AN-797 APPLICATION NOTE One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 Tel: 781/329-4700 Fax: 781/461-3113 www.analog.com Quad Precision Op Amp Evaluation Board by Giampaolo Marino and Steve Ranta INTRODUCTION Using the superposition principle, we write The EVAL-PRAOPAMP-4R/4RU is an evaluation board V = R6 / R4 V R4/R V + ( ) ( ) HP 1 JUMP LP which accommodates quad op amps in SOIC and TSSOP packages. It is meant to provide the user with multiple 1 + R4/ R6 + R4/ R / ( ) ( ) JUMP V BP choices and extensive flexibility for different applica - 1 + R / R5 ( ) JUMP1 tions circuits and configurations. Since A2 and A3 are integrators, we have: This board is not intended to be used with high frequency components or high speed amplifiers. However, it pro vides V = 1/R8 C6s V and V = 1/ R13 C8 V ( ) ( ) BP HP LP BP the user with many combinations for various circuit After extended simplifications we have the following types including active lfi ters, instrumentation amplifiers, expressions: composite amplifier, and external frequency compensa - tion circuits. For examples of application circuits refer to ADI amplifier data sheets under the applications section. = R4/R / R8 C6 R13 C8 ( ) ( ) o JUMP This application note will present different implementa- and tions of filter design using quad amplifiers. 1 + R /R5 R4 R8 C6/R R13 C8 / ( ) ( ) JUMP1 JUMP Q = STATE VARIABLE FILTER (INVERTING) 1 + R4/ R6 + R4/R ( ) The SV lfi ter uses two integrators and a summing am plifier JUMP to provide second-order low-pass, band-pass, and high- H = R4/R6, H = 1 + R /R5 / ( ) OHP OBP JUMP1 pass responses. A fourth op amp can be used to combine 1 + R6 /R + R6 /R4 , H = R /R6 ( ) JUMP OLP JUMP the existing responses and synthesize the notch or the all-pass responses. From the above expressions we observe that Q depends This filter circuit can be easily constructed using the on the resistor ratio R /R5. We therefore expect Q JUMP1 quad EVAL PRAOPAMP board and following the sche- to be much less sensitive to resistance tolerances and matic in Figure 1. drift. Indeed, with proper component selection and circuit construction, the SV filter can easily yield de pend- able Qs in the range of hundreds. For best results, it is advisable to use metal film resistors and polystyrene or polycarbonate capacitors, and properly bypass the op amp supplies. R * JUMP C6 C8 R6 R4 R8 R13 V V V CC CC CC V A IN 2 6 9 A1 A2 A3 V+ V+ V+ 1 7 8 V = V A V = V B V = V C HP OUT BP OUT LP OUT V V V 3 5 10 V V V R * EE EE EE R5 JUMP1 *R AND R IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD. JUMP JUMP1 Figure 1. State Variable Filter (Inverting) REV. 0AN-797 AN-797 The SV lfi ter is usually implemented with R4 = R6 = R , THE BIQUAD FILTER JUMP R8 = R13 = R and C6 = C8 = C. So, the above expressions This particular filter implementation consists of two simplify to integrators. The third op amp is a unity gain inverting amplifier whose sole purpose is to provide polarity rever sal. = 1/RC Q = 1/3 1 + R /R5 ( ) o JUMP1 Unlike the SV lfi ter, the biquad yields only two significant H = 1 H = Q H = 1 OHP OBP OLP responses however, since all its op amps are operated in The filter is tuned as follows: the inverting mode, the circuit is immune from common- 1. Adjust R6 for desired magnitude of the response mode limitations. of interest. From the circuit above we have: 2. Adjust R8 or R13 to tune . o 3. Adjust the ratio R /R5 to tune Q. H = R4/ R6 H = R /R6 JUMP1 OBP OLP JUMP = 1/ R8 R C6 C4 Q = R4 C4 / R8 R C6 ( ) ( ) o JUMP JUMP STATE VARIABLE FILTER (NONINVERTING) Another popular type of state variable filter can be The biquad lfi ter is usually implemented with R8 = R = R JUMP formed by just moving the input signal from the C4 = C6 = C, after which the above expressions simplify as: inverting to the noninverting side of A1, resulting in the H = R4/ R6 H = R/R6 noninverting state variable filter. See Figure 2. OBP OLP = 1/RC Q = R4/ R o By properly choosing the following components: This filter is tuned as follows: R4 = R6 R8 = R13 R5 = R C6 = C8 JUMP 1. Adjust R8 or R to tune . JUMP o It can be shown that: 2. Adjust R4 to tune Q. = 1/R8C6 Q = 1+ R /R3 3. Adjust R6 for desired values H or of H . o JUMP1 OBP OLP H = 1/Q H = 1 H = 1/Q OHP OBP OLP JUMP V A TO V C IN OUT R6 C6 C8 R4 R8 R13 V V V CC CC CC 2 6 9 A1 A2 A3 V+ V+ V+ 1 7 8 V = V A V = V B V = V C HP OUT BP OUT LP OUT 3 V 5 V 10 V V R * V V EE EE EE R5 JUMP +V A IN R3 *R IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD. JUMP Figure 2. State Variable Filter (Noninverting) R * JUMP R4 C4 C6 R8 R13 R16 V V V CC CC CC R6 2 6 9 A1 A2 A3 V+ V+ V+ 1 7 8 V A V = V A V = V B V = V C HP OUT BP OUT LP OUT IN V V V 3 5 10 V V V EE EE EE *R IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD. JUMP Figure 3. Biquad Filter 2 REV. 0 REV. 0 3