High Precision, Low Noise OPERATIONAL AMPLIFIERS

Transcription

1 For most current data sheet and other product information, visit OPA47 OPA7 OPA47 OPA7 OPA7 OPA7 OPA7 OPA7 OPA47 OPA OPA OPA4 High Precision, Low Noise OPERATIONAL AMPLIFIERS FEATURES LOW NOISE: nv/ Hz WIDE BANDWIDTH: OPA7: MHz,.V/µs OPA: MHz, V/µs SETTLING TIME: 5µs (significant improvement over OP-7) HIGH CMRR: db HIGH OPEN-LOOP GAIN: db LOW INPUT BIAS CURRENT: na max LOW OFFSET VOLTAGE: 75µV max WIDE SUPPLY RANGE: ±.5V to ±V OPA7 REPLACES OP-7, LT7, MAX47 OPA REPLACES OP-7, LT7, MAX47 SINGLE, DUAL, AND QUAD VERSIONS APPLICATIONS DATA ACQUISITION TELECOM EQUIPMENT GEOPHYSICAL ANALYSIS VIBRATION ANALYSIS SPECTRAL ANALYSIS PROFESSIONAL AUDIO EQUIPMENT ACTIVE FILTERS POWER SUPPLY CONTROL DESCRIPTION The OPA7 and OPA series op amps combine low noise and wide bandwidth with high precision to make them the ideal choice for applications requiring both ac and precision dc performance. The OPA7 is unity gain stable and features high slew rate (.V/µs) and wide bandwidth (MHz). The OPA is optimized for closed-loop gains of 5 or greater, and offers higher speed with a slew rate of V/µs and a bandwidth of MHz. The OPA7 and OPA series op amps are ideal for professional audio equipment. In addition, low quiescent current and low cost make them ideal for portable applications requiring high precision. The OPA7 and OPA series op amps are pinfor-pin replacements for the industry standard OP-7 and OP-7 with substantial improvements across the board. The dual and quad versions are available for space savings and per-channel cost reduction. The OPA7, OPA, OPA7, and OPA are available in DIP- and SO- packages. The OPA47 and OPA4 are available in DIP-4 and SO-4 packages with standard pin configurations. Operation is specified from 4 C to +5 C. OPA47, OPA4 SPICE Model available for OPA7 at Out A 4 Out D Trim In +In V 4 OPA7, OPA DIP-, SO- 7 5 Trim V+ Output NC Out A In A +In A V OPA7, OPA A 7 4 B 5 V+ Out B In B +In B In A +In A V+ +In B In B Out B A D B C DIP-4, SO-4 9 In D +In D V +In C In C Out C DIP-, SO- International Airport Industrial Park Mailing Address: PO Box 4, Tucson, AZ 574 Street Address: 7 S. Tucson Blvd., Tucson, AZ 57 Tel: (5) 74- Twx: Internet: Cable: BBRCORP Telex: -49 FAX: (5) 9-5 Immediate Product Info: () Burr-Brown Corporation PDS-494B Printed in U.S.A. May, 999

