AN-763 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com Dual Universal Precision Op Amp Evaluation Board by Giampaolo Marino and Steve Ranta The EVAL-PRAOPAMP-2R/2RU/2RM is an evaluation board that accommodates dual op amps in SOIC, TSSOP, and MSOP packages. It provides the user with multiple choices and extensive flexibility for different application 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 amplifiers, and external frequency compensation circuits. Several examples of application circuits are given in this application note. TWO STAGE BAND-PASS FILTER Choosing equal capacitor values minimizes the sensitivity and also simplifies the expression for fC to (3) The value of Q determines the peaking of the gain versus frequency (generally ringing in time domain). Commonly chosen values for Q are near unity. Setting Q = 1/÷2 yields minimum gain peaking and minimum ringing. Use Equation 3 to determine the values for R1 and R2. For example, set Q = 1/÷2, R1/R2 = 2 in the circuit example, and pick R1 = 5 k and R2 = 10 k for simplicity.The second stage is a low-pass filter whose corner frequency can be determined in a similar fashion: C3 680pF R2 10k R3 = R4 = R V– V– C2 10nF + V1 – 6 C1 10nF 5 R1 20k 4 7 R3 33k 8 1/2 OP2177 V+ R4 33k C4 330pF 2 3 4 1 VOUT 8 1/2 OP2177 V+ Figure 1. KRC Filter The low offset voltage and high CMRR makes the OP2177 a great choice for precision filters such as the KRC filter shown in Figure 1. This particular filter implementation offers the flexibility to tune the gain and the cut-off frequency independently. Since the common-mode voltage into the amplifier varies with the input signal in the KRC filter circuit, a high CMRR amplifier such as the OP2177 is required to minimize distortion. Furthermore, the low offset voltage of the OP2177 allows a wider dynamic range when the circuit gain is chosen to be high. The circuit in Figure 1 consists of two stages.The first stage is a simple high-pass filter whose corner frequency fC is 1 2π C1C2R1R2 (1) and whose Q =K R1 R2 K = is the dc gain. REV. B (2) and Q = 1/ 2 C3 C4 HALF-WAVE, FULL-WAVE RECTIFIER Rectifying circuits are used in a multitude of applications. One of the most popular uses is in the design of regulated power supplies where a rectifier circuit is used to convert an input sinusoid to a unipolar output voltage. There are some potential problems for amplifiers used in this manner. When the input voltage VIN is negative, the output is zero. When the magnitude of VIN is doubled at the input of the op amp, this voltage could exceed the power supply voltage which would damage the amplifiers permanently.The op amp must come out of saturation when VIN is negative. This delays the output signal because the amplifier needs time to enter its linear region.The AD8510/AD8512/AD8513 have very fast overdrive recovery time, which makes them a great choice for rectification of transient signals.The symmetry of the positive and negative recovery time is also very important in keeping the output signal undistorted. AN-763 R2 10k HIGH GAIN COMPOSITE AMPLIFIER R3 10k VIN 3V p-p 6 3 R1 1k + 2 – 8 1/2 AD8512 R2 99k R1 1k 5V 1 2/2 4 AD8512 8 5 VEE 7 VCC OUT B (HALF WAVE) AD8603 V– V+ 4 VIN 5V VEE VCC OUT A (HALF WAVE) R3 1k Figure 2a. Half-Wave and Full-Wave Rectifier U5 V+ AD8541 V– R4 99k Figure 3. High Gain Composite Amplifier VOLTAGE (1V/DIV) A composite amplifier can provide a very high gain in applications where high closed-loop dc gain is needed. The high gain achieved by the composite amplifier comes at the expense of a loss in phase margin. Placing a small capacitor, CF, in the feedback loop and in parallel with R2 improves the phase margin. For the circuit of Figure 3, picking a CF = 50 pF will yield a phase margin of about 45. R2 100k R1 1k TIME (1ms/DIV) Figure 2b. Half-Wave Rectifier Signal (Output A) VIN AD8603 V– V+ VEE R3 1k VCC V+ V– VCC C2 R4 100 AD8541 VEE C3 VOLTAGE (1V/DIV) Figure 4. Low Power Composite Amplifier A composite amplifier can be used to optimize the dc and ac characteristic. Figure 4 shows an example using the AD8603 and the AD8541 that offers too many circuit advantages.The bandwidth is increased substantially and the input offset voltage and noise of the AD8541 becomes insignificant since they are divided by the high gain of the AD8603. The circuit offers a high bandwidth, a high output current, and a very low power consumption of less than 100 A. TIME (1ms/DIV) Figure 2c. Full-Wave Rectifier Signal (Output B) Figure 2a is a typical representation of a rectifier circuit. The first stage of the circuit is a half-wave rectifier. When the sine wave applied at the input is positive, the output follows the input response. During the negative cycle of the input, the output tries to swing negative to follow the input, but the power supplies restrains it to zero. Similarly, the second stage is a follower during the positive cycle of the sine wave and an inverter during the negative cycle. Figure 2b and Figure 2c represents the signal response of the circuit at Output A and Output B, respectively. –2– REV. B AN-763 EXTERNAL COMPENSATION TECHNIQUES Series Resistor Compensation The use of external compensation networks may be required to optimize certain applications. Figure 5a is a typical representation of a series resistor compensation to stabilize an op amp driving capacitive loads.The stabilizing effect of the series resistor can be thought of as a means to isolate 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 sufficient to prevent local resonance. The disadvantage of this technique is a reduction in gain accuracy and extra distortion when driving nonlinear loads. R2 CL Snubber Network Another way to stabilize an op amp driving a capacitive load is the use of a snubber, as shown in Figure 6a. This method has the significant advantage of not reducing the output swing because there is no 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 RS and CS combination can be determined experimentally. VOUT VIN VOUT RS CL RL CS Figure 6a. Snubber Network RL VIN RL = 10k CL = 500pF VOLTAGE (200mV/DIV) Figure 5a. Series Resistor Compensation VOLTAGE (200mV/DIV) RL = 10k CL = 2nF GND TIME (1s/DIV) Figure 6b. Cap Load Drive Without Snubber RL = 10k CL = 500pF RS = 100 CS = 1nF TIME (10s/DIV) VOLTAGE (200mV/DIV) Figure 5b. Cap Load Drive Without Resistor VOLTAGE (200mV/DIV) RL = 10k RS = 200 CL = 2nF CS = 0.47F GND TIME (1s/DIV) Figure 6c. Cap Load Drive with Snubber TIME (10s/DIV) Figure 5c. Cap Load Drive with Resistor REV. B –3– AN-763 C4 C4 R4 R4 V VEE EE 11 R2 R2 –V1 –V1 –INA –INA G1 G1 R6 R6 11 11 22 Rt1 Rt1 44 +INA +INA G2 G2 R7 R7 VO1 11 VO1 R RSS11 C1 C1 R RLL11 V VOUT OUTAA C CLL11 G5 11 G5 C CSS11 R5 R5 C3 C3 Rt2 Rt2 A R8 R8 DUT DUT 88 11 11 DUTA DUTA 11 33 G1 G1 +V1 +V1 AMPLIFIER A AMPLIFIER C2 C2 R3 R3 G2 G2 G3 G3 R1 R1 11 V VCC CC C5 C5 Figure 7. Dual Universal Precision Op Amp Evaluation Board Electrical Schematic C7 C7 R10 R10 AMPLIFIER AMPLIFIER B –V2 1 –V2 1 –INB –INB G3 1 G3 1 R12 R12 66 Rt3 Rt3 55 G3 G3 +V2 1 +V2 1 +INB +INB G4 1 G4 1 G4 G4 R13 R13 Rt4 Rt4 DUTB DUTB 77 VO2 VO2 11 R14 R14 DUT DUT RR S2 S2 CC S2 S2 R11 R11 C6 C6 B RR L2 L2 2 CC L2L VV BB OUT OUT G6 G6 11 G6 G6 R9 R9 C8 C8 Figure 8. Dual Universal Precision Op Amp Evaluation Board Figure 9. Layout Patterns © 2012 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN05284-0-9/12(B) REV. –4– B

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