an-692 application note one technology way ? p.o. b ox 9106 ? n orwood , ma 02062-9106 ? t el : 781/329-4700 ? f ax : 781/326-8703 ? www.analog.com introduction the EVAL-PRAOPAMP evaluation board accommodates single op amps in many packages. it is meant to provide the user with multiple choices and extensive fexibility for different applications circuits and confgurations. for evaluation in smaller packages, a converter is needed; board-specifc packages will be available soon. this board is not intended to be used with high frequency compo - nents or high speed amplifers; however, it provides the user with many combinations for various circuit types including active flters, difference amplifers, and exter - nal frequency compensation circuits. a few examples of applications circuits are given in this application note. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 1. simple low-pass filter ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 2. universal precision op amp evaluation board by giampaolo marino and soufane bendaoud low pass filter figure 1 is a typical representation of a frst order low pass flter. this circuit has a 6 db per octave roll-off after a close loop C3 db point defned by fc. gain below this frequency is defned as the magnitude of r7 to r2. the circuit might be considered as an ac integrator for frequencies well above fc; however, the time domain response is that of a single rc rather than an integral. fc = 1/(2 r7 c7); C3 db frequency f l = 1/(2 r2 c7); unity gain frequency acl = C(r7/r2); close loop gain r6 should be chosen equal to the parallel combination between r7 and r2 in order to minimize errors due to bias currents. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 3. difference amplifer difference amplifier and performance optimization figure 3 shows an op amp confgured as a difference amplifer. the difference amplifer is the complement of the summing amplifer and allows the subtraction of two voltages or the cancellation of a signal common to both inputs. the circuit shown in figure 3 is useful as a computational amplifer, in making a differential to sin - gle-ended conversion or in rejecting a common-mode signal. the output voltage v out may be thought as being made up of two separate components: 1. a component v out 1 due to v in 1 acting alone (v in 2 short-circuited to ground.) 2. a component v out 2 due to v in 2 acting alone (v in 1 short-circuited to ground.) rev. 0
11/24/03 3:00 pm akos C2 C an-692 C3 C an-692 the algebraic sum of these two components should be equal to v out . by applying the principles expressed in bullets 1 and 2 and by letting r4 = r2 and r7 = r6, then: v out 1 = v in 1 r7/r2 v out 2 = Cv in 2 r7/r2 v out = v out 1 + v out 2 = ( v in 1 C v in 2) r7/r1 difference amplifiers are commonly used in high accuracy circuits to improve the common-mode rejec - tion ratio, typically known as cmrr. for this type of application, cmrr depends upon how tightly matched resistors are used; poorly matched resis - tors result in a low value of cmrr. to see how this works, consider a hypothetical source of error for resistor r7 (1 C error). using the superposi - tion principle and letting r4 = r2 and r7 = r6, the output voltage would be as follows: v r r r r r r error vd r r r error out = ? + + ? ? ? ? ? ? ? ? ? ? ? ? ? ? + + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 7 2 1 2 2 7 2 7 2 7 2 7 v v v dd in in = ? 2 1 from this equation, a cm and a dm can be defned as follows: a cm = r7/(r7 C r2) error a dm = r7/r2 {1 C [(r2+2r7/r2+r7) error/2]} these equations demonstrate that when there is not an error in the resistor values, the a cm = 0 and the amplifer 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 amplifer selected for this job. as mentioned above, errors introduced by resistor mismatch can be a big drawback of discrete differential amplifers, but there are different ways to optimize this circuit confguration: 1. the differential gain is directly related to the ratio r7/ r2; therefore, one way to optimize the performance of this circuit is to place the amplifer in a high gain confguration. when larger values for resistors r7 and r6 and smaller values for resistor r2 and r4 are selected, the higher the gain, the higher the cmrr. for example, when r7 = r6 = 10 k , and r2 = r4 = 1 k , and error = 0.1%, cmrr improves to better than 80 db. for high gain confguration, select amplifers with very low ib and very high gain (such as the ad8551, ad8571, ad8603, and ad8605) to reduce errors. 2. select resistors that have much tighter tolerance and accuracy. the more closely they are matched, the better the cmrr. for example, if a cmrr of 90 db is needed, then match resistors to approximately 0.02%. current to voltage converter current may be measured in two ways with an opera - tional amplifer. current can be converted to a voltage with a resistor and then amplifed, or current can be injected directly into a summing node. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 4. current to voltage converter figure 4 is a typical representation of a current-to-voltage transducer. the input current is fed directly into the sum - ming node and the amplifer output voltage changes to exactly the same current from the summing node through r7. the scale factor of this circuit is r7 volts per amps. the only conversion error in this circuit is i bias , which is summed algebraically with i in 1. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 5. bistable multivibrator figure 6. output response generation of square waveforms using bistable multivibrator a square waveform can be simply generated by arrang - ing the amplifer for a bistable multivibrator to switch states periodically as figure 6 shows. once the output of the amplifer reaches one of two pos - sible levels, such as l+, capacitor c9 charges toward this level through resistor r7. the voltage across c9, which is applied to the negative input terminal of the ampli - fer denoted as vC, then rises exponentially toward l+ with a time constant = c9r7. meanwhile, the voltage rev. 0 rev. 0 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
C2 C an-692 C3 C an-692 at the positive input terminal of the amplifer denoted as v+ = bl+. this continues until the capacitor voltage reaches the positive threshold v th , at which point the bistable multivibrator switches to the other stable state in which v o = lC and v+ = blC. the capacitor then begins to discharge, and its voltage vC decreases exponentially toward lC. this continues until vC reaches the negative threshold v tl , at which time the bistable multivibrator switches to the positive output state, and the cycle repeats itself. it is important to note that the frequency of the square wave being generated f o depends only on the external components being used. any variation in l+ will cause v+ to vary in proportion, ensuring the same transition time and the same oscillation frequency. the maximum operating frequency is determined by the amplifier speed, which can be increased signifcantly by using faster devices. the lowest operating frequency depends on the practical upper limits set by r7 and c9. using the name convention outlined on the pra opamp evaluation board, the following circuit should be con - nected as follows: b = r4/(r4 + r9); feedback factor (noninverting input) t = 2r7 c9 ln((1 + b)/(1 C b)); period of oscillation f o = 1/t; oscillation frequency ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 7. series resistor compensation figure 8. capacitive load drive without resistor figure 9. capacitive load drive with resistor external compensation techniques series resistor compensation the use of external compensation networks may be required to optimize certain applications. figure 7 is a typical representation of a series resistor compensation for stabilizing an op amp driving capacitive load. the stabilizing effect of the series resistor can be thought of as a means of isolating the op amp output and the feedback network from the capacitive load. the required amount of series resistance depends on the part used, but values of 5 to 50 are usually suffcient to prevent local resonance. the disadvantages of this technique are a reduction in gain accuracy and extra distortion when driving nonlinear loads. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 10. snubber network figure 11. capacitive load drive without snubber rev. 0 rev. 0 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
e04568C0C11/03(0) C4 C ? 2003 analog devices, inc. all rights reserved. trademarks and registered trademarks are the property of their respective owners. an-692 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 12. capacitive load drive with the snubber snubber network another way to stabilize an op amp driving a capacitive load is with the use of a snubber, as shown in figure 10. this method presents the signifcant advantage of not reducing the output swing because there is not any isolation resistor in the signal path. also, the use of the snubber does not degrade the gain accuracy or cause extra distortion when driving a nonlinear load. the exact r s and c s combinations can be determined experimentally. adapters for specific packages can be found at the following urls: www.enplas.com www.adapters.com www.emulation.com ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? figure 13. EVAL-PRAOPAMP electrical schematic figure 14. EVAL-PRAOPAMP board layout patterns rev. 0
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