2 SPECIFICATIONS: V S = ±5V to ±5V OPA7 Series At T A = +5 C, and R L = kω, unless otherwise noted. Boldface limits apply over the specified temperature range, T A = 4 C to +5 C. OPA7P, U OPA7P, U OPA7PA, UA OPA7PA, UA OPA47PA, UA PARAMETER CONDITION MIN TYP MAX MIN TYP MAX UNITS OFFSET VOLTAGE Input Offset Voltage V OS ±5 ±75 ± ± µv OT A = 4 C to +5 Cver Temperature ± ± µv vs Temperature dv OS /dt ±. ±. ±. ± µv/ C vs Power Supply PSRR V S = ±.5V to ±V ±.5 ± µv/v T A = 4 C to +5 C ± µv/v vs Time. µv/mo Channel Separation (dual, quad) dc. µv/v f = khz, R L = 5kΩ db INPUT BIAS CURRENT Input Bias Current I B ±.5 ± na T A = 4 C to +5 C ± na Input Offset Current I OS ±.5 ± na T A = 4 C to +5 C ± na NOISE Input Voltage Noise, f =.Hz to Hz 9 nvp-p 5 nvrms Input Voltage Noise Density, f = Hz e n.5 nv/ Hz f = Hz nv/ Hz f = khz nv/ Hz Current Noise Density, f = khz i n.4 pa/ Hz INPUT VOLTAGE RANGE Common-Mode Voltage Range V CM (V )+ (V+) V Common-Mode Rejection CMRR V CM = (V )+V to (V+) V db T A = 4 C to +5 C db INPUT IMPEDANCE Differential 7 Ω pf Common-Mode V CM = (V )+V to (V+) V 9 Ω pf OPEN-LOOP GAIN Open-Loop Voltage Gain A OL V O = (V )+V to (V+) V, R L = kω db T A = 4 C to +5 C db V O = (V )+.5V to (V+).5V, R L = Ω db T A = 4 C to +5 C db FREQUENCY RESPONSE Gain Bandwidth Product GBW MHz Slew Rate SR. V/µs Settling Time:.% G =, V Step, C L = pf 5 µs.% G =, V Step, C L = pf 5. µs Overload Recovery Time V IN G = V S. µs Total Harmonic Distortion + Noise THD+N f = khz, G =, V O =.5Vrms.5 % OUTPUT Voltage Output R L = kω (V )+ (V+) V T A = 4 C to +5 C R L = kω (V )+ (V+) V R L = Ω (V )+.5 (V+).5 V T A = 4 C to +5 C R L = Ω (V )+.5 (V+).5 V Short-Circuit Current I SC ±45 ma Capacitive Load Drive C LOAD See Typical Curve POWER SUPPLY Specified Voltage Range V S ±5 ±5 V Operating Voltage Range ±.5 ± V Quiescent Current (per amplifier) I Q I O = ±.7 ±. ma T A = 4 C to +5 C I O = ±4. ma TEMPERATURE RANGE Specified Range 4 +5 C Operating Range C Storage Range 5 +5 C Thermal Resistance θ JA SO- Surface Mount 5 C/W DIP- C/W DIP-4 C/W SO-4 Surface Mount C/W Specifications same as OPA7P, U. OPA7, 7, 47 OPA,, 4

3 SPECIFICATIONS: V S = ±5V to ±5V OPA Series At T A = +5 C, and R L = kω, unless otherwise noted. Boldface limits apply over the specified temperature range, T A = 4 C to +5 C. OPAP, U OPAP, U OPAPA, UA OPAPA, UA OPA4PA, UA PARAMETER CONDITION MIN TYP MAX MIN TYP MAX UNITS OFFSET VOLTAGE Input Offset Voltage V OS ±5 ±75 ± ± µv OT A = 4 C to +5 Cver Temperature ± ± µv vs Temperature dv OS /dt ±. ±. ±. ± µv/ C vs Power Supply PSRR V S = ±.5V to ±V ±.5 ± µv/v T A = 4 C to +5 C ± µv/v vs Time. µv/mo Channel Separation (dual, quad) dc. µv/v f = khz, R L = 5kΩ db INPUT BIAS CURRENT Input Bias Current I B ±.5 ± na T A = 4 C to +5 C ± na Input Offset Current I OS ±.5 ± na T A = 4 C to +5 C ± na NOISE Input Voltage Noise, f =.Hz to Hz 9 nvp-p 5 nvrms Input Voltage Noise Density, f = Hz e n.5 nv/ Hz f = Hz nv/ Hz f = khz nv/ Hz Current Noise Density, f = khz i n.4 pa/ Hz INPUT VOLTAGE RANGE Common-Mode Voltage Range V CM (V )+ (V+) V Common-Mode Rejection CMRR V CM = (V )+V to (V+) V db T A = 4 C to +5 C db INPUT IMPEDANCE Differential 7 Ω pf Common-Mode V CM = (V )+V to (V+) V 9 Ω pf OPEN-LOOP GAIN Open-Loop Voltage Gain A OL V O = (V )+V to (V+) V, R L = kω db T A = 4 C to +5 C db V O = (V )+.5V to (V+).5V, R L = Ω db T A = 4 C to +5 C db FREQUENCY RESPONSE Minimum Closed-Loop Gain 5 V/V Gain Bandwidth Product GBW MHz Slew Rate SR V/µs Settling Time:.% G = 5, V Step, C L = pf, C F =pf.5 µs.% G = 5, V Step, C L = pf, C F =pf µs Overload Recovery Time V IN G = V S. µs Total Harmonic Distortion + Noise THD+N f = khz, G = 5, V O =.5Vrms.5 % OUTPUT Voltage Output R L = kω (V )+ (V+) V T A = 4 C to +5 C R L = kω (V )+ (V+) V R L = Ω (V )+.5 (V+).5 V T A = 4 C to +5 C R L = Ω (V )+.5 (V+).5 V Short-Circuit Current I SC ±45 ma Capacitive Load Drive C LOAD See Typical Curve POWER SUPPLY Specified Voltage Range V S ±5 ±5 V Operating Voltage Range ±.5 ± V Quiescent Current (per amplifier) I Q I O = ±.7 ±. ma T A = 4 C to +5 C I O = ±4. ma TEMPERATURE RANGE Specified Range 4 +5 C Operating Range C Storage Range 5 +5 C Thermal Resistance θ JA SO- Surface Mount 5 C/W DIP- C/W DIP-4 C/W SO-4 Surface Mount C/W Specifications same as OPAP, U. OPA7, 7, 47 OPA,, 4

