EVAL-PRAOPAMP-2RZ

AN-763 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Dual Universal Precision Op Amp Evaluation Board INTRODUCTION The EVAL-PRAOPAMP-2RZ, EVAL-PRAOPAMP-2RMZ, and EVAL-PRAOPAMP-2CPZ are universal precision evaluation boards that accommodate dual op amps in 8-pin SOIC, MSOP, and LFCSP packages, respectively. For the exposed pad connection for the LFCSP package, see the appropriate product data sheet. 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 ADA4077-2, is required to minimize distortion. Furthermore, the low offset voltage of the ADA4077-2 allows a wider dynamic range when the circuit gain is chosen to be high. These PRAOPAMP evaluation boards provide multiple choices and extensive flexibility for different application circuits and configurations. The circuit shown in Figure 1 consists of two stages. The first stage is a simple high-pass filter with a corner frequency, fC, of These boards are not intended to be used with high frequency components or high speed amplifiers. However, they provide 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 provided in this application note. Q=K C2 10nF C1 10nF + V1 – R1 20kΩ 6 4 7 R3 33kΩ R4 33kΩ 1 2πC R1R2 V+ VOUT C4 330pF V+ 05284-001 8 1/2 ADA4077-2 1 3 (3) The value of Q determines the peaking of the gain vs. frequency (generally ringing in the time domain). Commonly chosen values for Q are near unity. 4 8 1/2 ADA4077-2 5 (2) Choosing equal capacitor values minimizes the sensitivity and simplifies the expression for fC to V– 2 R1 R2 where K is the dc gain. C3 680pF V– (1) and TWO STAGE BAND-PASS FILTER R2 10kΩ 1 2π C1C2R1R2 Figure 1. KRC Filter The low offset voltage and high CMRR makes the ADA4077-2 a great choice for precision filters, such as the KRC filter shown in Figure 1. 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 and R1/R2 = 2 in the circuit example, and pick R1 = 5 kΩ and R2 = 10 kΩ for simplicity. The second stage is a low-pass filter whose corner frequency can be determined in a similar fashion. R3 = R4 = R This particular filter implementation offers the flexibility to tune the gain and the cut-off frequency independently. fC = 1 2π × R C3C4 and Q = 1/ 2 Rev. C | Page 1 of 8 C3 C4 AN-763 Application Note TABLE OF CONTENTS Introduction ...................................................................................... 1 High Gain Composite Amplifier .....................................................3 Two Stage Band-Pass Filter ............................................................. 1 External Compensation Techniques ...............................................4 Revision History ............................................................................... 2 Snubber Network ...............................................................................5 Half Wave, Full Wave Rectifier ....................................................... 3 REVISION HISTORY 10/13—Rev. B to Rev. C Updated Format .................................................................. Universal Replaced All Figures ......................................................................... 1 Changed EVAL-PRAOPAMP-2R/2RU/2RM to EVALPRAOPAMP-2RZ, EVAL-PRAOPAMP-2RMZ, and EVALPRAOPAMP-2CPZ Throughout .................................................... 1 Deleted Authors Names and added Introduction Section Heading .............................................................................................. 1 Changes to Two Stage Band-Pass Filter Section ........................... 1 Changes to Half Wave, Full Wave Rectifier Section..................... 3 Changes to High Gain Composite Amplifier Sections ................ 