OPA 268 0 ® OPA OPA2680 268 0 Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable TM FEATURES APPLICATIONS ● ● ● ● ● ● ● ● ● ● ● ● ● ● WIDEBAND +5V OPERATION: 220MHz (G = 2) HIGH OUTPUT CURRENT: 150mA OUTPUT VOLTAGE SWING: ±4.0V HIGH SLEW RATE: 1800V/µs LOW SUPPLY CURRENT: 6.4mA/Ch. LOW DISABLED CURRENT: 300µA/Ch. ENABLE/DISABLE TIME: 25ns/100ns DESCRIPTION VIDEO LINE DRIVING xDSL LINE DRIVER/RECEIVER HIGH SPEED IMAGING CHANNELS ADC BUFFERS PORTABLE INSTRUMENTS TRANSIMPEDANCE AMPLIFIERS ACTIVE FILTERS The OPA2680’s low 6.4mA/ch. supply current is precisely trimmed at 25°C. This trim, along with low temperature drift, guarantees lower maximum supply current than competing products. System power may be reduced further using the optional disable control pin (SO-14 package only). Leaving this disable pin open, or holding it high, will operate the OPA2680N normally. If pulled low, the OPA2680N supply current drops to less than 600µA while the output goes into a high impedance state. The OPA2680 represents a major step forward in unity gain stable, voltage feedback op amps. A new internal architecture provides slew rate and full power bandwidth previously found only in wideband current feedback op amps. A new output stage architecture delivers high currents with a minimal headroom requirement. These combine to give exceptional single supply operation. Using a single +5V supply, the OPA2680 can deliver a 1V to 4V output swing with over 100mA drive current and 150MHz bandwidth. This combination of features makes the OPA2680 an ideal RGB line driver or single supply ADC input driver. OPA2680 RELATED PRODUCTS SINGLES DUALS TRIPLES Voltage Feedback OPA680 OPA2680 OPA3680 Current Feedback OPA681 OPA2681 OPA3681 Fixed Gain OPA682 OPA2682 OPA3682 +5V 0.1µF 1.5kΩ REFB 1.5kΩ +3.5V 50Ω 200Ω +1.5V REFT 0.1µF +2.5V +5V 0.1µF 3.2kΩ 1/2 OPA2680 50Ω ADS822 10pF +1.5V 800Ω IN 1/2 OPA2680 1.6kΩ 1.6kΩ 10-Bit 40MSPS 2.5VCM ±1VDIFF Clock 400Ω 400Ω 50Ω VIN 0V to +1V IN 10pF Single-Supply, DC-Coupled, Single-to-Differential ADC Driver nternational Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 1997 Burr-Brown Corporation PDS-1433C Printed in U.S.A. October, 1999 SPECIFICATIONS: VS = ±5V RF = 402Ω, RL = 100Ω, and G = +2, (Figure 1 for AC performance only), unless otherwise noted. OPA2680U, N TYP PARAMETER AC PERFORMANCE (Figure 1) Small-Signal Bandwidth Gain-Bandwidth Product Bandwidth for 0.1dB Gain Flatness Peaking at a Gain of +1 Large Signal Bandwidth Slew Rate Rise/Fall Time Settling Time to 0.02% 0.1% Harmonic Distortion 2nd Harmonic 3rd Harmonic Input Voltage Noise Input Current Noise Differential Gain Differential Phase Channel-to-Channel Crosstalk DC PERFORMANCE(4) Open-Loop Voltage Gain (AOL) Input Offset Voltage Average Offset Voltage Drift Input Bias Current Average Bias Current Drift (magnitude) Input Offset Current Average Offset Current Drift INPUT Common-Mode Input Range (CMIR)(5) Common-Mode Rejection Ratio (CMRR) Input Impedance Differential-Mode Common-Mode OUTPUT Voltage Output Swing Current Output, Sourcing Current Output, Sinking Closed-Loop Output Impedance DISABLE (SO-14 Only) Power Down Supply Current (+VS) Disable Time Enable Time Off Isolation Output Capacitance in Disable Turn On Glitch Turn Off Glitch Enable Voltage Disable Voltage Control Pin Input Bias Current (VDIS) POWER SUPPLY Specified Operating Voltage Maximum Operating Voltage Range Max Quiescent Current Min Quiescent Current Power Supply Rejection (+PSRR) THERMAL CHARACTERISTICS Specified Operating Range U, N Package Thermal Resistance, θJA U SO-8 N SO-14 CONDITIONS +25°C G = +1, VO = 0.5Vp-p, RF = 25Ω G = +2, VO = 0.5Vp-p G = +10, VO = 0.5Vp-p G ≥ 10 G = +2, VO < 0.5Vp-p VO < 0.5Vp-p G = +2, VO = 5Vp-p G = +2, 4V Step G = +2, VO = 0.5V Step G = +2, VO = 5V Step G = +2, VO = 2V Step G = +2, VO = 2V Step 400 220 30 300 30 4 175 1800 1.4 2.8 12 8 G = +2, f = 5MHz, VO = 2Vp-p RL = 100Ω RL ≥ 500Ω RL = 100Ω RL ≥ 500Ω f > 1MHz f > 1MHz G = +2, NTSC, VO = 1.4Vp, RL = 150 G = +2, NTSC, VO = 1.4Vp, RL = 150 f = 5MHz –68 –80 –80 –88 4.8 2.5 0.05 0.03 –70 VO = 0V, RL = 100Ω VCM = 0V VCM = 0V VCM = 0V VCM = 0V VCM = 0V VCM = 0V GUARANTEED +25°C(2) 0°C to 70°C(3) –40°C to +85°C(3) 210 20 200 200 20 200 190 20 200 1400 1200 900 –63 –70 –75 –85 5.3 2.8 –62 –68 –73 –83 5.9 3.0 54 58 ±1.0 ±4.5 +8 +14 ±0.1 ±0.7 VCM = ±1V ±3.5 59 ±3.4 VCM = 0 VCM = 0 190 || 0.6 3.2 || 0.9 No Load 100Ω Load VO = 0 VO = 0 G = +2, f = 100kHz ±4.0 ±3.9 +190 –150 0.03 Disabled Low VDIS = 0, Both Channels G = +2, RL = 150Ω, VIN = 0 G = +2, RL = 150Ω, VIN = 0 VDIS = 0, Each Channel +160 –135 MHz MHz MHz MHz MHz dB MHz V/µs ns ns ns ns typ min min min typ typ typ min typ typ typ typ C B B B C C C C C C C C –60 –65 –70 –80 6.1 3.6 dBc dBc dBc dBc nV/√Hz pA/√Hz % deg dBc max max max max max max typ typ typ , B B B B B C C C 52 ±5.2 ±10 +19 –70 ±1 ±1 50 ±6.0 ±10 +32 –150 ±1.2 ±1.5 dB mV µV/°C µA nA/°C µA nA/°C min max max max max max max A A B A B A B ±3.3 53 ±3.2 52 V dB min min A A kΩ || pF MΩ || pF typ typ C C V V mA mA Ω min min min min typ A A A A C typ typ typ typ typ typ typ min max max C C C C C C C A A A ±3.7 ±3.6 +140 –130 ±3.6 ±3.3 +80 –80 3.5 1.7 160 3.6 1.6 160 3.7 1.5 160 ±6 ±6 14.0 12.0 58 ±6 14.4 10.6 56 V V mA mA dB typ max max min min C A A A A –40 to +85 °C typ C 125 100 °C/W °C/W typ typ C C ±5 VS = ±5V VS = ±5V Input Referred ±3.8 ±3.7 MIN/ TEST MAX LEVEL(1) µA ns ns dB pF mV mV V V µA –600 100 25 70 4 ±50 ±20 3.3 1.8 100 G = +2, 5MHz 56 UNITS 12.8 12.8 65 13.6 12.0 60 Junction-to-Ambient NOTES: (1) Test Levels: (A) 100% tested at 25°C. Over temperature limits by characterization and simulation. (B) Limits set by characterization and simulation. (C) Typical value only for information. (2) Junction temperature = ambient for 25°C guaranteed specifications. (3) Junction temperature = ambient at low temperature limit: junction temperature = ambient +23°C at high temperature limit for over temperature guaranteed specifications. (4) Current is considered positive-out-of node. VCM is the input common-mode voltage. (5) Tested < 3dB below minimum CMRR specification at ±CMIR limits. ® OPA2680 2 SPECIFICATIONS: VS = +5V RF = 402Ω, RL = 100Ω to VS /2, G = +2, (Figure 2 for AC performance only), unless otherwise noted. OPA2680U, N TYP CONDITIONS +25°C G = +1, VO < 0.5Vp-p, RF = ±25Ω G = +2, VO < 0.5Vp-p G = +10, VO < 0.5Vp-p G ≥ 10 G = +2, VO < 0.5Vp-p VO < 0.5Vp-p G = +2, VO = 2Vp-p G = +2, 2V Step G = +2, VO = 0.5V Step G = +2, VO = 2V Step G = +2, VO = 2V Step G = +2, VO = 2V Step G = +2, f = 5MHz, VO = 2Vp-p RL = 100Ω to VS /2 RL ≥ 500Ω to VS /2 RL = 100Ω to VS /2 RL ≥ 500Ω to VS /2 f > 1MHz f > 1MHz G = +2, NTSC, VO = 1.4Vp, RL = 150 to VS /2 G = +2, NTSC, VO = 1.4Vp, RL = 150 to VS /2 300 220 25 250 20 5 200 1000 1.6 2.0 12 8 PARAMETER AC PERFORMANCE (Figure 2) Small-Signal Bandwidth Gain-Bandwidth Product Bandwidth for 0.1dB Gain Flatness Peaking at a Gain of +1 Large Signal Bandwidth Slew Rate Rise/Fall Time Settling Time to 0.02% 0.1% Harmonic Distortion 2nd Harmonic 3rd Harmonic Input Voltage Noise Input Current Noise Differential Gain Differential Phase DC PERFORMANCE(4) Open-Loop Voltage Gain Input Offset Voltage Average Offset Voltage Drift Input Bias Current Average Bias Current Drift (magnitude) Input Offset Current Average Offset Current Drift INPUT Least Positive Input