Antialiasing Filters Smoothing or Reconstruction Filters Linear Phase Filtering for Data Communication Phase Locked Loops DESCRIPTIO Active RC, th Order Lowpass Filter Family The LTC -/LTC- are a family of extremely easy-to-use, active RC lowpass filters with rail-to-rail inputs and outputs and low DC offset suitable for systems with a resolution of up to bits. The LTC-, with a single resistor value, gives a unity-gain Butterworth response. The LTC-, with a single resistor value, gives a unity-gain Bessel response. The proprietary architecture of these parts allows for a simple resistor calculation: R = k (khz/f C ); f C = Cutoff Frequency where f C is the desired cutoff frequency. For many applications, this formula is all that is needed to design a filter. By simply utilizing different valued resistors, gain and other responses are achieved. The LTC-X features a low power mode, for the lower frequency applications, where the supply current is reduced by an order of magnitude and a near zero power shutdown mode. The LTC-Xs are available in the narrow SSOP- package (SO- footprint)., LTC and LT are registered trademarks of Linear Technology Corporation. FilterCAD is trademark of Linear Technology Corporation. TYPICAL APPLICATIO Single.V, Hz to khz Butterworth Lowpass Filter.V R R R LTC- B A R R V OT R k k k FREQEY (Hz) ( TA R ) Frequency Response R = M f C = Hz R = k k TA M

2 ABSOLTE MAXIMM RATINGS W W W (Note ) Total Supply Voltage ( to V )... V Maximum Input Voltage at Any Pin... (V.V) V PIN ( +.V) Power Dissipation... mw Operating Temperature Range LTCC... C to C LTCI... C to C Storage Temperature Range... C to C Lead Temperature (Soldering, sec)... C PACKAGE/ORDER INFORMATION A V TOP VIEW GN PACKAGE -LEAD PLASTIC SSOP B EN T JMAX = C, θ JA = C/ W NOTE: PINS LABELED ARE NOT CONNECTED INTERNALLY AND SHOLD BE CONNECTED TO THE SYSTEM GROND W ORDER PART NMBER LTC-CGN LTC-CGN LTC-IGN LTC-IGN GN PART MARKING I I ELECTRICAL CHARACTERISTICS The denotes specifications which apply over the full operating temperature range, otherwise specifications are T A = C. V S = Single.V, EN pin to logic low, Gain =, R FIL = R = R = R = R = R = R, specifications apply to both the high speed (HS) and low power () modes unless otherwise noted. Consult factory for Military grade parts. PARAMETER CONDITIONS MIN TYP MAX NITS Specifications for Both LTC- and LTC- Total Supply Voltage (V S ), HS Mode V Total Supply Voltage (V S ), Mode. V Output Voltage Swing High (B Pin) V S = V, f C =.khz, R FIL = k, R L = k to GND.. V HS Mode V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V V S = ±V, f C =.khz, R FIL = k, R L = k to GND.. V Output Voltage Swing Low (B Pin) V S = V, f C =.khz, R FIL = k, R L = k to GND.. V HS Mode V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V V S = ±V, f C =.khz, R FIL = k, R L = k to GND.. V Output Swing High (B Pin) V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V Mode V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V V S = ±V, f C =.khz, R FIL = k, R L = k to GND.. V Output Swing Low (B Pin) V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V Mode V S =.V, f C =.khz, R FIL = k, R L = k to GND.. V V S = ±V, f C =.khz, R FIL = k, R L = k to GND.. V DC Offset Voltage, HS Mode V S = V, f C =.khz, R FIL = k ±. ± mv (Section A Only) V S =.V, f C =.khz, R FIL = k ±. ± mv V S = ±V, f C =.khz, R FIL = k ±. ± mv DC Offset Voltage, Mode V S =.V, f C =.khz, R FIL = k ± ± mv (Section A Only) V S =.V, f C =.khz, R FIL = k ± ± mv V S = ±V, f C =.khz, R FIL = k ± ± mv DC Offset Voltage, HS Mode V S = V, f C =.khz, R FIL = k ±. ± mv (Input to Output, Sections A, B Cascaded) V S =.V, f C =.khz, R FIL = k ±. ± mv V S = ±V, f C =.khz, R FIL = k ±. ± mv DC Offset Voltage, Mode V S =.V, f C =.khz, R FIL = k ± ± mv (Input to Output, Sections A, B Cascaded) V S =.V, f C =.khz, R FIL = k ± ± mv V S = ±V, f C =.khz, R FIL = k ± ± mv

