57664391956299AN_797_0| Application Note
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AN-797 APPLICATION NOTE
Quad Precision Op Amp Evaluation Board
by Giampaolo Marino and Steve Ranta
INTRODUCTION The EVAL-PRAOPAMP- 4R/4RU is an evaluation board which accommodates quad op amps in SOIC and TSSOP packages. It is meant to provide the user with multiple choices and extensive flexibility for different applications circuits and configurations. This board is not intended to be used with high frequency components or high speed amplifiers. However, it provides the user with many combinations for various circuit types including active filters, instrumentation amplifiers, composite amplifier, and external frequency compensation circuits. For examples of application circuits refer to ADI amplifier data sheets under the applications section. This application note will present different implementations of filter design using quad amplifiers. STATE VARIABLE FILTER (INVERTING) The SV filter uses two integrators and a summing amplifier to provide second-order low-pass, band-pass, and highpass responses. A fourth op amp can be used to combine the existing responses and synthesize the notch or the all-pass
responses. This filter circuit can be easily constructed using the quad EVAL_PRAOPAMP board and following the schematic in Figure 1.
Using the superposition principle, we write VHP = ( R6 / R4) V1 (R4 / R JUMP ) VLP + 1 + (R4 / R6 ) + (R4 / R JUMP ) / VBP 1 + (R JUMP1 / R5 )
Since A2 and A3 are integrators, we have: VBP = ( 1/ R8 C6s) VHP and VLP = ( 1/ R13 C8) VBP After extended simplifications we have the following expressions: o = and (1 + R JUMP1 / R5 ) R4 R8 C6 /R JUMP R13 C8 Q = (1 + R4 / R6 + R4 /R JUMP ) HOHP = R4 / R6 , HOBP = (1 + R JUMP1 / R5 ) /
(R4 / R JUMP ) / (R8 C6 R13 C8)
(
) /
(1 + R6 / R JUMP + R6 / R4) , HOLP = R JUMP / R6
From the above expressions we observe that Q depends on the resistor ratio RJUMP1/R5. We therefore expect Q to be much less sensitive to resistance tolerances and drift. Indeed, with proper component selection and circuit construction, the SV filter can easily yield dependable 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.
RJUMP* R6 VINA 2 V+ 3 V VEE R4 VCC A1 6 1 VHP = VOUTA V+ 5 V VEE R8 VCC A2 9 7 VBP = VOUTB V+ 10 V VEE C6 R13 C8 VCC A3
8
VLP = VOUTC
R5
RJUMP1*
*RJUMP AND RJUMP1 IS AN EXTERNAL COMPONENT, NOT PROVIDED ON THE APPLICATION BOARD.
Figure 1. State Variable Filter (Inverting)
REV. 0
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