Voltage(5) Most Positive Input Voltage(5) Common-Mode Rejection Ratio (CMRR) Input Impedance Differential-Mode Common-Mode OUTPUT Most Positive Output Voltage Least Positive Output Voltage Current Output, Sourcing Current Output, Sinking Closed-Loop Output Impedance DISABLE (SO-14 Only) Power Down Supply Current (+VS) Disable Time Enable Time Off Isolation Output Capacitance in Disable Turn On Glitch Turn Off Glitch Enable Voltage Disable Voltage Control Pin Input Bias Current (VDIS) POWER SUPPLY Specified Single Supply Operating Voltage Maximum Single Supply Operating Voltage Max Quiescent Current Min Quiescent Current Power Supply Rejection (+PSRR) TEMPERATURE RANGE Specification: U, N Thermal Resistance, θJA U SO-8 N SO-14 VO = 2.5V, RL = 100Ω to 2.5V VCM = 2.5V VCM = 2.5V VCM = 2.5V VCM = 2.5V VCM = 2.5V VCM = 2.5V GUARANTEED +25°C(2) 0°C to 70°C(3) –40°C to +85°C(3) 120 20 200 160 19 190 140 18 180 700 670 550 –55 –66 –66 –76 5.3 2.8 –54 –63 –64 –74 6.0 3.0 54 typ min min min typ typ typ min typ typ typ typ C B B B C C C B C C C C –51 –59 –62 –71 6.2 3.4 dBc dBc dBc dBc nV/√Hz pA/√Hz % deg max max max max max max typ typ B B B B B B C C 52 ±7 –10 +18 –52 ±1.0 ±0.5 50 ±8.5 –12 +32 –52 ±1.2 ±1.0 dB mV µV/°C µA nA/°C µA nA/°C min max max max max max max A A B A B A B 1.7 3.3 53 1.8 3.2 52 V V dB max min min A A A kΩ || pF MΩ || pF typ typ C C V V V V mA mA Ω min min max max min min typ A A A A A A C µA ns ns dB pF mV mV V V µA typ typ typ typ typ typ typ min max typ C C C C C C C A A C V V mA mA dB typ max max min typ C B A A C –40 to +85 °C typ C 125 100 °C/W °C/W typ typ C C –60 –70 –72 –80 5 2.5 0.06 0.03 ±6.0 +8 +15 ±0.1 ±0.6 VCM = 2.5V ±0.5V 1.5 3.5 59 1.6 3.4 56 VCM = 2.5V VCM = 2.5V 92 || 1.4 2.2 || 1.5 No Load RL = 100Ω to 2.5V No Load RL = 100Ω to 2.5V 4 3.9 1 1.1 +150 –110 0.03 G = +2, 5MHz G = +2, RL = 150Ω, VIN = VS /2 G = +2, RL = 150Ω, VIN = VS /2 VDIS = 0, Each Channel –500 100 25 65 4 ±50 ±20 3.3 1.8 100 3.8 3.7 1.2 1.3 +110 –75 3.6 3.5 1.4 1.5 +110 –70 3.5 3.4 1.5 1.7 +60 –50 3.5 1.7 3.6 1.6 3.7 1.5 12 12.0 8.0 12 12.0 8.0 12 12.0 7.6 5 VS = +5V VS = +5V Input Referred MIN/ TEST MAX LEVEL(1) MHz MHz MHz MHz MHz dB MHz V/µs ns ns ns ns 58 ±2.0 G =+2, f = 100kHz Disabled Low VDIS = 0, Both Channels UNITS 10.2 10.2 55 Junction-to-Ambient NOTES: (1) Test Levels: (A) 100% tested at 25°C. Over temperature limits by characterization and simulation. (B) Limits set by characterization and simulation. (C) Typical value only for information. (2) Junction temperature = ambient for 25°C guaranteed specifications. (3) Junction temperature = ambient at low temperature limit: Junction temperature = ambient +23°C at high temperature limit for over temperature guaranteed specifications. (4) Current is considered positive-out-of node. VCM is the input common-mode voltage. (5) Tested < 3dB below minimum CMRR specification at ±CMIR limits. ® 3 OPA2680 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATIONS Power Supply .............................................................................. ±6.5VDC Internal Power Dissipation ..................................... See Thermal Analysis Differential Input Voltage .................................................................. ±1.2V Input Voltage Range ............................................................................ ±VS Storage Temperature Range: U, N ................................ –40°C to +125°C Lead Temperature (soldering, 10s) .............................................. +300°C Junction Temperature (TJ ) ........................................................... +175°C Top View SO-8 Out A –In A ELECTROSTATIC DISCHARGE SENSITIVITY 1 2 +In A 3 –VS 4 A B 8 +VS 7 Out B 6 –In B 5 +In B SO-14 Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. Burr-Brown Corporation recommends that all integrated circuits be handled and stored using appropriate ESD protection methods. 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 published specifications. –In A 1 14 Out A +In A 2 13 NC DISA 3 12 NC –VS 4 11 +VS DISB 5 10 NC +In B 6 9 NC –In B 7 8 Out B PACKAGE/ORDERING INFORMATION PRODUCT PACKAGE PACKAGE DRAWING NUMBER(1) OPA2680U SO-8 Surface Mount 182 –40°C to +85°C OPA2680U " " " " SO-14 Surface mount 235 –40°C to +85°C OPA2680N " " " " " OPA2680N " SPECIFIED TEMPERATURE RANGE PACKAGE MARKING ORDERING NUMBER(2) TRANSPORT MEDIA OPA2680U OPA2680U/2K5 OPA2680N OPA2680N/2K5 Rails Tape and Reel Rails Tape and Reel NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “OPA2680U/2K5” will get a single 2500-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. ® OPA2680 4 TYPICAL PERFORMANCE CURVES: VS = ±5V At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 1. SMALL-SIGNAL FREQUENCY RESPONSE 6 VO = 0.5Vp-p VO = 1Vp-p VO = 2Vp-p 12 0 9 G = +2 –3 –6 –9 G = +5 –12 –15 6 Gain (3dB/div) Normalized Gain (3dB/div) 3 LARGE-SIGNAL FREQUENCY RESPONSE 15 G = +1 RF = 25Ω 3 VO = 7Vp-p 0 –3 VO = 4Vp-p –6 G = +10 –18 –9 –21 –12 –24 –15 0.5 10 100 500 0.5 Frequency (MHz) SMALL-SIGNAL PULSE RESPONSE G = +2 VO = 0.5Vp-p 300 G = +2 VO = 5Vp-p +3 Output Voltage (1V/div) Output Voltage (100mV/div) 500 LARGE-SIGNAL PULSE RESPONSE 200 100 0 –100 –200 +2 +1 0 –1 –2 –3 –300 –4 –400 Time (5ns/div) Time (5ns/div) LARGE-SIGNAL DISABLE/ENABLE RESPONSE 4.0 2.0 0 0 –10 –20 Crosstalk (10dB/div) VDIS VDIS (2V/div) CHANNEL-TO-CHANNEL CROSSTALK 5.0 Output Voltage 2.0 1.6 VO (0.4V/div) 100 +4 400 Each Channel SO-14 Package Only 1.2 0.8 0.4 10 Frequency (MHz) G = +2 VIN = +1V –30 –40 –50 –60 –70 –80 0 –90 –100 Time (50ns/div) 1 10 100 Frequency (MHz) ® 5 OPA2680 TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT) At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 1. 5MHz 2nd HARMONIC DISTORTION vs OUTPUT VOLTAGE 5MHz 3rd HARMONIC DISTORTION vs OUTPUT VOLTAGE –60 RL = 100Ω –65 3rd Harmonic Distortion (dBc) 2nd Harmonic Distortion (dBc) –60 RL = 200Ω –70 –75 RL = 500Ω –80 –85 –65 –70 RL = 100Ω –75 RL = 200Ω –80 –85 RL = 500Ω –90 0.1 1 –90 10 0.1 10MHz 2nd HARMONIC DISTORTION vs OUTPUT VOLTAGE 10MHz 3rd HARMONIC DISTORTION vs OUTPUT VOLTAGE –60 3rd Harmonic Distortion (dBc) –65 RL = 100Ω –70 RL = 200Ω –75 RL = 500Ω –80 –85 RL = 200Ω –65 RL = 100Ω –70 –75 –80 RL = 500Ω –85 –90 –90 0.1 1 0.1 10 1 10 Output Voltage Swing (Vp-p) Output Voltage Swing (Vp-p) 20MHz 2nd HARMONIC DISTORTION vs OUTPUT VOLTAGE 20MHz 3rd HARMONIC DISTORTION vs OUTPUT VOLTAGE –50 –50 –55 3rd Harmonic Distortion (dBc) RL = 100Ω 2nd Harmonic Distortion (dBc) 10 Output Voltage Swing (Vp-p) –60 2nd Harmonic Distortion (dBc) 1 Output Voltage Swing (Vp-p) RL = 200Ω –60 RL = 500Ω –65 –70 –75 –80 RL = 200Ω –55 RL = 100Ω –60 –65 –70 RL = 500Ω –75 –80 0.1 1 10 0.1 Output Voltage Swing (Vp-p) ® OPA2680 1 Output Voltage Swing (Vp-p) 6 10 TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT) At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 1. 