3 ELECTRICAL CHARACTERISTICS The denotes specifications which apply over the full operating temperature range, otherwise specifications are T A = C. V S = Single.V, EN pin to logic low, Gain =, R FIL = R = R = R = R = R = R, specifications apply to both the high speed (HS) and low power () modes unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX NITS DC Offset Voltage Drift, HS Mode V S = V, f C =.khz, R FIL = k µv/ C (Input to Output, Sections A, B Cascaded) V S =.V, f C =.khz, R FIL = k µv/ C V S = ±V, f C =.khz, R FIL = k µv/ C DC Offset Voltage Drift, Mode V S =.V, f C =.khz, R FIL = k µv/ C (Input to Output, Sections A, B Cascaded) V S =.V, f C =.khz, R FIL = k µv/ C V S = ±V, f C =.khz, R FIL = k µv/ C Voltage V S =.V, f C =.khz, R FIL = k... V Power Supply Current, HS Mode V S = V, f C =.khz, R FIL = k. ma V S =.V, f C =.khz, R FIL = k. ma V S = ±V, f C =.khz, R FIL = k ma Power Supply Current, Mode V S =.V, f C =.khz, R FIL = k.. ma V S =.V, f C =.khz, R FIL = k.. ma V S = ±V, f C =.khz, R FIL = k.. ma Shutdown Mode Supply Current V S =.V, f C =.khz, R FIL = k µa EN Input V S = V. V Logic Low Level V S =.V V V S = ±V V EN Input V S = V. V Logic High Level V S =.V. V V S = ±V. V V S = V. V Logic Low Level V S =.V V V S = ±V V V S = V. V Logic High Level V S =.V. V V S = ±V. V LTC- Transfer Function Characteristics Cutoff Frequency Range, f C V S = V. khz HS Mode V S =.V. khz (Note ) V S = ±V. khz Cutoff Frequency Range, f C V S =.V.. khz Mode V S =.V.. khz (Note ) V S = ±V.. khz Cutoff Frequency Accuracy, HS Mode V S = V, R FIL = k. ±.. % f C =.khz V S =.V, R FIL = k. ±.. % V S = ±V, R FIL = k. ±.. % Cutoff Frequency Accuracy, HS Mode V S = V, R FIL = k ±.. % V S =.V, R FIL = k ±.. % V S = ±V, R FIL = k ±.. % Cutoff Frequency Accuracy, Mode V S =.V, R FIL = k ±. % f C =.khz V S =.V, R FIL = k ±. % V S = ±V, R FIL = k ±. % Cutoff Frequency Temperature Coefficient (Note ) ± ppm/ C Passband Gain, HS Mode, f C =.khz Test Frequency =.khz (. f C ).. db V S =.V, R FIL = k Test Frequency =.khz (. f C ).. db

4 ELECTRICAL CHARACTERISTICS The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = C. V S = Single.V, EN pin to logic low, Gain =, R FIL = R = R = R = R = R = R, specifications apply to both the high speed (HS) and low power () modes unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX NITS Stopband Gain, HS Mode, f C =.khz Test Frequency =.khz ( f C ). d B V S =.V, R FIL = k Test Frequency =.khz ( f C ) db Passband Gain, HS Mode, Test Frequency =.khz (. f C ).. db V S =.V, R FIL = k Test Frequency = khz (. f C ).. db Stopband Gain, HS Mode, Test Frequency = khz (. f C ).. db V S =.V, R FIL = k Test Frequency = khz (. f C ).. db Passband Gain, Mode, f C =.khz Test Frequency =.khz (. f C ).. db V S =.V, R FIL = k Test Frequency =.khz (. f C )... db Stopband Gain, Mode, f C =.khz Test Frequency =.khz ( f C ) db V S =.V, R FIL = k Test Frequency =.khz ( f C ). db LTC- Transfer Function Characteristics Cutoff Frequency Range, f C V S = V. khz HS Mode V S =.V. khz (Note ) V S = ±V. khz Cutoff Frequency Range, f C V S =.V.. khz Mode V S =.V.. khz (Note ) V S = ±V.. khz Cutoff Frequency Accuracy, HS Mode V S = V, R FIL = k ±. % f C =.khz V S =.V, R FIL = k ±. % V S = ±V, R FIL = k ±. % Cutoff Frequency Accuracy, HS Mode V S = V, R FIL = k ± % V S =.V, R FIL = ± % V S = ±V, R FIL = k ± % Cutoff Frequency Accuracy, Mode V S =.V, R FIL = k ± % f C =.khz V S =.V, R FIL = k ± % V S = ±V, R FIL = k ± % Cutoff Frequency Temperature Coefficient (Note ) ± ppm/ C Passband Gain, HS Mode, f C =.khz Test Frequency =.khz (. f C )... db V S =.V, R FIL = k Test Frequency =.khz (. f C )... db Stopband Gain, HS Mode, f C =.khz Test Frequency =.khz ( f C ). db V S =.V, R FIL = k Test Frequency =.khz ( f C ). db Passband Gain, HS Mode, Test Frequency =.khz (. f C )... db V S =.V, R FIL = k Test Frequency = khz (. f C )... db Stopband Gain, HS Mode, Test Frequency = khz (. f C ). db V S =.V, R FIL = k Test Frequency = khz (. f C ). db Passband Gain, Mode, f C =.khz Test Frequency =.khz (. f C )... db V S =.V, R FIL = k Test Frequency =.khz (. f C )... db Stopband Gain, Mode, f C =.khz Test Frequency =.khz ( f C ). db V S =.V, R FIL = k Test Frequency =.khz ( f C ). db Note : Absolute Maximum Ratings are those value beyond which the life of a device may be impaired. Note : The minimum cutoff frequency of the LTC is arbitrarily listed as Hz. The limit is arrived at by setting the maximum resistor value limit at MΩ. The LTC can be used with even larger valued resistors. When using very large values of resistance careful layout and thorough assembly practices are required. There may also be greater DC offset at high temperatures when using such large valued resistors. Note : The cutoff frequency temperature drift at low frequencies is as listed. At higher cutoff frequencies (approaching.khz in low power mode and approaching khz in high speed mode) the internal amplifier s bandwidth can effect the cutoff frequency. At these limits the cutoff frequency temperature drift is ±ppm/ C.