4 ABSOLUTE MAXIMUM RATINGS () Supply Voltage... ±V Signal Input Terminals, Voltage... (V ).7V to (V+) +.7V Current... ma Output Short-Circuit ()... Continuous Operating Temperature C to +5 C Storage Temperature... 5 C to +5 C Junction Temperature... 5 C Lead Temperature (soldering, s)... C NOTE: () Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. () Short-circuit to ground, one amplifier per package. ELECTROSTATIC DISCHARGE SENSITIVITY This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. PACKAGE/ORDERING INFORMATION OFFSET OFFSET PACKAGE VOLTAGE VOLTAGE DRIFT DRAWING TEMPERATURE ORDERING TRANSPORT PRODUCT max, µv max, µv/ C PACKAGE NUMBER () RANGE NUMBER () MEDIA OPA7 Series Single OPA7PA ± ± DIP- 4 C to +5 C OPA7PA Rails OPA7P ±75 ±. DIP- 4 C to +5 C OPA7P Rails OPA7UA ± ± SO- Surface Mount 4 C to +5 C OPA7UA Rails " " " " " " OPA7UA/K5 Tape and Reel OPA7U ±75 ±. SO- Surface Mount 4 C to +5 C OPA7U Rails " " " " " " OPA7U/K5 Tape and Reel Dual OPA7PA ± ± DIP- 4 C to +5 C OPA7PA Rails OPA7P ±75 ±. DIP- 4 C to +5 C OPA7P Rails OPA7UA ± ± SO- Surface Mount 4 C to +5 C OPA7UA Rails " " " " " " OPA7UA/K5 Tape and Reel OPA7U ±75 ±. SO- Surface Mount 4 C to +5 C OPA7U Rails " " " " " " OPA7U/K5 Tape and Reel Quad OPA47PA ± ± DIP-4 4 C to +5 C OPA47PA Rails OPA47UA ± ± SO-4 Surface Mount 5 4 C to +5 C OPA47UA Rails " " " " " " OPA47UA/K5 Tape and Reel OPA Series Single OPAPA ± ± DIP- 4 C to +5 C OPAPA Rails OPAP ±75 ±. DIP- 4 C to +5 C OPAP Rails OPAUA ± ± SO- Surface Mount 4 C to +5 C OPAUA Rails " " " " " " OPAUA/K5 Tape and Reel OPAU ±75 ±. SO- Surface Mount 4 C to +5 C OPAU Rails " " " " " " OPAU/K5 Tape and Reel Dual OPAPA ± ± DIP- 4 C to +5 C OPAPA Rails OPAP ±75 ±. DIP- 4 C to +5 C OPAP Rails OPAUA ± ± SO- Surface Mount 4 C to +5 C OPAUA Rails " " " " " " OPAUA/K5 Tape and Reel OPAU ±75 ±. SO- Surface Mount 4 C to +5 C OPAU Rails " " " " " " OPAU/K5 Tape and Reel Quad OPA4PA ± ± DIP-4 4 C to +5 C OPA4PA Rails OPA4UA ± ± SO-4 Surface Mount 5 4 C to +5 C OPA4UA Rails " " " " " " OPA4UA/K5 Tape and Reel NOTE: () For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. () Products followed by a slash (/) are only available in Tape and Reel in the quantities indicated (e.g. /K5 indicates 5 devices per reel). Ordering 5 pieces of OPA7UA/K5 will get a single 5 piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. OPA7, 7, 47 OPA,, 4 4