2nd HARMONIC DISTORTION vs FREQUENCY 3rd HARMONIC DISTORTION vs FREQUENCY –40 VO = 2Vp-p RL = 100Ω –45 –50 G = +10 –55 –60 VO = 2Vp-p RL = 100Ω –45 3rd Harmonic Distortion (dBc) 2nd Harmonic Distortion (dBc) –40 G = +5 –65 –70 –75 –80 G = +2 –85 –50 G = +10 –55 –60 –65 G = +5 –70 –75 –80 G = +2 –85 –90 –90 0.1 1 10 20 0.1 1 Frequency (MHz) 10 20 Frequency (MHz) INPUT VOLTAGE AND CURRENT NOISE DENSITY TWO-TONE, 3rd-ORDER SPURIOUS LEVEL 100 –40 10 Voltage Noise 3rd-Order Spurious Level (dBc) Voltage Noise (nV/√Hz) Current Noise (pA/√Hz) 50MHz 4.8nV/√Hz Current Noise 2.5pA/√Hz –50 –60 20MHz –70 10MHz –80 Load Power at matched 50Ω load 1 –90 100 1k 10k 100k 1M 10M –8 –6 –4 Frequency (Hz) 0 2 4 6 8 10 FREQUENCY RESPONSE vs CAPACITIVE LOAD RECOMMENDED RS vs CAPACITIVE LOAD 12 70 9 Gain-to-Capacitive Load (3dB/div) 80 60 50 RS (Ω) –2 Single-Tone Load Power (dBm) 40 30 20 10 CL = 10pF G = +2 CL = 22pF 6 3 CL = 47pF 0 VIN –3 –6 1/2 OPA2680 402Ω –9 –12 RS VO CL 1kΩ CL = 100pF 402Ω –15 1kΩ is optional –18 0 10 0 100 100MHz 200MHz Frequency (20MHz/div) Capacitive Load (pF) ® 7 OPA2680 TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT) At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 1. OPEN-LOOP GAIN AND PHASE 90 60 0 –30 Open-Loop Phase 80 –PSRR 70 +PSRR 50 –60 Open-Loop Gain 40 –90 30 –120 20 –150 10 –180 0 –210 10 –10 –240 0 –20 60 CMRR 50 40 30 20 10k 100k 1M 10M –270 10k 100M 100k 1M COMPOSITE VIDEO dG/dP 75Ω 402Ω Input Offset Voltage (mV) Input Bias and Offset Current (µA) With 1.3kΩ Pulldown Optional 1.3kΩ Pulldown dP dG 0.1 402Ω 0.075 –5V 0.05 dG dP 0.025 0 10 IB 5 VIO 0 IOS –5 –10 –15 1 2 3 4 –40 –20 0 Number of 150Ω Loads OUTPUT VOLTAGE AND CURRENT LIMITATIONS 5 3 VO (Volts) 2 1 1W Internal Power Limit One Channel Only 25Ω Load Line 50Ω Load Line –1 100Ω Load Line –2 –3 –4 60 80 100 120 140 SUPPLY AND OUTPUT CURRENT vs TEMPERATURE Output Current Limited 0 40 200 Output Current (50mA/div) 4 20 Ambient Temperature (°C) 20.0 Sourcing Output Current Sinking Output Current 150 15.0 Quiescent Supply Current 100 10.0 50 5.0 1W Internal Power Limit Output Current Limit –5 0 –300 –200 –100 0 100 200 300 IO (mA) –20 0 20 40 60 80 Ambient Temperature (°C) ® OPA2680 0 –40 8 100 120 140 Supply Current (5.0mA/div) dG/dP (%/degrees) TYPICAL DC DRIFT OVER TEMPERATURE Video Loads 1/2 OPA2680 0.125 1G No Pulldown Video In 0.15 100M 15 +5V 0.175 10M Frequency (Hz) Frequency (Hz) 0.2 Open-Loop Phase (degrees) 70 Open-Loop Gain (dB) Power Supply Rejection Ratio (dB) Common-Mode Rejection Ratio (dB) CMRR AND PSRR vs FREQUENCY 100 TYPICAL PERFORMANCE CURVES: VS = +5V At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 2. SMALL-SIGNAL FREQUENCY RESPONSE 6 VO = 0.5Vp-p 9 0 VO = 1Vp-p 6 G = +2 –3 Gain (3dB/div) Normalized Gain (3dB/div) G = +1 RF = 25Ω VO = 0.5Vp-p 3 LARGE-SIGNAL FREQUENCY RESPONSE 12 –6 –9 G = +5 –12 –15 3 VO = 2Vp-p 0 –3 VO = 3Vp-p –6 –9 G = +10 –18 –12 –21 –15 –24 –18 0.5 10 100 500 0.5 10 Frequency (MHz) SMALL-SIGNAL PULSE RESPONSE 4.1 G = +2 VO = 0.5Vp-p 2.8 Output Voltage (400mV/div) Output Voltage (100mV/div) 500 LARGE-SIGNAL PULSE RESPONSE 2.9 2.7 2.6 2.5 2.4 2.3 2.2 G = +2 VO = 2Vp-p 3.7 3.3 2.9 2.5 2.1 1.7 1.3 2.1 0.9 Time (5ns/div) Time (5ns/div) RECOMMENDED RS vs CAPACITIVE LOAD FREQUENCY RESPONSE vs CAPACITIVE LOAD 70 12 Gain-to-Capacitive Load (3dB/div) Noise Gain = 2.6 60 50 RS (Ω) 100 Frequency (MHz) 40 30 20 10 0 CL = 47pF Signal Gain = +2 Noise Gain = 2.6 9 CL = 10pF CL = 22pF 6 CL = 100pF 3 +5V 0 –3 0.1µF 714Ω VI –6 58Ω 714Ω 714Ω 1/2 OPA2680 RS VO CL –9 402Ω –12 402Ω –15 0.1µF –18 1 10 100 0 Capacitive Load (pF) 100MHz 200MHz Frequency (20MHz/div) ® 9 OPA2680 TYPICAL PERFORMANCE CURVES: VS = +5V At TA = +25°C, G = +2, RF = 402Ω, and RL = 100Ω, unless otherwise noted. See Figure 2. 2nd HARMONIC DISTORTION vs FREQUENCY 3rd HARMONIC DISTORTION vs FREQUENCY –40 VO = 2Vp-p RL = 100Ω to VS/2 –45 3rd Harmonic Distortion (dBc) 2nd Harmonic Distortion (dBc) –40 G = +10 –50 –55 G = +5 –60 –65 G = +2 –70 VO = 2Vp-p RL = 100Ω to VS/2 –45 –50 G = +10 –55 G = +5 –60 –65 –70 –75 G = +2 –75 –80 0.1 1 10 20 0.1 1 Frequency (MHz) –40 VO = 2Vp-p VO = 2Vp-p –45 –50 3rd Harmonic Distortion (dBc) 2nd Harmonic Distortion (dBc) 20 3rd HARMONIC DISTORTION vs FREQUENCY 2nd HARMONIC DISTORTION vs FREQUENCY –40 RL = 100Ω –55 –60 RL = 200Ω –65 –70 RL = 500Ω –75 –45 –50 RL = 500 –55 –60 RL = 200 –65 RL = 100 –70 –75 –80 –80 0.1 1 10 0.1 20 1 10 20 Frequency (MHz) Frequency (MHz) TWO-TONE, 3RD-ORDER SPURIOUS LEVEL CLOSED-LOOP OUTPUT IMPEDANCE vs FREQUENCY 10 –40 dBc = dB Below Carrier –45 +5V 50MHz Output Impedance (Ω) 3rd-Order Spurious Level (dBc) 10 Frequency (MHz) –50 –55 20MHz –60 –65 –70 200Ω 1/2 OPA2680 ZO 1 –5V 402Ω 402Ω 0.1 10MHz –75 Load Power at Matched 50Ω Load 0.01 –80 –14 –12 –10 –8 –6 –4 –2 0 2 10k Single-Tone Load Power (dBm) 1M Frequency (Hz) ® OPA2680 100k 10 10M 100M 200Ω source resistance seen at the inverting input (see the DC Accuracy and Offset Control section). In addition to the usual power supply decoupling capacitors to ground, a 0.1µF capacitor is included between the two power supply pins. In practical PC board layouts, this optional-added capacitor will typically improve the 2nd harmonic distortion performance by 3dB to 6dB. APPLICATIONS INFORMATION WIDEBAND VOLTAGE FEEDBACK OPERATION The OPA2680 provides an exceptional combination of high output power capability in a dual, wideband, unity gain stable voltage feedback op amp using a new high slew rate input stage. Typical differential input stages used for voltage feedback op amps are designed to steer a fixed-bias current to the compensation capacitor, setting a limit to the achievable slew rate. The OPA2680 uses a new input stage which places the transconductance element between two input buffers, using their output currents as the forward signal. As the error voltage increases across the two inputs, an increasing current is delivered to the compensation capacitor. This provides very high slew rate (1800V/µs) while consuming relatively low quiescent current (6.4mA/ch.). This exceptional full power performance comes at the price of a slightly higher input noise voltage than alternative architectures. The 4.8nV/√Hz input voltage noise for the OPA2680 is exceptionally low for this type of input stage. Figure 2 shows the AC-coupled, gain of +2, single supply circuit configuration which is the basis of the +5V Specifications and Typical Performance Curves. Though not a “railto-rail” design, the OPA2680 requires minimal input and output voltage headroom compared to other very wideband voltage feedback op amps. It will deliver a 3Vp-p output swing on a single +5V supply with >150MHz bandwidth. The key requirement of broadband single-supply operation is to maintain input and output signal swings within the useable voltage ranges at both the input and the output. The circuit of Figure 2 establishes an input midpoint bias using a simple resistive divider from the +5V supply (two 698Ω resistors). Separate networks would be required at each input. The input signal is then AC-coupled into the midpoint voltage bias. The input voltage can swing to within 1.5V of either supply pin, giving a 2Vp-p input signal range centered between the supply pins. The input impedance matching resistor (59Ω) used for testing is adjusted to give a 50Ω input load when the parallel combination of the biasing divider network is included. Again, an additional resistor (50Ω in this case) is included directly in series with the non-inverting input. This minimum recommended value provides part of the DC source resistance matching for the non-inverting input bias current. It is also used to form a simple parasitic pole to roll off the frequency response at very high frequencies (>500MHz) using the input parasitic capacitance. The gain resistor (RG) is