5 TYPICAL PERFOR A CE CHARACTERISTICS W.. Output Voltage Swing High vs Load Resistance V S = SINGLE.V.. Output Voltage Swing High vs Load Resistance V S = SINGLE V.. Output Voltage Swing High vs Load Resistance V S = ±V OTPT VOLTAGE (V).... HS MODE MODE OTPT VOLTAGE (V)... HS MODE MODE OTPT VOLTAGE (V)... HS MODE MODE.... k k k LOAD RESISTAE LOAD TO GROND (Ω). k k k LOAD RESISTAE LOAD TO GROND (Ω). k k k LOAD RESISTAE LOAD TO GROND (Ω) G G G Output Voltage Swing Low vs Load Resistance Output Voltage Swing Low vs Load Resistance Output Voltage Swing Low vs Load Resistance OTPT VOLTAGE (V)..... V S = SINGLE.V HS MODE MODE k k k LOAD RESISTAE LOAD TO GROND (Ω) OTPT VOLTAGE (V)..... V S = SINGLE V HS MODE MODE k k k LOAD RESISTAE LOAD TO GROND (Ω) OTPT VOLTAGE (V) HS MODE V S = ±V. MODE. k k k LOAD RESISTAE LOAD TO GROND (Ω) G G G. THD + Noise vs Input Voltage THD + Noise vs Input Voltage THD + Noise vs Input Voltage.V SPPLY V SPPLY ±V SPPLY f C =.khz LOW POWER MODE f IN = khz INPT VOLTAGE (V P-P )..V SPPLY V SPPLY f C =.khz HIGH SPEED MODE f IN = khz ±V SPPLY INPT VOLTAGE (V P-P )..V SPPLY V SPPLY HIGH SPEED MODE f IN = khz ±V SPPLY INPT VOLTAGE (V P-P ) G G G

6 TYPICAL PERFOR A CE CHARACTERISTICS THD + Noise vs Input Frequency THD + Noise vs Input Frequency THD + Noise vs Input Frequency V S = SINGLE.V LOW POWER MODE f C =.khz V P-P V P-P W V S = SINGLE.V HIGH SPEED MODE f C =.khz V P-P V P-P V S = SINGLE V HIGH SPEED MODE V P-P V P-P INPT FREQEY (khz) INPT FREQEY (khz) INPT FREQEY (khz) G G G THD + Noise vs Input Frequency THD + Noise vs Input Frequency THD + Noise vs Input Frequency V S = SINGLE V LOW POWER MODE f C =.khz V P-P V P-P V P-P V S = SINGLE V HIGH SPEED MODE f C =.khz V P-P V P-P V P-P V S = SINGLE V HIGH SPEED MODE V P-P V P-P V P-P INPT FREQEY (khz) INPT FREQEY (khz) INPT FREQEY (khz) G G G THD + Noise vs Input Frequency THD + Noise vs Input Frequency THD + Noise vs Input Frequency V S = ±V LOW POWER MODE f C =.khz V P-P V P-P V P-P V S = ±V HIGH SPEED MODE f C =.khz V P-P V P-P V P-P V S = ±V HIGH SPEED MODE V P-P V P-P INPT FREQEY (khz) INPT FREQEY (khz) V P-P INPT FREQEY (khz) G G G