5 TYPICAL PERFORMANCE CURVES At T A = +5 C, R L = kω, and V S = ±5V, unless otherwise noted. A OL (db) OPEN-LOOP GAIN/PHASE vs FREQUENCY 4 OPA7 4 4 G φ 4.. k k k M M M Phase ( ) A OL (db) OPEN-LOOP GAIN/PHASE vs FREQUENCY 4 OPA 4 G 4 φ 4.. k k k M M M Phase ( ) 4 POWER SUPPLY AND COMMON-MODE REJECTION RATIO vs FREQUENCY k INPUT VOLTAGE AND CURRENT NOISE SPECTRAL DENSITY vs FREQUENCY PSRR, CMRR (db) 4 - PSRR +CMRR +PSRR Voltage Noise (nv/ Hz) Current Noise (fa/ Hz) k k Current Noise Voltage Noise. k k k M. k k. TOTAL HARMONIC DISTORTION + NOISE vs FREQUENCY V OUT =.5Vrms OPA7. TOTAL HARMONIC DISTORTION + NOISE vs FREQUENCY V OUT =.5Vrms OPA THD+Noise (%).. G =, R L = kω THD+Noise (%).. G =, R L = kω. k k k. k k 5k 5 OPA7, 7, 47 OPA,, 4

6 TYPICAL PERFORMANCE CURVES (CONT) At T A = +5 C, R L =kω, and V S = ±5V, unless otherwise noted. INPUT NOISE VOLTAGE vs TIME 4 CHANNEL SEPARATION vs FREQUENCY 5nV/div s/div Channel Separation (db) Dual and quad devices. G =, all channels. Quad measured Channel A to D, or B to C; other combinations yield similiar or improved rejection. 4 k k k M 4 VOLTAGE NOISE DISTRIBUTION (Hz) OFFSET VOLTAGE PRODUCTION DISTRIBUTION Typical distribution of packaged units. Percent of Units (%) Percent of Amplifiers (%) Noise (nv/ Hz) Offset Voltage (µv) Percent of Amplifiers (%) 4 OFFSET VOLTAGE DRIFT PRODUCTION DISTRIBUTION Typical distribution of packaged units. Offset Voltage Change (µv) 4 4 WARM-UP OFFSET VOLTAGE DRIFT.5..5 Offset Voltage Drift (µv)/ C Time from Power Supply Turn-On (s) OPA7, 7, 47 OPA,, 4

7 TYPICAL PERFORMANCE CURVES (CONT) At T A = +5 C, R L = kω, and V S = ±5V, unless otherwise noted. A OL, CMRR, PSRR (db) A OL, CMRR, PSRR vs TEMPERATURE A OL 5 4 CMRR PSRR 9 7 OPA Temperature ( C) A OL, CMRR, PSRR (db) A OL, CMRR, PSRR vs TEMPERATURE 5 A OL 4 CMRR PSRR 9 7 OPA Temperature ( C). INPUT BIAS CURRENT vs TEMPERATURE SHORT-CIRCUIT CURRENT vs TEMPERATURE Input Bias Current (na) Short-Circuit Current (ma) 5 4 +I SC I SC Temperature ( C) Temperature ( C) 5. QUIESCENT CURRENT vs TEMPERATURE. QUIESCENT CURRENT vs SUPPLY VOLTAGE Quiescent Current (ma) ±V ±5V ±V ±V ±5V ±.5V Quiescent Current (ma) Temperature ( C) Supply Voltage (±V) 7 OPA7, 7, 47 OPA,, 4

