AN-732 APPLICATION NOTE One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 Tel: 781/329-4700 Fax: 781/326-8703 www.analog.com Universal Precision Op Amp Evaluation Board in SOIC Package by Giampaolo Marino, Sou ane Bendaoud, and Steve Ranta INTRODUCTION LOW-PASS FILTER The EVAL-PRAOPAMP-1R is an evaluation board which Figure 1 is a typical representation of a rst-order low- accommodates single op amps in SOIC packages. It is pass lter. This circuit has a 6 dB per octave roll-off meant to provide the user with multiple choices and after a close-loop 3 dB point de ned by f . Gain below C extensive exibility for different applications circuits this frequency is de ned as the magnitude of R7 to R2. and con gurations. This board is not intended to be The circuit might be considered as an ac integrator for used with high frequency components or high speed frequencies well above f however, the time domain C ampli ers. However, it provides the user with many response is that of a single RC, rather than an integral. combinations for various circuit types including active f = 1/(2 R7 C7) 3 dB frequency C lters, differential ampli ers, and external frequency f = 1/(2 R2 C7) unity gain frequency L compensation circuits. A few examples of application circuits are given in this application note. Acl = (R7/R2) close loop gain R6 should be chosen equal to the parallel combination C7 between R7 and R2 in order to minimize errors due to R7 bias currents. R7 R2 V IN V OUT R2 V 1 IN R6 V R4 OUT V 2 IN R6 60 Figure 2. Difference Ampli er f C 40 DIFFERENCE AMPLIFIER AND PERFORMANCE OPTIMIZATION Figure 2 shows an op amp con gured as a difference 20 ampli er. The difference ampli er is the complement of the summing ampli er, and allows the subtraction f L of two voltages or the cancellation of a signal common 0 to both inputs. The circuit shown in Figure 2 is useful as a computational ampli er in making a differential R7/R2 = 100 20 to single-ended conversion or in rejecting a common- f 10f 100f 1000f 10000f mode signal. The output voltage V is comprised of OUT RELATIVE FREQUENCY two separate components: Figure 1. Simple Low-Pass Filter 1. A component V 1 due to V 1 acting alone (V 2 OUT IN IN short-circuited to ground.) 2. A component V 2 due to V 2 acting alone (V 1 OUT IN IN short-circuited to ground.) REV. A GAIN (dB)AN-732 The algebraic sum of these two components should be CURRENT-TO-VOLTAGE CONVERTER equal to V . By applying the principles expressed in Current may be measured in two ways with an opera- OUT the output voltage V components, and by letting R4 tional ampli er. Current can be converted to a voltage OUT = R2 and R7 = R6, then: with a resistor and then ampli ed or injected directly into a summing node. V 1 = V 1 R7/R2 OUT IN R7 V 2 = V 2 R7/R2 OUT IN V = V 1 + V 2 = ( V 1 V 2) R7/R1 OUT OUT OUT IN IN I 1 Difference amplifiers are commonly used in high IN V OUT accuracy circuits to improve the common-mode rejec- R6 tion ratio, typically known as CMRR. V =I 1 R7 OUT IN For this type of application, CMRR depends upon how tightly matched resistors are used poorly matched resis- Figure 3. Current-to-Voltage Converter tors result in a low value of CMRR. Figure 3 is a typical representation of a current-to-voltage To see how this works, consider a hypothetical source transducer. The input current is fed directly into the sum- of error for resistor R7 (1 error). Using the superposi- ming node and the ampli er output voltage changes to tion principle and letting R4 = R2 and R7 = R6, the output exactly the same current from the summing node through voltage would be as follows: R7. The scale factor of this circuit is R7 volts per amps. The only conversion error in this circuit is I , which is BIAS R7 RR22+ 7 error 1 summed algebraically with I 1. IN R2 RR27+ 2 V = OUT R7 R7 VD + error RR27+ C9 V V VV=21V R4 OUT V+ DD IN IN From this equation, A and A can be de ned as CM DM R9 follows: A = R7/(R7 R2) error Figure 4. Bistable Multivibrator CM A = R7/R2 1 (R2+2R7/R2+R7) error/2 DM These equations demonstrate that when there is not an L+ BL+=V error in the resistor values, the A = 0 and the ampli er TH CM responds only to the differential voltage being applied to its inputs under these conditions, the CMRR of the circuit becomes highly dependent on the CMRR of the ampli er selected for this job. As mentioned above, errors introduced by resistor mismatch can be a big drawback of discrete differential BL=V TL ampli ers, but there are different ways to optimize this L circuit con guration: 1. The differential gain is directly related to the ratio R7/ R2 therefore, one way to optimize the performance Figure 5. Output Response of this circuit is to place the ampli er in a high gain con guration. When larger values for resistors R7 and GENERATION OF SQUARE WAVEFORMS USING A R6 and smaller values for resistors R2 and R4 are se- BISTABLE MULTIVIBRATOR lected, the higher the gain, the higher the CMRR. For A square waveform can be simply generated by arrang- example, when R7 = R6 = 10 k , and R2 = R4 = 1 k , and ing the ampli er for a bistable multivibrator to switch error = 0.1%, CMRR improves to better than 80 dB. For states periodically as Figure 5 shows. high gain con guration, select ampli ers with very Once the output of the ampli er reaches one of two pos- low I and very high gain (such as the AD8551, BIAS sible levels, such as L+, capacitor C9 charges toward this AD8571, AD8603, and AD8605) to reduce errors. level through resistor R7. The voltage across C9, which 2. Select resistors that have much tighter tolerance and is applied to the negative input terminal of the ampli- accuracy. The more closely they are matched, the better er denoted as V, then rises exponentially toward L+ the CMRR. For example, if a CMRR of 90 dB is needed, with a time constant = C9R7. Meanwhile, the voltage then match resistors to approximately 0.02%. 2 REV. A