7 TYPICAL PERFOR A CE CHARACTERISTICS W THD + Noise vs Output Load THD + Noise vs Output Load Output Voltage Noise vs Cutoff Frequency MODE, V P-P SIGNAL HS MODE, V P-P SIGNAL MODE, V P-P SIGNAL V S = SINGLE V f C =.khz f IN = khz HS MODE, V P-P SIGNAL V P-P, khz V P-P, khz V S = SINGLE V HIGH SPEED MODE f IN = khz, khz V P-P, khz V P-P, khz TOTAL INTEGRATED NOISE (µvrms) T A = C MODE HS MODE OTPT LOAD RESISTAE LOAD TO GROND (kω) OTPT LOAD RESISTAE LOAD TO GROND (kω) k. f C (Hz) G G G THD + Noise vs Output Load THD + Noise vs Output Load Stopband Gain vs Input Frequency MODE, V P-P SIGNAL HS MODE, V P-P SIGNAL V S = ±V f C =.khz f IN = khz MODE, V P-P SIGNAL HS MODE, V P-P SIGNAL OTPT LOAD RESISTAE LOAD TO GROND (kω) G V P-P, khz V P-P, khz V P-P, khz V P-P, khz V S = ±V HIGH SPEED MODE f IN = khz, khz OTPT LOAD RESISTAE LOAD TO GROND (kω) G LTC- LTC- k k M M M FREQEY (Hz) G CROSSTALK (db) Crosstalk Rejection vs Frequency DAL SECOND ORDER BTTERWORTH f C =.khz HS OR MODE CROSSTALK (db) Crosstalk Rejection vs Frequency DAL SECOND ORDER BTTERWORTH HIGH SPEED MODE FREQEY (khz) G k k k M FREQEY (Hz) G

8 PIN FTIONS (Pin ): Low Power. The LTC-X has two operating modes. Most applications use the part s High Speed operating mode. Some lower frequency, lower gain applications can take advantage of the Low Power mode. When placed in the Low Power mode, the supply current is nearly an order of magnitude lower than the High Speed mode. Refer to the Applications Information section for more information on the Low Power mode. The LTC-X is in the High Speed mode when the input is at a logic high level or is open-circuited. A small pull-up current source at the input defaults the LTC-X to the High Speed mode if the pin is left open. The part is in the Low Power mode when the pin is pulled to a logic low level or connected to V., (Pins, ): Summing Pins. These pins are a summing point for signals fed forward and backward. Capacitance on the or pin will cause excess peaking of the frequency response near the cutoff frequency. The three external resistors for each section should be located as close as possible to the summing pin to minimize this effect. Refer to the Applications Information section for more details. (Pins,,,, ): These pins are not connected internally. For best performance, they should be connected to ground., (Pins, ): Inverting Input. Each of the INV pins is an inverting input of an op amp. Note that the INV pins are high impedance, sensitive nodes of the filter and very susceptible to coupling of unintended signals. Capacitance on 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. A, B (Pins, ): Lowpass Output. These pins are the rail-to-rail outputs of an op amp. Each output is designed to drive a nominal net load of kω and pf. Refer to the Applications Information section for more details on output loading effects. (Pin ): Analog Ground. The pin is the midpoint of an internal resistive voltage divider developing a potential halfway between the and V pins. The equivalent series resistance is nominally kω. This serves as an internal ground reference. Filter performance will reflect the quality of the analog signal ground. An analog ground plane surrounding the package is recommended. The analog ground plane should be connected to any digital ground at a single point. Figures and show the proper connections for dual and single supply operation. V, (Pins, ): The V and pins should be bypassed with capacitors to an adequate analog ground or ground plane. These capacitors should be connected as closely as possible to the supply pins. Low noise linear supplies are recommended. Switching supplies are not recommended as they will decrease the filter s dynamic range. Refer to Figures and for the proper connections for dual and single supply operation. EN (Pin ): ENABLE. When the EN input goes high or is open-circuited, the LTC-X enters a shutdown state and only junction leakage currents flow. The pin, the A output and the B output assume high impedance states. If an input signal is applied to a complete filter circuit while the LTC-X is in shutdown, some signal will normally flow to the output through passive components around the inactive part. A small internal pull-up current source at the EN input defaults the LTC to the shutdown state if the EN pin is left floating. Therefore, the user must connect the EN pin to V (or a logic low) to enable the part for normal operation.