AC-coupled, giving the circuit a DC gain of +1, which puts the input DC bias voltage (2.5V) on the output as well. The output voltage can swing to within 1V of either supply pin while delivering >100mA output Figure 1 shows the DC-coupled, gain of +2, dual power supply circuit configuration used as the basis of the ±5V Specifications and Typical Performance Curves. This is for one channel. The other channel is connected similarly. For test purposes, the input impedance is set to 50Ω with a resistor to ground and the output impedance is set to 50Ω with a series output resistor. Voltage swings reported in the specifications are taken directly at the input and output pins, while output powers (dBm) are at the matched 50Ω load. For the circuit of Figure 1, the total effective load will be 100Ω || 804Ω. The disable control line (SO-14 package only) is typically left open to guarantee normal amplifier operation. Two optional components are included in Figure 1. An additional resistor (175Ω) is included in series with the noninverting input. Combined with the 25Ω DC source resistance looking back towards the signal generator, this gives an input bias current cancelling resistance that matches the +5V 0.1µF +5V +VS 6.8µF + 0.1µF 50Ω Source 175Ω VI 50Ω VO 1/2 OPA2680 50Ω 0.1µF 50Ω Load 50Ω DIS VD VI 698Ω 59Ω 0.1µF 6.8µF 698Ω DIS VD + 1/2 OPA2680 VO 100Ω VS/2 RF 402Ω RF 402Ω RG 402Ω RG 402Ω + 6.8µF 0.1µF 0.1µF –5V FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specification and Test Circuit. FIGURE 2. AC-Coupled, G = +2, Single Supply Specification and Test Circuit. ® 11 OPA2680 current. A demanding 100Ω load to a midpoint bias is used in this characterization circuit. The new output stage circuit used in the OPA2680 can deliver large bipolar output currents into this midpoint load with minimal crossover distortion, as shown in the +5V supply, 3rd harmonic distortion plots. 73 VO = 2Vp-p, 10MHz 72 SFDR (dBc) 71 SINGLE SUPPLY A/D CONVERTER INTERFACE Most modern, high performance analog-to-digital converters (such as the Burr-Brown ADS8xx and ADS9xx series) operate on a single +5V (or lower) power supply. It has been a considerable challenge for single supply op amps to deliver a low distortion input signal at the ADC input for signal frequencies exceeding 5MHz. The high slew rate, exceptional output swing and high linearity of the OPA2680 make it an ideal single supply ADC driver. The circuit on the front page shows one possible interface particularly suited to DC-coupled pulse digitization requirements. Figure 3 shows the AC-coupled test circuit of Figure 2 modified for a capacitive (A/D) load and with an optional output pull-down resistor (RB). 70 69 68 67 66 65 0 1 2 3 4 5 6 7 High frequency DDS DACs require a low distortion output amplifier to retain their SFDR performance into real-world loads. A differential output drive implementation is shown in Figure 5. The diagram shows the signal output current(s) connected into the virtual ground summing junction(s) of the OPA2680, which is set up as a transimpedance stage or “I-V converter”. If the DAC requires its outputs terminated to a compliance voltage other than ground for operation, the appropriate voltage level may be applied to the noninverting inputs of the OPA2680. The DC gain for this circuit is equal to RF. At high frequencies, the DAC output capacitance (CD in Figure 5) will produce a zero in the noise gain for the OPA2680 that may cause peaking in the closed-loop frequency response. CF is added across RF to compensate for this noise gain peaking. To achieve a flat Power supply decoupling not shown 698Ω 50Ω RS 30Ω 1/2 OPA2680 1Vp-p 59Ω 698Ω 2.5V DC ±1V AC 50pF 402Ω 402Ω RB 0.1µF FIGURE 3. Single-Supply ADC Input Driver. One of Two Channels. ® OPA2680 10 HIGH PERFORMANCE DAC TRANSIMPEDANCE AMPLIFIER +5V 0.1µF 9 FIGURE 4. SFDR vs IB. The OPA2680 in the circuit of Figure 3 provides >200MHz bandwidth for a 2Vp-p output swing. Minimal 3rd harmonic distortion or two-tone, 3rd-order intermodulation distortion will be observed due to the very low crossover distortion in the OPA2680 output stage. The limit of output Spurious Free Dynamic Range (SFDR) will be set by the 2nd harmonic distortion. Without RB, the circuit of Figure 3 measured at 10MHz shows an SFDR of 65dBc. This may be improved by pulling additional DC bias current (IB) out of the output stage through the optional RB resistor to ground (the output midpoint is at 2.5V for Figure 3). Adjusting IB gives the improvement in SFDR shown in Figure 4. SFDR improvement is achieved for IB values up to 6mA, with worse performance for higher values. Using the dual OPA2680 in an IQ receiver channel will give matched AC performance through high frequencies. VI 8 Output Pull-Down Current (mA) 12 IB ADC Input WIDEBAND VIDEO MULTIPLEXING transimpedance frequency response, the pole in each feedback network should be set to: One common application for video speed amplifiers which include a disable pin is to wire multiple amplifier outputs together, then select which one of several possible video inputs to source onto a single line. This simple “Wired-OR Video Multiplexer” can be easily implemented using the OP2680N (SO-14 package only) as shown in Figure 6. 1/2πRFCF = √GBP/4πRFCD which will give a corner frequency f–3dB of approximately: f–3dB = √GBP/(2πRFCD) Typically, channel switching is performed either on sync or retrace time in the video signal. The two inputs are approximately equal at this time. The “make-before-break” disable characteristic of the OPA2680N ensures that there is always one amplifier controlling the line when using a wired-OR circuit like that shown in Figure 6. Since both inputs may be on for a short period during the transition between channels, the outputs are combined through the output impedance matching resistors (82.5Ω in this case). When one channel is disabled, its feedback network forms part of the output impedance and slightly attenuates the signal in getting out onto the cable. The gain and output matching resistor have been slightly increased to get a signal gain of +1 at the matched load and provide a 75Ω output impedance to the cable. The video multiplexer connection (Figure 6) also insures that the maximum differential voltage across the inputs of the unselected channel does not exceed the rated ±1.2V maximum for standard video signal levels. 