8 TYPICAL PERFORMANCE CURVES (CONT) At T A = +5 C, R L = kω, and V S = ±5V, unless otherwise noted.. OPA7 SLEW RATE vs TEMPERATURE OPA SLEW RATE vs TEMPERATURE Slew Rate (µv/v) Positive Slew Rate Negative Slew Rate Slew Rate (µv/v) 4.5 R LOAD = kω C LOAD = pf R LOAD = kω C LOAD = pf Temperature ( C) Temperature ( C) 5 I B (na) CHANGE IN INPUT BIAS CURRENT vs POWER SUPPLY VOLTAGE Curve shows normalized change in bias current with respect to V S = ±V. Typical I B may range from na to +na at V S = ±V. I B (na) CHANGE IN INPUT BIAS CURRENT vs COMMON-MODE VOLTAGE Curve shows normalized change in bias current with respect to V CM = V. Typical I B may range from na to +na at V CM = V. V S = ±5V V S = ±5V Supply Voltage (V) Common-Mode Voltage (V) 5 Settling Time (µs) SETTLING TIME vs CLOSED-LOOP GAIN V S = ±5V, V Step C L = 5pF R L = kω OPA7.%.% OPA.%.% ± ± ± Gain (V/V) Output Voltage Swing (V) OUTPUT VOLTAGE SWING vs OUTPUT CURRENT 5 C 5 C 5 C 4 C 55 C V+ (V+) V (V+) V (V+) V 55 C 5 C 5 C 4 C (V ) +V 5 C (V ) +V (V ) +V V 4 5 Output Current (ma) OPA7, 7, 47 OPA,, 4

9 TYPICAL PERFORMANCE CURVES (CONT) At T A = +5 C, R L = kω, and V S = ±5V, unless otherwise noted. 5 MAXIMUM OUTPUT VOLTAGE vs FREQUENCY V S = ±5V OPA7 7 OPA7 SMALL-SIGNAL OVERSHOOT vs LOAD CAPACITANCE Gain = + Output Voltage (Vp-p) 5 5 V S = ±5V Overshoot (%) 5 4 k k k Load Capacitance (pf) LARGE-SIGNAL STEP RESPONSE G =, C L = 5pF SMALL-SIGNAL STEP RESPONSE G = +, C L = pf V/div 5mV/div 5µs/div 4ns/div SMALL-SIGNAL STEP RESPONSE G = +, C L = 5pF 5mV/div Gain = + Gain = Gain = k k k M M OPA7 OPA7 OPA7 4ns/div 9 OPA7, 7, 47 OPA,, 4

10 TYPICAL PERFORMANCE CURVES (CONT) At T A = +5 C, R L = kω, and V S = ±5V, unless otherwise noted. 5 MAXIMUM OUTPUT VOLTAGE vs FREQUENCY V S = ±5V OPA 7 OPA SMALL-SIGNAL OVERSHOOT vs LOAD CAPACITANCE Output Voltage (Vp-p) 5 5 V S = ±5V Overshoot (%) 5 4 G = ± k k k Load Capacitance (pf) LARGE-SIGNAL STEP RESPONSE G =, C L = pf SMALL-SIGNAL STEP RESPONSE G = +, C L = pf, R L =.kω mv/div µs/div 5ns/div SMALL-SIGNAL STEP RESPONSE G = +, C L = 5pF, R L =.kω mv/div 5V/div G = G = + k k k M M OPA OPA OPA 5ns/div OPA7, 7, 47 OPA,, 4