9 PIN FTIONS Dual Supply Power and Ground Connections Single Supply Power and Ground Connections ANALOG GROND PLANE V LTC-X A B ANALOG GROND PLANE + LTC-X A B SINGLE POINT SYSTEM GROND DIGITAL GROND PLANE (IF ANY) SINGLE POINT SYSTEM GROND DIGITAL GROND PLANE (IF ANY) PF PF BLOCK DIAGRA W R R R R R R V OT SHTDOWN SWITCH CA CB k k CA + A CB + B SHTDOWN SWITCH EN V LTC-X PATENT PENDING BD

10 APPLICATIONS INFORMATION Functional Description W The are a family of easy-to-use, th order lowpass filters with rail-to-rail operation. The LTC-, with a single resistor value, gives a unity-gain filter approximating a Butterworth response. The LTC-, with a single resistor value, gives a unity-gain filter approximating a Bessel (linear phase) response. The proprietary architecture of these parts allows for a simple unity-gain resistor calculation: R = k(khz/f C ) where f C is the desired cutoff frequency. For many applications, this formula is all that is needed to design a filter. For example, a khz filter requires a.k resistor. In practice, a.k resistor would be used as this is the closest E, % value available. The LTC-X is constructed with two nd order sections. The output of the first section (section A) is simply fed into the second section (section B). Note that section A and section B are similar, but not identical. The parts are designed to be simple and easy to use. By simply utilizing different valued resistors, gain, other transfer functions and higher cutoff frequencies are achieved. For these applications, the resistor value calculation gets more difficult. The tables of formulas provided later in this section make this task much easier. For best results, design these filters using FilterCAD Version. (or newer) or contact the Linear Technology Filter Applications group for assistance. Cutoff Frequency (f C ) and Gain limitations The LTC-X has both a maximum f C limit and a minimum f C limit. The maximum f C limit (khz in High Speed mode and.khz in the Low Power mode) is set by the speed of the LTC-X s op amps. At the maximum f C, the gain is also limited to unity. A minimum f C is dictated by the practical limitation of reliably obtaining large valued, precision resistors. As the desired f C decreases, the resistor value required increases. When f C is Hz, the resistors are M. Obtaining a reliable, precise M resistance between two points on a printed circuit board is somewhat difficult. For example, a M resistor with only MΩ of stray, layout related resistance in parallel, yields a net effective resistance of.m and an error of %. Note that the gain is also limited to unity at the minimum f C. At intermediate f C, the gain is limited by one of the two reasons discussed above. For best results, design filters with gain using FilterCAD Version (or newer) or contact the Linear Technology Filter Applications Group for assistance. While the simple formula and the tables in the applications section deliver good approximations of the transfer functions, a more accurate response is achieved using FilterCAD. FilterCAD calculates the resistor values using an accurate and complex algorithm to account for parasitics and op amp limitations. A design using FilterCAD will always yield the best possible design. By using the FilterCAD design tool you can also achieve filters with cutoff frequencies beyond khz. Cutoff frequencies up to khz are attainable. Contact the Linear Technology Filter Applications Group for a copy the FilterCAD software. FilterCAD can also be downloaded from our website at DC Offset, Noise and Gain Considerations The LTC-X is DC offset trimmed in a -step manner. First, section A is trimmed for minimum DC offset. Next, section B is trimmed to minimize the total DC offset (section A plus section B). This method is used to give the minimum DC offset in unity gain applications and most higher gain applications. For gains greater than unity, the gain should be distributed such that most of the gain is taken in section A, with section B at a lower gain (preferably unity). This type of gain distribution results in the lowest noise and lowest DC offset. For high gain, low frequency applications, all of the gain is taken in section A, with section B set for unity-gain. In this configuration, the noise and DC offset is dominated by those of section A. At higher frequencies, the op amps finite bandwidth limits the amount of gain that section A can reliably achieve. The gain is more evenly distributed in this case. The noise and DC offset of section A is now multiplied by the gain of section B. The result is slightly higher noise and offset.