50Ω 1/2 OPA2680 High Speed DAC VO = IO RF RF1 CF1 IO CD1 RF2 CD2 IO CF2 1/2 OPA2680 50Ω VO = IO RF GBP → Gain Bandwidth Product (Hz) for the OPA2680 The section on Disable Operation shows the turn-on and turn-off switching glitches using a 0V input for a single channel is typically less than ±50mV. Where two outputs are switched (as shown in Figure 6), the output line is always under the control of one amplifier or the other due to the “make-before-break” disable timing. In this case, the switching glitches for two 0V inputs drop to <20mV. FIGURE 5. High Speed DAC—Differential Transimpedance Amplifier +5V 2kΩ VDIS +5V 50Ω Video 1 DISA 1/2 OPA2680 75Ω 340Ω 82.5Ω –5V 402Ω 75Ω Cable 340Ω RG-59 402Ω 75Ω Load +5V 50Ω Video 2 82.5Ω 1/2 OPA2680 DISB 75Ω –5V 2kΩ FIGURE 6. Two-Channel Video Multiplexer (SO-14 package only). ® 13 OPA2680 HIGH SPEED DELAY CIRCUIT with TD = (1/360) • (dφ/df) = delay of the op amp itself. The values of resistors RF and RG should be equal and low to avoid parasitic effects. If the all-pass filter is designed for very low delay times, include parasitic board capacitances to calculate the correct delay time. Simulating this application using the PSPICE model of the OPA2680 will allow this design to be tuned. The OPA2680 makes an ideal amplifier for a variety of active filter designs. Shown in Figure 7 is a circuit that utilizes the two amplifiers within the dual OPA2680 to design a two-stage analog group delay adjustment circuit. For simplicity, the circuit assumes a dual supply (±5V) operation, but it can also be modified to operate on signal supply. The input to the first filter stage is driven by the wideband buffer amplifier, BUF601 to isolate the signal input from the filter network. The BUF601 features an openloop design, holding its output impedance at 4Ω for frequencies in excess of 200MHz. DIFFERENTIAL RECEIVER/DRIVER A very versatile application for a dual operational amplier is the differential amplifier configuration shown in Figure 8. With both amplifiers of the OPA2680 connected for non-inverting operation, the circuit provides a high input impedance while the gain can easily be set by just one resistor, RG. When operated in low gains, the output swing may be limited as a result of the common-mode input swing limits of the amplifier itself. An interesting modification of this circuit is to place a capacitor in series with the RG. Now the DC gain for each side is reduced to +1, while the AC gain still follows the standard transfer function of G = 1 + 2RF/RG. This might be advantageous for applications processing only a frequency band which excludes DC or very low frequencies. An input DC voltage resulting from input bias currents will not be amplified by the AC gain and can be kept low. This circuit can be used as a differential line receiver, driver, or as an interface to a differential input A/D converter. Each of the two filter stages is a 1st-order filter with a voltage gain of +1. The delay time through one filter is given by Equation 1. Equation 1: tGR0 = 2RC For a more accurate analysis of the circuit consider the group delay for the amplifiers. For example, in the case of the OPA2680, the group delay in the bandwidth from 1MHz to 100MHz is approximately 1.0ns. To account for this, modify the transfer function, which now comes out to be: Equation 2: tGR = 2 (2RC +TD) C VIN BUF601 C 1/2 OPA2680 1/2 OPA2680 R VOUT R RG 402Ω RF 402Ω RG 402Ω RF 402Ω FIGURE 7. Two Stage, All-Pass Network. 50Ω VI RO 1/2 OPA2680 RF 402Ω RG 50Ω VDIFF = 1 + RF 402Ω RO 1/2 OPA2680 VI FIGURE 8. High Speed Differential Receiver. ® OPA2680 14 2RF RG VI – VI DESIGN-IN TOOLS A good rule of thumb is to target the parallel combination of RF and RG (Figure 1) to be less than approximately 300Ω. The combined impedance RF || RG interacts with the inverting input capacitance, placing an additional pole in the feedback network and thus, a zero in the forward response. Assuming a 2pF total parasitic on the inverting node, holding RF || RG < 300Ω will keep this pole above 250MHz. By itself, this constraint implies that the feedback resistor RF can increase to several kΩ at high gains. This is acceptable as long as the pole formed by RF and any parasitic capacitance appearing in parallel is kept out of the frequency range of interest. DEMONSTRATION BOARDS Several PC boards are available to assist in the initial evaluation of circuit performance using the OPA2680 in its two package styles. All of these are available free as an unpopulated PC board delivered with descriptive documentation. The summary information for these boards is shown below: PRODUCT PACKAGE BOARD PART NUMBER OPA2680U OPA2680N 8-Lead SO-8 SO-14 DEM-OPA26xU DEM-OPA26xN ORDERING NUMBER MKT-352 MKT-353 BANDWIDTH VS GAIN: NON-INVERTING OPERATION Voltage feedback op amps exhibit decreasing closed-loop bandwidth as the signal gain is increased. In theory, this relationship is described by the Gain Bandwidth Product (GBP) shown in the specifications. Ideally, dividing GBP by the non-inverting signal gain (also called the Noise Gain, or NG) will predict the closed-loop bandwidth. In practice, this only holds true when the phase margin approaches 90°, as it does in high gain configurations. At low gains (increased feedback factor), most amplifiers will exhibit a more complex response with lower phase margin. The OPA2680 is compensated to give a slightly peaked response in a noninverting gain of 2 (Figure 1). This results in a typical gain of +2 bandwidth of 220MHz, far exceeding that predicted by dividing the 300MHz GBP by 2. Increasing the gain will cause the phase margin to approach 90° and the bandwidth to more closely approach the predicted value of (GBP/NG). At a gain of +10, the 30MHz bandwidth shown in the Typical Specifications agrees with that predicted using the simple formula and the typical GBP of 300MHz. Contact the Burr-Brown Applications support line to request any of these boards. MACROMODELS AND APPLICATIONS SUPPORT Computer simulation of circuit performance using SPICE is often useful when analyzing the performance of analog circuits and systems. This is particularly true for Video and RF amplifier circuits where parasitic capacitance and inductance can have a major effect on circuit performance. A SPICE model for the OPA2680 is available through either the Burr-Brown Internet web page (http://www.burrbrown.com) or as one model on a disk from the Burr-Brown Applications Department (1-800-548-6132). The Application Department is also available for design assistance at this number. These models do a good job of predicting smallsignal AC and transient performance under a wide variety of operating conditions. They do not do as well in predicting the harmonic distortion or dG/dP characteristics. These models do not attempt to distinguish between the package types in their small-signal AC performance. The frequency response in a gain of +2 may be modified to achieve exceptional flatness simply by increasing the noise gain to 2.5. One way to do this, without affecting the +2 signal gain, is to add an 804Ω resistor across the two inputs in the circuit of Figure 1. A similar technique may be used to reduce peaking in unity gain (voltage follower) applications. For example, by using a 402Ω feedback resistor along with a 402Ω resistor across the two op amp inputs, the voltage follower response will be similar to the gain of +2 response of Figure 2. Reducing the value of the resistor across the op amp inputs will further limit the frequency response due to increased noise gain. OPERATING SUGGESTIONS OPTIMIZING RESISTOR VALUES Since the OPA2680 is a unity gain stable voltage feedback op amp, a wide range of resistor values may be used for the feedback and gain setting resistors. The primary limits on these values are set by dynamic range (noise and distortion) and parasitic capacitance considerations. For a non-inverting unity gain follower application, the feedback connection should be made with a 25Ω resistor, not a direct short. This will isolate the inverting input capacitance from the output pin and improve the frequency response flatness. Usually, the feedback resistor value should be between 200Ω and 1.5kΩ. Below 200Ω, the feedback network will present additional output loading which can degrade the