11 APPLICATIONS INFORMATION The OPA7 and OPA series are precision op amps with very low noise. The OPA7 series is unity-gain stable with a slew rate of.v/µs and MHz bandwidth. The OPA series is optimized for higher-speed applications with gains of 5 or greater, featuring a slew rate of V/µs and MHz bandwidth. Applications with noisy or high impedance power supplies may require decoupling capacitors close to the device pins. In most cases,.µf capacitors are adequate. OFFSET VOLTAGE AND DRIFT The OPA7 and OPA series have very low offset voltage and drift. To achieve highest dc precision, circuit layout and mechanical conditions should be optimized. Connections of dissimilar metals can generate thermal potentials at the op amp inputs which can degrade the offset voltage and drift. These thermocouple effects can exceed the inherent drift of the amplifier and ultimately degrade its performance. The thermal potentials can be made to cancel by assuring that they are equal at both input terminals. In addition: Keep thermal mass of the connections made to the two input terminals similar. Locate heat sources as far as possible from the critical input circuitry. Shield op amp and input circuitry from air currents such as those created by cooling fans. OPERATING VOLTAGE OPA7 and OPA series op amps operate from ±.5V to ±V supplies with excellent performance. Unlike most op amps which are specified at only one supply voltage, the OPA7 series is specified for real-world applications; a single set of specifications applies over the ±5V to ±5V supply range. Specifications are guaranteed for applications between ±5V and ±5V power supplies. Some applications do not require equal positive and negative output voltage swing. Power supply voltages do not need to be equal. The OPA7 and OPA series can operate with as little as 5V between the supplies and with up to V between the supplies. For example, the positive supply could be set to 5V with the negative supply at 5V or vice-versa. In addition, key parameters are guaranteed over the specified temperature range, 4 C to +5 C. Parameters which vary significantly with operating voltage or temperature are shown in the Typical Performance Curves. OFFSET VOLTAGE ADJUSTMENT The OPA7 and OPA series are laser-trimmed for very low offset and drift so most applications will not require external adjustment. However, the OPA7 and OPA (single versions) provide offset voltage trim connections on pins and. Offset voltage can be adjusted by connecting a potentiometer as shown in Figure. This adjustment should be used only to null the offset of the op.µf kω 7 OPA7.µF FIGURE. OPA7 Offset Voltage Trim Circuit. amp. This adjustment should not be used to compensate for offsets created elsewhere in the system since this can introduce additional temperature drift. INPUT PROTECTION Back-to-back diodes (see Figure ) are used for input protection on the OPA7 and OPA. Exceeding the turn-on threshold of these diodes, as in a pulse condition, can cause current to flow through the input protection diodes due to the amplifier s finite slew rate. Without external current-limiting resistors, the input devices can be destroyed. Sources of high input current can cause subtle damage to the amplifier. Although the unit may still be functional, important parameters such as input offset voltage, drift, and noise may shift. Input V+ V FIGURE. Pulsed Operation. 4 + Trim range exceeds offset voltage specification OPA7 and OPA single op amps only. Use offset adjust pins only to null offset voltage of op amp. See text. R F 5Ω OPA7 Output When using the OPA7 as a unity-gain buffer (follower), the input current should be limited to ma. This can be accomplished by inserting a feedback resistor or a resistor in series with the source. Sufficient resistor size can be calculated: R X = V S /ma R SOURCE where R X is either in series with the source or inserted in the feedback path. For example, for a V pulse (V S = V), total loop resistance must be 5Ω. If the source impedance is large enough to sufficiently limit the current on its own, no additional resistors are needed. The size of any external resistors must be carefully chosen since they will increase noise. See the Noise Performance section of this data sheet for further information on noise calculation. Figure shows an example implementing a currentlimiting feedback resistor. OPA7, 7, 47 OPA,, 4