11 APPLICATIONS INFORMATION W Output Loading: Resistive and Capacitive The op amps of the LTC-X have a rail-to-rail output stage. To obtain maximum performance, the output loading effects must be considered. Output loading issues can be divided into resistive effects and capacitive effects. Resistive loading affects the maximum output signal swing and signal distortion. If the output load is excessive, the output swing is reduced and distortion is increased. All of the output voltage swing testing on the LTC-X is done with R = k and a k load resistor. For best undistorted output swing, the output load resistance should be greater than k. Capacitive loading on the output reduces the stability of the op amp. If the capacitive loading is sufficiently high, the stability margin is decreased to the point of oscillation at the output. Capacitive loading should be kept below pf. Good, tight layout techniques should be maintained at all times. These parts should not drive long traces and must never drive a long coaxial cable. When probing the LTC-X, always use a x probe. Never use a x probe. A standard x probe has a capacitance of pf to pf while a x probe s capacitance can be as high as pf. The use of a x probe will probably cause oscillation. For larger capacitive loads, a series isolation resistor can be used between the part and the capacitive load. If the load is too great, a buffer must be used. Layout Precautions The LTC-X is an active RC filter. The response of the filter is determined by the on-chip capacitors and the external resistors. Any external, stray capacitance in parallel with an on-chip capacitor, or to an AC ground, can alter the transfer function. Capacitance to an AC ground is the most likely problem. Capacitance on the A or B pins does not affect the transfer function but does affect the stability of the op amps. Capacitance on the and pins will affect the transfer function somewhat and will also affect the stability of the op amps. Capacitance on the and pins alters the transfer function of the filter. These pins are the most sensitive to stray capacitance. Stray capacitance on these pins results in peaking of the frequency response near the cutoff frequency. Poor layout can give.db to db of excess peaking. To minimize the effects of parasitic layout capacitance, all of the resistors for section A should be placed as close as possible to the pin. Place the R resistor first so that it is as close as possible to the pin on one end and as close as possible to the pin on the other end. se the same strategy for the layout of section B, keeping all of the resistors as close as possible to the node and first placing R between the and pins. It is also best if the signal routing and resistors are on the same layer as the part without any vias in the signal path. Figure illustrates a good layout using the LTC-X with surface mount size resistors. An even tighter layout is possible with smaller resistors. R R R LTC-X R Figure. PC Board Layout R R V OT F Single Pole Sections and Odd Order Filters The LTC is configured to naturally form even ordered filters (nd, th, th and th). With a little bit of work, single pole sections and odd order filters are easily achieved. To form a single pole section you simply use the op amp, the on-chip C capacitor and two external resistors as shown in Figure. This gives an inverting section with the gain set by the R-R ratio and the pole set by the R-C time constant. You can use this pole with a nd order section to form a noninverting gain rd order filter or as a stand alone inverting gain single pole filter. Figure illustrates another way of making odd order filters. The R input resistor is split into two parts with an additional capacitor connected to ground in between the resistors. This TEE network forms a single real pole. RB

12 APPLICATIONS INFORMATION W should be much larger than RA to minimize the interaction of this pole with the nd order section. This circuit is useful in forming dual rd order filters and th order filters with a single LTC part. By cascading two parts, th order and th order filters are achieved. RA C P RB S R INV R C R R V OT C (OPEN) + S INV C C + / LTC RA RB F P = π RA RB C P RA + RB ( ) F / LTC R DC GAIN = R F P = π R C LTC-: CA =.pf, CB =.pf LTC-: CA = pf, CB =.pf Figure F You can also use the TEE network in both sections of the part to make a th order filter. This th order filter does not conform exactly to the textbook responses. Textbook responses (Butterworth, Bessel, Chebyshev etc.) all have three complex pole pairs. This filter has two complex pole pairs and two real poles. The textbook response always has one section with a low Q value between. and.. By replacing this low Q section with two real poles (two real poles are the same mathematically as a complex pole pair with a Q of.) and tweaking the Q of the other two complex pole pair sections you end up with a filter that is indistinguishable from the textbook filter. The Typical Applications section illustrates a khz, th order pseudo- Butterworth filter. FilterCAD is a valuable tool for custom filter design and tweaking textbook responses. (OPEN) S C / LTC INV Figure What To Do with An nused Section If the LTC is used as a nd or rd order filter, one of the sections is not used. Do not leave this section unconnected. If the section is left unconnected, the output is left to float and oscillation may occur. The unused section should be connected as shown in Figure with the INV pin connected to the pin and the S pin left open. + C Figure F

13 APPLICATIONS INFORMATION th Order Filter Responses sing the LTC- R R R LTC- A B F V OT Figure. th Order Filter Connections (Power Supply, Ground, EN and Connections Not Shown for Clarity). Table Shows Resistor Values W R R R BTTERWORTH.dB RIPPLE CHEBYSHEV.dB RIPPLE CHEBYSHEV NORMALIZED TO f C = Hz. FREQEY (Hz).. Fa Figure a. Frequency Response BTTERWORTH.dB RIPPLE CHEBYSHEV.dB RIPPLE CHEBYSHEV OTPT VOLTAGE (V).... BTTERWORTH.dB RIPPLE CHEBYSHEV.dB RIPPLE CHEBYSHEV NORMALIZED TO f C = Hz. FREQEY (Hz) NORMALIZED TO f C = Hz.... TIME (s).. Fb FC Figure b. Passband Frequency Response Figure c. Step Response Table. Resistor Values, Normalized to khz Cutoff Frequency (f C ), Figure. The Passband Gain, of the th Order LTC- Lowpass Filter, Is Set to nity. (Note ).db RIPPLE.dB RIPPLE BTTERWORTH CHEBYSHEV CHEBYSHEV Mode Max f C.kHz khz khz HS Mode Max f C khz khz khz R = R = k(khz/f C ).k(khz/f C ).k(khz/f C ) R = k(khz/f C ).k(khz/f C ).k(khz/f C ) R = R = k(khz/f C ) k(khz/f C ).k(khz/f C ) R = k(khz/f C ).k(khz/f C ).k(khz/f C ) Example: In HS mode,.db ripple Chebyshev, khz cutoff frequency, R = R = k.k (%), R =.k.k (%), R = R = k k (%), R =.k.k (%) Note : The resistor values listed in this table provide good approximations of the listed transfer functions. For the optimal resistor values, higher gain or other transfer functions, use FilterCAD Version. (or newer) or contact the Linear Technology Filter Applications group for assistance.