3 Rev. C | Page 2 of 8 Application Note AN-763 HALF WAVE, FULL WAVE RECTIFIER VOLTAGE (1V/DIV) 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. 05284-004 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. TIME (1mS/DIV) Figure 4. Full Wave Rectifier Signal (Output B) The ADA4610-2 has a very fast overdrive recovery time, which makes it 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. R3 10kΩ 5V VIN 3V p-p 3 R1 1kΩ + 2 – 6 4 2/2 ADA4610-2 5 8 8 1/2 ADA4610-2 1 7 OUT B (FULL WAVE) HIGH GAIN COMPOSITE AMPLIFIER 4 R2 99kΩ R1 1kΩ 05284-002 5V OUT A (HALF WAVE) VEE VCC Figure 2. Half Wave and Full Wave Rectifier V– V+ VIN 1/2 ADA4661-2 VEE VCC R3 1kΩ VOLTAGE (1V/DIV) U5 1/2 V+ ADA4661-2 V– R4 99kΩ 05284-005 R2 10kΩ Figure 2 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 3 and Figure 4 represents the signal response of the circuit at Output A and Output B, respectively. Figure 5. High Gain Composite Amplifier 05284-003 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. TIME (1mS/DIV) Figure 3. Half Wave Rectifier Signal (Output A) Placing a small capacitor, CF, in the feedback loop and in parallel with R2 improves the phase margin. For the circuit of Figure 5, picking a CF = 50 pF yields a phase margin of about 45°. Rev. C | Page 3 of 8 AN-763 Application Note R2 100kΩ R2 VOUT V– V+ CL R3 1kΩ VCC V+ V– VCC C2 RL VIN 05284-007 VIN VEE R4 100Ω Figure 7. Series Resistor Compensation 1/2 AD8657 VEE C3 05284-006 R1 1kΩ 1/2 AD8657 RL = 10kΩ CL = 2nF GND 05284-008 A composite amplifier can be used to optimize the dc and ac characteristics. Figure 6 shows an example using the AD8657, which offers many circuit advantages. The bandwidth is increased substantially and the input offset voltage and noise of the AD8657 becomes insignificant because they are divided by the high gain of the amplifier. The circuit offers a high bandwidth, a high output current, and a very low power consumption of less than 100 μA. VOLTAGE (200mV/DIV) Figure 6. Low Power Composite Amplifier EXTERNAL COMPENSATION TECHNIQUES TIME (10µs/DIV) Series Resistor Compensation Figure 8. Capacitor Load Drive Without Resistor RL = 10kΩ RS = 200Ω CL = 2nF CS = 0.47µF GND 05284-009 VOLTAGE (200mV/DIV) The use of external compensation networks may be required to optimize certain applications. Figure 7 shows 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 Ω to 50 Ω 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. TIME (10µs/DIV) Figure 9. Capacitor Load Drive with Resistor Rev. C | Page 4 of 8 Application Note AN-763 SNUBBER NETWORK Another way to stabilize an op amp driving a capacitive load is through the use of a snubber as shown in Figure 10. 05284-011 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. VOLTAGE (200mV/DIV) RL = 10kΩ CL = 500pF VOUT CL RL VIN CS TIME (1µs/DIV) Figure 11. Capacitor Load Drive Without Snubber 05284-010 RS RL = 10kΩ CL = 500pF RS = 100Ω CS = 1nF 05284-012 VOLTAGE (200mV/DIV) Figure 10. Snubber Network TIME (1µs/DIV) Figure 12. Capacitor Load Drive with Snubber Rev. C | Page 5 of 8 AN-763 Application Note C4 R4 VEE 1 AMPLIFIER A R2 –V1 1 –INA G1 1 R6 2 Rt1 C2 4 DUTA 1 3 R8 DUT 8 RS1 C1 G1 +V1 1 RL1 CL1 1 CS1 R5 R3 G2 G5 G3 R1 C3 Rt2 VO1 VOUTA 1 VCC 05284-013 +INA G2 1 R7 1 C5 Figure 13. Dual Universal Precision Op Amp Evaluation Board Electrical Schematic C7 R10 AMPLIFIER B R12 6 Rt3 DUTB 7 R14 1 5 RS2 DUT RL2 CL2 G3 +INB G4 1 G4 R13 Rt4 CS2 R11 1 G6 G6 C6 R9 C8 Figure 14. Dual Universal Precision Op Amp Evaluation Board 05284-015 +V2 1 VO2 VOUTB 05284-014 –V2 1 –INB G3 1 Figure 15. Dual SOIC Layout Patterns Rev. C | Page 6 of 8 Application Note AN-763 NOTES Rev. C | Page 7 of 8 AN-763 Application Note NOTES ©2005–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN05284-0-10/13(C) Rev. C | Page 8 of 8