AN-735 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 SOT-23 Package by Giampaolo Marino, Soufiane Bendaoud, and Steve Ranta INTRODUCTION LOW-PASS FILTER The EVAL-PRAOPAMP-1RJ is an evaluation board which Figure 1 is a typical representation of a first-order low- accommodates single op amps in SOT-23 packages. It pass filter. This circuit has a 6 dB per octave roll-off is meant to provide the user with multiple choices and after a close-loop 3 dB point defined by f . Gain below C extensive flexibility for different applications circuits this frequency is defined as the magnitude of R7 to R2. and configurations. 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 amplifiers. 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 filters, differential amplifiers, 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 between R7 and R2 in order to minimize errors due to bias currents. Figure 2. Difference Ampliefi r DIFFERENCE AMPLIFIER AND PERFORMANCE OPTIMIZATION Figure 2 shows an op amp configured as a difference amplifier. The difference amplifier is the complement of the summing amplifier, and allows the subtraction of two voltages or the cancellation of a signal common to both inputs. The circuit shown in Figure 2 is useful as a computational amplifier in making a differential to single-ended conversion or in rejecting a common- mode signal. The output voltage V is comprised of OUT 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 AN-735 AN-735 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 amplifier. Current can be converted to a voltage OUT = R2 and R7 = R6, then: with a resistor and then amplified or injected directly into a summing node. V 1 = V 1 R7/R2 OUT IN V 2 = V 2 R7/R2 OUT IN V = V 1 + V 2 = ( V 1 V 2) R7/R1 OUT OUT OUT IN IN 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- 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 amplifier 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 R2 + 2R7 error 1 - summed algebraically with I . IN R2 R2 + R7 2 V = OUT R7 VD + error R2 + R7 V = V 2 -V 1 DD IN IN From this equation, A and A can be defined as CM DM 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 error in the resistor values, the A = 0 and the ampliefi r 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 ampliefi r selected for this job. As mentioned above, errors introduced by resistor mismatch can be a big drawback of discrete differential amplifiers, but there are different ways to optimize this circuit configuration: 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 amplifier in a high gain configuration. When larger values for resistors R7 GENERATION OF SQUARE WAVEFORMS USING A and R6 and smaller values for resistor R2 and R4 are BISTABLE MULTIVIBRATOR selected, the higher the gain, the higher the CMRR. A square waveform can be simply generated by arrang- For example, when R7 = R6 = 10 k, and R2 = R4 = 1 k, ing the amplifier for a bistable multivibrator to switch and error = 0.1%, CMRR improves to better than 80 dB. states periodically as Figure 5 shows. For high gain configuration, select amplifiers with Once the output of the amplifier reaches one of two pos - very low Ib and very high gain (such as the AD8551, 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 fier 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 REV. A 3