14 APPLICATIONS INFORMATION W th Order Filter Responses sing the LTC- R R R LTC- A B R R R F V OT Figure. th Order Filter Connections (Power Supply, Ground, EN and Connections Not Shown for Clarity). Table Shows Resistor Values BESSEL TRANSITIONAL GASSIAN TO db TRANSITIONAL GASSIAN TO db NORMALIZED TO f C = Hz. FREQEY (Hz) Fa Figure a. Frequency Response OTPT VOLTAGE (V) TIME (s) BESSEL TRANSITIONAL GASSIAN TO db TRANSITIONAL GASSIAN TO db NORMALIZED TO f C = Hz Figure b. Step Response.. Fb OTPT VOLTAGE (V)... BESSEL TRANSITIONAL GASSIAN TO db TRANSITIONAL GASSIAN TO db NORMALIZED TO f C = Hz... TIME (s) Fc. Figure c. Step Response Settling Table. Resistor Values, Normalized to khz Cutoff Frequency (f C ), Figure. The Passband Gain, of the th Order LTC- Lowpass Filter, Is Set to nity. (Note ) TRANSITIONAL TRANSITIONAL BESSEL GASSIAN TO db GASSIAN TO db Mode Max f C.kHz khz khz HS Mode Max f C khz khz khz R = R = k(khz/f C ).k(khz/f C ) k(khz/f C ) R = k(khz/f C ).k(khz/f C ).k(khz/f C ) R = R = k(khz/f C ).k(khz/f C ).k(khz/f C ) R = k(khz/f C ).k(khz/f C ).k(khz/f C ) Note : The resistor values listed in this table provide good approximations of the listed transfer functions. For the optimal resistor values, higher gain or other transfer functions, use FilterCAD Version. (or newer) or contact the Linear Technology Filter Applications group for assistance.

15 TYPICAL APPLICATIO S ±V,.mA Supply Current, khz, th Order,.dB Ripple Chebyshev Lowpass Filter Frequency Response k V LTC- k k B A k.k k TA ENABLE V OT V FREQEY (khz) TA Single.V, ma Supply Current, khz th Order Butterworth Lowpass Filter.V k k k LTC- A B k.k k.k k k LTC- B A V OT k k k TA ENABLE Frequency Response FREQEY (khz) TA

16 TYPICAL APPLICATIO S khz, th Order Pseudo-Butterworth Frequency Response R A.k R B.k C pf R.k R.k LTC- A B R A.k R B.k R.k R.k.V V OT k k FREQEY (Hz) M C pf TA TAa TEXTBOOK BTTERWORTH PSEDO-BTTERWORTH f O = khz Q =. f O = khz Q =. f O = khz Q =. f O = khz Q =. f O = khz Q =. f O = khz Real Poles f O = khz Real Poles The complex, nd order section of the textbook design with the lowest Q is replaced with two real first order poles. The Q of another section is slightly altered such that the final filter s response is indistinguisable from a textbook Butterworth response. Other Pseudo Filter Response Coefficients (All f O Are Normalized for a Hz Filter Cutoff) BESSEL.dB RIPPLE CHEBYSHEV.dB RIPPLE CHEBYSHEV TRANSITIONAL GASSIAN TO db TRANSITIONAL GASSIAN TO db f O..... Q..... f O..... Q..... f O..... f O..... The f O and Q values listed above can be entered in FilterCAD s Enhanced Design window as a custom response filter. After entering the coefficients, FilterCAD will produce a schematic of the circuit. The procedure is as follows:. After starting FilterCAD, select the Enhanced Design window.. Select the Custom Response and set the custom F C to Hz.. In the Coefficients table, go to the Type column and click on the types listed and set the column with two types and two types. This sets up a template of a th order filter with two nd order lowpass sections and two st order lowpass sections.. Enter the f O and Q coefficients as listed above. For a Butterworth filter, use the same coefficients as the example circuit above except set all of the f O to Hz.. Set the custom F C to the desired cutoff frequency. This will automatically multiply all of the f O coefficients. You have now finished the design of the filter and you can click on the frequency response or step response buttons to verify the filter s response.. Click on the Implement button to go on to the filter implementation stage.. In the Enhanced Implement window, click on the Active RC button to choose the LTC- part. You are now done with the filter s implementation. Click on the schematic button to view the resulting circuit.