harmonic distortion performance of the OPA2680. Above 1.5kΩ, the typical parasitic capacitance (approximately 0.2pF) across the feedback resistor may cause unintentional band-limiting in the amplifier response. The OPA2680 exhibits minimal bandwidth reduction going to single supply (+5V) operation as compared with ±5V. This is because the internal bias control circuitry retains nearly constant quiescent current as the total supply voltage between the supply pins is changed. INVERTING AMPLIFIER OPERATION Since the OPA2680 is a general purpose, wideband voltage feedback op amp, all of the familiar op amp application circuits are available to the designer. Inverting operation is one of the more common requirements and offers ® 15 OPA2680 several performance benefits. Figure 9 shows a typical inverting configuration where the I/O impedances and signal gain from Figure 1 are retained in an inverting circuit configuration. ance is added in series with RG for calculating the noise gain (NG). The resultant NG is 2.8 for Figure 9, as opposed to only 2 if RM could be eliminated as discussed above. The bandwidth will therefore be slightly lower for the gain of –2 circuit of Figure 9 than for the gain of +2 circuit of Figure 1. The third important consideration in inverting amplifier design is setting the bias current cancellation resistor on the non-inverting input (RB). If this resistor is set equal to the total DC resistance looking out of the inverting node, the output DC error, due to the input bias currents, will be reduced to (Input Offset Current) • RF. If the 50Ω source impedance is DC-coupled in Figure 9, the total resistance to ground on the inverting input will be 228Ω. Combining this in parallel with the feedback resistor gives the RB = 146Ω used in this example. To reduce the additional high frequency noise introduced by this resistor, it is sometimes bypassed with a capacitor. As long as RB <350Ω, the capacitor is not required since the total noise contribution of all other terms will be less than that of the op amp’s input noise voltage. As a minimum, the OPA2680 requires an RB value of 50Ω to damp out parasitic-induced peaking—a direct short to ground on the non-inverting input runs the risk of a very high frequency instability in the input stage. +5V + 0.1µF 6.8µF 0.1µF RB 146Ω 50Ω RO 50Ω 50Ω Load VO = –2 VI RF 402Ω RG 200Ω Source VO 1/2 OPA2680 VI RM 67Ω 0.1µF + 6.8µF –5V OUTPUT CURRENT AND VOLTAGE The OPA2680 provides output voltage and current capabilities that are unsurpassed in a low cost monolithic op amp. Under no-load conditions at +25°C, the output voltage typically swings closer than 1V to either supply rail; the guaranteed swing limit is within 1.2V of either rail. Into a 15Ω load (the minimum tested load), it is guaranteed to deliver more than ±135mA. FIGURE 9. Gain of –2 Example Circuit. In the inverting configuration, three key design consideration must be noted. The first is that the gain resistor (RG) becomes part of the signal channel input impedance. If input impedance matching is desired (which is beneficial whenever the signal is coupled through a cable, twisted pair, long PC board trace or other transmission line conductor), RG may be set equal to the required termination value and RF adjusted to give the desired gain. This is the simplest approach and results in optimum bandwidth and noise performance. However, at low inverting gains, the resultant feedback resistor value can present a significant load to the amplifier output. For an inverting gain of –2, setting RG to 50Ω for input matching eliminates the need for RM but requires a 100Ω feedback resistor. This has the interesting advantage that the noise gain becomes equal to 2 for a 50Ω source impedance—the same as the non-inverting circuits considered above. However, the amplifier output will now see the 100Ω feedback resistor in parallel with the external load. In general, the feedback resistor should be limited to the 200Ω to 1.5kΩ range. In this case, it is preferable to increase both the RF and RG values as shown in Figure 8, and then achieve the input matching impedance with a third resistor (RM) to ground. The total input impedance becomes the parallel combination of RG and RM. The specifications described above, though familiar in the industry, consider voltage and current limits separately. In many applications, it is the voltage • current, or V-I product, which is more relevant to circuit operation. Refer to the “Output Voltage and Current Limitations” plot in the Typical Performance Curves. The X and Y axes of this graph show the zero-voltage output current limit and the zero-current output voltage limit, respectively. The four quadrants give a more detailed view of the OPA2680’s output drive capabilities, noting that the graph is bounded by a “Safe Operating Area” of 1W maximum internal power dissipation for each channel separately. Superimposing resistor load lines onto the plot shows that the OPA2680 can drive ±2.5V into 25Ω or ±3.5V into 50Ω without exceeding the output capabilities or the 1W dissipation limit. A 100Ω load line (the standard test circuit load) shows the full ±3.9V output swing capability, as shown in the typical specifications. The minimum specified output voltage and current specifications over temperature are set by worst-case simulations at the cold temperature extreme. Only at cold startup will the output current and voltage decrease to the numbers shown in the guaranteed tables. As the output transistors deliver power, their junction temperatures will increase, decreasing their VBE’s (increasing the available output voltage swing) and increasing their current gains (increasing the available output current). In steady-state operation, the available output The second major consideration, touched on in the previous paragraph, is that the signal source impedance becomes part of the noise gain equation and influences the bandwidth. For the example in Figure 9, the RM value combines in parallel with the external 50Ω source impedance, yielding an effective driving impedance of 50Ω || 67Ω = 28.6Ω. This imped® OPA2680 16 voltage and current will always be greater than that shown in the over-temperature specifications since the output stage junction temperatures will be higher than the minimum specified operating ambient. +5V 50Ω 175Ω 50Ω To maintain maximum output stage linearity, no output short-circuit protection is provided. This will not normally be a problem since most applications include a series matching resistor at the output that will limit the internal power dissipation if the output side of this resistor is shorted to ground. However, shorting the output pin directly to the adjacent positive power supply pin (8-pin packages) will, in most cases, destroy the amplifier. Including a small series resistor (5Ω) in the power supply line will protect against this. Always place the 0.1µF decoupling capacitor directly on the supply pins. RNG Power supply decoupling not shown. R 1/2 OPA2680 VO CLOAD 402Ω 402Ω –5V FIGURE 10. Capacitive Load Driving with Noise Gain Tuning. This gain of +2 circuit includes a noise gain tuning resistor across the two inputs to increase the noise gain, increasing the unloaded phase margin for the op amp. Although this technique will reduce the required RS resistor for a given capacitive