12 INPUT BIAS CURRENT CANCELLATION The input bias current of the OPA7 and OPA series is internally compensated with an equal and opposite cancellation current. The resulting input bias current is the difference between with input bias current and the cancellation current. The residual input bias current can be positive or negative. When the bias current is cancelled in this manner, the input bias current and input offset current are approximately equal. A resistor added to cancel the effect of the input bias current (as shown in Figure ) may actually increase offset and noise and is therefore not recommended. Conventional Op Amp Configuration R Not recommended for OPA7 R Op Amp NOISE PERFORMANCE Figure 4 shows total circuit noise for varying source impedances with the op amp in a unity-gain configuration (no feedback resistor network, therefore no additional noise contributions). Two different op amps are shown with total circuit noise calculated. The OPA7 has very low voltage noise, making it ideal for low source impedances (less than kω). A similar precision op amp, the OPA77, has somewhat higher voltage noise but lower current noise. It provides excellent noise performance at moderate source impedance (kω to kω). Above kω, a FET-input op amp such as the OPA (very low current noise) may provide improved performance. The equation is shown for the calculation of the total circuit noise. Note that e n = voltage noise, i n = current noise, R S = source impedance, k = Boltzmann s constant =. J/K and T is temperature in K. For more details on calculating noise, see the insert titled Basic Noise Calculations. R B = R R External Cancellation Resistor.+ VOLTAGE NOISE SPECTRAL DENSITY vs SOURCE RESISTANCE Recommended OPA7 Configuration R R OPA7 No cancellation resistor. See text. Votlage Noise Spectral Density, E Typical at k (V/ Hz).E+.E+ E O R S OPA77 OPA7 OPA7 OPA77 Resistor Noise Resistor Noise E O = e n + (i n R S ) + 4kTR S.E+ k k k M Source Resistance, R S (Ω) FIGURE. Input Bias Current Cancellation. FIGURE 4. Noise Performance of the OPA7 in Unity- Gain Buffer Configuration. Design of low noise op amp circuits requires careful consideration of a variety of possible noise contributors: noise from the signal source, noise generated in the op amp, and noise from the feedback network resistors. The total noise of the circuit is the root-sum-square combination of all noise components. The resistive portion of the source impedance produces thermal noise proportional to the square root of the resistance. This function is shown plotted in Figure 4. Since the source impedance is usually fixed, select the op amp and the feedback resistors to minimize their contribution to the total noise. Figure 4 shows total noise for varying source impedances with the op amp in a unity-gain configuration (no feedback resistor network and therefore no additional noise contributions). The operational amplifier itself contributes both a voltage noise component and a current BASIC NOISE CALCULATIONS noise component. The voltage noise is commonly modeled as a time-varying component of the offset voltage. The current noise is modeled as the time-varying component of the input bias current and reacts with the source resistance to create a voltage component of noise. Consequently, the lowest noise op amp for a given application depends on the source impedance. For low source impedance, current noise is negligible and voltage noise generally dominates. For high source impedance, current noise may dominate. Figure 5 shows both inverting and noninverting op amp circuit configurations with gain. In circuit configurations with gain, the feedback network resistors also contribute noise. The current noise of the op amp reacts with the feedback resistors to create additional noise components. The feedback resistor values can generally be chosen to make these noise sources negligible. The equations for total noise are shown for both configurations. OPA7, 7, 47 OPA,, 4

13 Noise in Noninverting Gain Configuration R Noise at the output: R E R R O en e e = + inr es in RS R + + +( ) + +( ) + R R E O R Where e S = 4kTR S + = thermal noise of R S R V S R S R e = 4kTR = thermal noise of R R e = 4kTR = thermal noise of R Noise in Inverting Gain Configuration R Noise at the output: V S RS R E O O S E R = + R + R en e e inr es + + +( ) + R Where e S = 4kTR S = thermal noise of R S R + R S R e = 4kTR = thermal noise of R R + R S e = 4kTR = thermal noise of R For the OPA7 and OPA series op amps at khz, e n = nv/ Hz and i n =.4pA/ Hz. FIGURE 5. Noise Calculation in Gain Configurations. OPA7, 7, 47 OPA,, 4