17 TYPICAL APPLICATIO S khz, th Order,.dB Ripple Chebyshev Lowpass Filter Driving the LTC, -Bit ADC V R A.k V R B k C pf R.k R k LTC- A B R k R k R.k pf.ω.µf µf V + µf V µf ++ + A IN A IN V REF REFCOMP DV DD DGND V SS LTC AV DD AV DD SHDN CS CONVST RD BSY OV DD OGND + µf V Ω µf µp CONTROL LINES TO -BIT PARALLEL BS V OR V + µf + TA AMPLITDE (db) Point FFT of the Output Data f MPLE =.khz f IN = khz SINAD = db THD =.db.... FREQEY (khz) TAa

18 TYPICAL APPLICATIO S khz Wideband Bandpass th Order Bessel Lowpass at khz with Two Highpass Poles at.khz Yields a Wideband Bandpass Centered at khz V C pf R k R k R k LTC- A B R k R k TA V R k C pf V OT k k k M FREQEY (Hz) TAa To design these wideband bandpass filters with the LTC, start with a th order lowpass filter and add two highpass poles with the input, AC coupling capacitors. The lowpass cutoff frequency and highpass pole frequencies depend on the specific application. Some experimentation of lowpass and highpass frequencies is required to achieve the desired response. FilterCAD does not directly support this configuration. se the custom design window in FilterCAD get the desired response and then use FilterCAD to give the schematic for the lowpass portion of the filter. Calculate the two highpass poles using the following formulae: f O ( HPA)=, fo( HPB)= π R C π R C The design process is as follows:. After starting FilterCAD, select the Enhanced Design window.. Choose a th order Bessel or Butterworth lowpass filter response and set the cutoff frequency to the high frequency corner of the desired bandpass.. Click on the custom response button. This copies the lowpass coefficients into the custom design Coefficients table.. In the Coefficients table, the first two rows are the Type with the f O and Q as previously defined. Go to the third and fourth rows and click on the Type column (currently a hyphen is in this space). Change the Type of each of these rows to type HP. This sets up a template of a th order filter with two nd order lowpass sections and two st order highpass sections.. Change the frequency of the highpass (HP) poles to get the desired frequency response.. You may have to perform this loop several times before you close in on the correct response.. Once you have reached a satisfactory response, note the highpass pole frequencies. The HP highpass poles must now be removed from the Custom design coefficients table. After removing the highpass poles, click on the Implement button to go on to the filter implementation stage.. In the Enhanced Implement window, click on the Active RC button and choose the LTC- part. Click on the schematic button to view the resulting circuit.. You now have the schematic for the th order lowpass part of the design. Now calculate the capacitor values from the following formulae: C R f HPA C =, = π π R f HPB O ( ) O ( )

19 TYPICAL APPLICATIO S khz,.db Ripple, th Order Chebyshev with db of DC Gain V R.k R.k R.k LTC- A B R k R.k TA V R k V OT k k M FREQEY (Hz) TAa PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. GN Package -Lead Plastic SSOP (Narrow.) (LTC DWG # --)..* (..). (.) REF.. (..)..** (..).. (..). ±. (. ±.) TYP.. (..).. (..).. (..) * DIMENSION DOES NOT ILDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED." (.mm) PER SIDE ** DIMENSION DOES NOT ILDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED." (.mm) PER SIDE.. (..). (.) BSC GN (SSOP) 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.

20 TYPICAL APPLICATIO S Single.V, khz Bessel Lowpass Filter Frequency Response k k k LTC- A B k k.v V OT k ENABLE TA k k FREQEY (Hz) M TA RELATED PARTS PART NMBER DESCRIPTION COMMENTS LTC- -Pole Elliptic Lowpass, f C = MHz/.MHz No External Components, SO- LTC niversal Quad -Pole Active RC khz < f O < khz LTC- niversal Quad -Pole Active RC khz < f O < khz LTC- Low Power -Pole Delay Equalized Elliptic Lowpass f C < khz, One Resistor Sets f C, SO- LTC- -Pole Delay Equalized Elliptic Lowpass f C < khz, One Resistor Sets f C, SO- LTC- khz Continuous Time, Linear Phase Lowpass f C = khz, Differential In/Out Linear Technology Corporation McCarthy Blvd., Milpitas, CA - ()- FAX: () - f LT/TP K PRINTED IN LINEAR TECHNOLOGY CORPORATION