AN-733 APPLICATION NOTE One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 TTTelel: 781/329-4700 Fax: 781/326-8703 www.analog.com Universal Precision Op Amp Evaluation Board in MSOP Package by Giampaolo Marino, Soufi ane Bendaoud, and Steve Ranta INTRODUCTION LOW-PASS FILTER The EVAL-PRAOPAMP-1RM is an evaluation board which Figure 1 is a typical representation of a first-order low- accommodates single op amps in MSOP packages. It is pass filter. 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 fined by f . Gain below C extensive flexibility for different application circuits this frequency is de fined as the magnitude of R7 to R2. and con figurations. 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 fiers. 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 ampli fiers, 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 Ampl iefi r DIFFERENCE AMPLIFIER AND PERFORMANCE OPTIMIZATION Figure 2 shows an op amp con figured as a difference ampli fier. The difference ampli fier is the complement of the summing ampli fier, 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 ampli fier 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: 1. A component V 1 due to V 1 acting alone (V 2 OUT IN IN Figure 1. Simple Low-Pass Filter 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-733 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 ampl ifier: It can be converted to a voltage with a OUT = R2 and R7 = R6, then: resistor and then ampl ified, 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 ampl iefi r 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 summed algebraically with I . 1 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 de fined 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 ampl iefi 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 ampl iefi r selected for this job. As mentioned above, errors introduced by resistor mismatch can be a big drawback of discrete differential ampl ifiers, but there are different ways to optimize this circuit con figuration: 1. The differential gain is directly related to the ratio R7/R2 therefore, one way to optimize the perfor- Figure 5. Output Response mance of this circuit is to place the ampl ifier in a high gain con figuration. When larger values for resistors GENERATION OF SQUARE WAVEFORMS USING A R7 and R6 and smaller values for resistors R2 and R4 BISTABLE MULTIVIBRATOR are 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 ampl ifier 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 con figuration, select ampli fiers with Once the output of the ampl ifier reaches one of two pos- very 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 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