14 R MΩ R MΩ R 4kΩ R 7kΩ R kω R 4 9.9kΩ C 4 nf C nf C µf C µf U (OPA7) R 4.kΩ R 7 97.kΩ C.47µF U (OPA7) R 9 7kΩ R kω C 5.47µF U (OPA7) V OUT Input from Device Under Test R 5 4kΩ FIGURE..Hz to Hz Bandpass Filter Used to Test Wideband Noise of the OPA7 and OPA Series. Ω FIGURE 7. Noise Test Circuit. pf kω OPA7 V OUT Figure shows the.hz Hz bandpass filter used to test the noise of the OPA7 and OPA. The filter circuit was designed using Burr-Brown s FilterPro software (available at Figure 7 shows the configuration of the OPA7 and OPA for noise testing. Device Under Test USING THE OPA IN LOW GAINS The OPA family is intended for applications with signal gains of 5 or greater, but it is possible to take advantage of their high speed in lower gains. Without external compensation, the OPA has sufficient phase margin to maintain stability in unity gain with purely resistive loads. However, the addition of load capacitance can reduce the phase margin and destabilize the op amp. A variety of compensation techniques have been evaluated specifically for use with the OPA. The recommended configuration consists of an additional capacitor (C F ) in parallel with the feedback resistance, as shown in Figures and. This feedback capacitor serves two purposes in compensating the circuit. The op amp s input capacitance and the feedback resistors interact to cause phase shift that can result in instability. C F compensates the input capacitance, minimizing peaking. Additionally, at high frequencies, the closed-loop gain of the amplifier is strongly influenced by the ratio of the input capacitance and the feedback capacitor. Thus, C F can be selected to yield good stability while maintaining high speed. OPA7, 7, 47 OPA,, 4 4

15 Without external compensation, the noise specification of the OPA is the same as that for the OPA7 in gains of 5 or greater. With the additional external compensation, the output noise of the of the OPA will be higher. The amount of noise increase is directly related to the increase in high frequency closed-loop gain established by the C IN / C F ratio. Figures and show the recommended circuit for gains of + and, respectively. The figures suggest approximate values for C F. Because compensation is highly dependent on circuit design, board layout, and load conditions, C F should be optimized experimentally for best results. Figures 9 and show the large- and small-signal step responses for the G = + configuration with pf load capacitance. Figures and show the large- and smallsignal step responses for the G = configuration with pf load capacitance. pf 5pF kω kω kω kω OPA OPA kω pf kω pf FIGURE. Compensation of the OPA for G =+. FIGURE. Compensation for OPA for G =. 5mV/div 4ns/div FIGURE 9. Large-Signal Step Response, G = +, C LOAD = pf, Input Signal = 5Vp-p. 4ns/div FIGURE. Large-Signal Step Response, G =, C LOAD = pf, Input Signal = 5Vp-p. 5mV/div 5mV/div 5mV/div OPA OPA OPA ns/div FIGURE. Small-Signal Step Response, G = +, C LOAD = pf, Input Signal = 5mVp-p. OPA ns/div FIGURE. Small-Signal Step Response, G =, C LOAD = pf, Input Signal = 5mVp-p. 5 OPA7, 7, 47 OPA,, 4

16 .kω.nf.4kω dc Gain =.kω.5kω pf V IN nf OPA7.4kΩ.9kΩ nf OPA7.kΩ nf V OUT f N =.khz Q =. f N =.khz Q = 4.59 f = 7.kHz FIGURE 4. Three-Pole, khz Low Pass,.5dB Chebyshev Filter..µF pf Ω Dexter M Thermopile Detector kω OPA7 Output NOTE: Use metal film resistors and plastic film capacitor. Circuit must be well shielded to achieve low noise. Responsivity.5 x 4 V/W Output Noise µvrms,.hz to Hz TTL INPUT Input TTL In D D DG GAIN + kω S S 4.99kΩ 9.7kΩ 5Ω OPA7 Offset Trim 4.75kΩ kω +V CC Balance Trim Output 4.75kΩ FIGURE 5. Long-Wavelength Infrared Detector Amplifier. FIGURE. High Performance Synchronous Demodulator. OPA7, 7, 47 OPA,, 4

17 +5V.µF kω Audio In kω / OPA7 Ω Ω To Headphone / OPA7 This application uses two op amps in parallel for higher output current drive..µf 5V FIGURE 7. Headphone Amplifier. Bass Tone Control R 7.5kΩ R 5kΩ CW R 7.5kΩ R kω Midrange Tone Control C 94pF V IN R 4.7kΩ R 5 5kΩ CW R.7kΩ C.47µF Treble Tone Control R 7 7.5kΩ R 5kΩ CW R 9 7.5kΩ R kω C pf OPA7 V OUT FIGURE. Three-Band ActiveTone Control (bass, midrange and treble). 7 OPA7, 7, 47 OPA,, 4