load, it does increase the noise at the output. It also will decrease the loop gain, nominally decreasing the distortion performance. If, however, the dominant distortion mechanism arises from a high RS value, significant dynamic range improvement can be achieved using this technique. Figure 11 shows the required RS versus CLOAD parametric on noise gain using this technique. This is the circuit of Figure 10 with RNG adjusted to increase the noise gain (increasing the phase margin) then sweeping CLOAD and finding the required RS to get a flat frequency response. This plot also gives the required RS versus CLOAD for the OPA2680 operated at higher signal gains. DRIVING CAPACITIVE LOADS One of the most demanding and yet very common load conditions for an op amp is capacitive loading. Often, the capacitive load is the input of an A/D converter—including additional external capacitance which may be recommended to improve A/D linearity. A high speed, high open-loop gain amplifier like the OPA2680 can be very susceptible to decreased stability and closed-loop response peaking when a capacitive load is placed directly on the output pin. When the amplifier’s open-loop output resistance is considered, this capacitive load introduces an additional pole in the signal path that can decrease the phase margin. Several external solutions to this problem have been suggested. When the primary considerations are frequency response flatness, pulse response fidelity and/or distortion, the simplest and most effective solution is to isolate the capacitive load from the feedback loop by inserting a series isolation resistor between the amplifier output and the capacitive load. This does not eliminate the pole from the loop response, but rather shifts it and adds a zero at a higher frequency. The additional zero acts to cancel the phase lag from the capacitive load pole, thus increasing the phase margin and improving stability. 100 Series Resistor, RS (Ω) 90 The Typical Performance Curves show the recommended RS versus capacitive load and the resulting frequency response at the load. Parasitic capacitive loads greater than 2pF can begin to degrade the performance of the OPA2680. Long PC board traces, unmatched cables, and connections to multiple devices can easily exceed this value. Always consider this effect carefully, and add the recommended series resistor as close as possible to the OPA2680 output pin (see Board Layout Guidelines). 80 70 60 NG = 2 50 40 NG = 3 30 20 10 NG = 4 0 1 10 100 Capacitive Load (pF) FIGURE 11. Required RS vs Noise Gain. DISTORTION PERFORMANCE The criterion for setting this RS resistor is a maximum bandwidth, flat frequency response at the load. For the OPA2680 operating in a gain of +2, the frequency response at the output pin is already slightly peaked without the capacitive load requiring relatively high values of RS to flatten the response at the load. Increasing the noise gain will reduce the peaking as described previously. The circuit of Figure 10 demonstrates this technique, allowing lower values of RS to be used for a given capacitive load. The OPA2680 provides good distortion performance into a 100Ω load on ±5V supplies. Relative to alternative solutions, it provides exceptional performance into lighter loads and/or operating on a single +5V supply. Generally, until the fundamental signal reaches very high frequency or power levels, the 2nd harmonic will dominate the distortion with a negligible 3rd harmonic component. Focusing then on the 2nd harmonic, increasing the load impedance im® 17 OPA2680 proves distortion directly. Remember that the total load includes the feedback network; in the non-inverting configuration (Figure 1) this is sum of RF + RG, while in the inverting configuration, it is just RF. Also, providing an additional supply decoupling capacitor (0.1µF) between the supply pins (for bipolar operation) improves the 2nd-order distortion slightly (3dB to 6dB). The total output spot noise voltage can be computed as the square root of the sum of all squared output noise voltage contributors. Equation 3 shows the general form for the output noise voltage using the terms shown in Figure 12. Equation 3: EO = In most op amps, increasing the output voltage swing increases harmonic distortion directly. The new output stage used in the OPA2680 actually holds the difference between fundamental power and the 2nd and 3rd harmonic powers relatively constant with increasing output power until very large output swings are required (>4Vp-p). This also shows up in the two-tone, 3rd-order intermodulation spurious (IM3) response curves. The 3rd-order spurious levels are extremely low at low output power levels. The output stage continues to hold them low even as the fundamental power reaches very high levels. As the Typical Performance Curves show, the spurious intermodulation powers do not increase as predicted by a traditional intercept model. As the fundamental power level increases, the dynamic range does not decrease significantly. For 2 tones centered at 20MHz, with 10dBm/tone into a matched 50Ω load (i.e., 2Vp-p for each tone at the load, which requires 8Vp-p for the overall twotone envelope at the output pin), the Typical Performance Curves show 57dBc difference between the test tone powers and the 3rd-order intermodulation spurious powers. This exceptional performance improves further when operating at lower frequencies. DC ACCURACY AND OFFSET CONTROL The balanced input stage of a wideband voltage feedback op amp allows good output DC accuracy in a wide variety of applications. The power supply current trim for the OPA2680 gives even tighter control than comparable amplifiers. Although the high speed input stage does require relatively high input bias current (typically 14µA out of each input terminal), the close matching between them may be used to reduce the output DC error caused by this current. The total output offset voltage may be considerably reduced by matching the DC source resistances appearing at the two inputs. This reduces the output DC error due to the input bias currents to the offset current times the feedback resistor. Evaluating the configuration of Figure 1, using worst-case +25°C input offset voltage and current specifications, gives a worst-case output offset voltage equal to: EO ERS RF RG IBI 2 Evaluating these two equations for the OPA2680 circuit and component values shown in Figure 1 will give a total output spot noise voltage of 11nV/√Hz and a total equivalent input spot noise voltage of 5.5nV/√Hz. This is including the noise added by the bias current cancellation resistor (175Ω) on the non-inverting input. This total input-referred spot noise voltage is only slightly higher than the 4.8nV/√Hz specification for the op amp voltage noise alone. This will be the case as long as the impedances appearing at each op amp input are limited to the previously recommend maximum value of 300Ω. Keeping both (RF || RG) and the non-inverting input source impedance less than 300Ω will satisfy both noise and frequency response flatness considerations. Since the resistor-induced noise is relatively negligible, additional capacitive decoupling across the bias current cancellation resistor (RB) for the inverting op amp configuration of Figure 9 is not required. IBN 4kT RG 2 I R 2 4kTR F 2 E N = E NI 2 + ( I BN R S ) + 4kTR S + BI F + NG NG ENI √ 4kTRS ) + ( I BN R S ) + 4kTR S NG 2 + ( I BI R F ) + 4kTR F NG 2 Equation 4: High slew rate, unity gain stable, voltage feedback op amps usually achieve their slew rate at the expense of a higher input noise voltage. The 4.8nV/√Hz input voltage noise for the OPA2680 is, however, much lower than comparable amplifiers. The input-referred voltage noise, and the two input-referred current noise terms, combine to give low output noise under a wide variety of operating conditions. Figure 12 shows the op amp noise analysis model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/√Hz or pA/√Hz. 1/2 OPA2680 NI Dividing this expression by the noise gain (NG = (1+RF /RG)) will give the equivalent input-referred spot noise voltage at the non-inverting input, as shown in Equation 4. NOISE PERFORMANCE RS (E ±(NG • VOS(MAX)) ± (RF • IOS(MAX)) = ±(2 • 4.5mV) ± (402Ω • 0.7µA) = ±9.3mV √ 4kTRF 4kT = 1.6E –20J at 290°K – (NG = non-inverting signal gain) FIGURE 12. Op Amp Noise Analysis Model. ® OPA2680 18 A fine scale output offset null, or DC operating point adjustment, is often required. Numerous techniques are available for introducing DC offset control into an op amp circuit. Most of these techniques eventually reduce to adding a DC current through the feedback resistor. In selecting an offset trim method, one key consideration is the impact on the desired signal path frequency response. If the signal path is intended to be non-inverting, the offset control is best applied as an inverting summing signal to avoid interaction with the signal source. If the signal path is intended to be inverting, applying the offset control to the non-inverting input may be considered. However, the DC offset voltage on the summing junction will set up a DC current back into the source which must be considered. Applying an offset adjustment to the inverting op amp input can change the noise gain and frequency response flatness. For a DC-coupled inverting amplifier, Figure 13 shows one example of an offset adjustment technique that has minimal impact on the signal frequency response. In this case, the DC offsetting current is brought into the inverting input node through resistor values that are much larger than the signal path resistors. This will insure that the adjustment circuit has minimal effect on the loop gain and hence the frequency response. +VS 15kΩ Q1 25kΩ 1/2 OPA2680 eventually turning on those two diodes (≈100uA). At this point, any further current pulled out of VDIS goes through those diodes holding the emitter-base voltage of Q1 at approximately zero volts. This shuts off the collector current out of Q1, turning the amplifier off. The supply current in the disable mode are only those required to operate the circuit of Figure 14. Additional circuitry ensures that turn-on time occurs faster than turn-off time (make-before-break). When disabled, the output and input nodes go to a high impedance state. If the OPA2680 is operating in a gain of +1, this will show a very high impedance at the output and exceptional signal isolation. If operating at a gain greater than +1, the total feedback network resistance (RF + RG) will appear as the impedance looking back into the output, but the circuit will still show very high forward and reverse isolation. If configured as an inverting amplifier, the input and output will be connected through the feedback network resistance (RF + RG) and the isolation will be very poor as a result. VO –5V RG 500Ω +5V 5kΩ RF 1kΩ VI 20kΩ ±200mV Output Adjustment 10kΩ 0.1µF 5kΩ VO VI =– RF RG –VS FIGURE 14. Simplified Disable Control Circuit. Supply Decoupling Not Shown 328Ω IS Control VDIS +5V 0.1µF 110kΩ One key parameter in disable operation is the output glitch when switching in and out of the disabled mode. Figure 15 shows these glitches for the circuit of Figure 1 with the input signal at 0V. The glitch waveform at the output pin is plotted along with the DIS pin voltage. = –2 –5V FIGURE 13. DC-Coupled, Inverting Gain of –2, with Offset Adjustment. DISABLE OPERATION (SO-14 Package Only) 40 Output Voltage (20mV/div) The OPA2680N provides an optional disable feature that may be used either to reduce system power or to implement a simple channel multiplexing operation. If the DIS control pin is left unconnected, the OPA2680N will operate normally. To disable, the control pin must be asserted LOW. Figure 14 shows a simplified internal circuit for the disable control feature. In normal operation, base current to Q1 is provided through the 110kΩ resistor, while the emitter current through the 15kΩ resistor sets up a voltage drop that is inadequate to turn on the two diodes in Q1’s emitter. As VDIS is pulled LOW, additional current is pulled through the 15kΩ resistor 20 Output Voltage (0V Input) 0 –20 –40 4.8V VDIS 0.2V Time (20ns/div) FIGURE 15. Disable/Enable Glitch. ® 19 OPA2680 The transition edge rate (dv/dt) of the DIS control line will influence this glitch. For the plot of Figure 15, the edge rate was reduced until no further reduction in glitch amplitude was observed. This approximately 1V/ns maximum slew rate may be achieved by adding a simple RC filter into the DIS pin from a higher speed logic line. If extremely fast transition logic is used, a 2kΩ series resistor between the logic gate and the DIS input pin will provide adequate bandlimiting using just the parasitic input capacitance on the DIS pin while still ensuring adequate logic level swing. opened in all of the ground and power planes around those pins. Otherwise, ground and power planes should be unbroken elsewhere on the board. b) Minimize the distance (<0.25") from the power supply pins to high frequency 0.1µF decoupling capacitors. At the device pins, the ground and power plane layout should not be in close proximity to the signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling capacitors. The power supply connections should always be decoupled with these capacitors. An optional supply decoupling capacitor (0.1µF) across the two power supplies (for bipolar operation) will improve 2nd harmonic distortion performance. Larger (2.2µF to 6.8µF) decoupling capacitors, effective at lower frequency, should also be used on the main supply pins. These may be placed somewhat farther from the device and may be shared among several devices in the same area of the PC board. THERMAL ANALYSIS Due to the high output power capability of the OPA2680, heatsinking or forced airflow may be required under extreme operating conditions. Maximum desired junction temperature will set the maximum allowed internal power dissipation as described below. In no case should the maximum junction temperature be allowed to exceed 175°C. c) Careful selection and placement of external components will preserve the high frequency performance of the OPA2680. Resistors should be a very low reactance type. Surface-mount resistors work best and allow a tighter overall layout. Metal film or carbon composition axiallyleaded resistors can also provide good high frequency performance. Again, keep their leads and PC board traces as short as possible. Never use wirewound type resistors in a high frequency application. Since the output pin and inverting input pin are the most sensitive to parasitic capacitance, always position the feedback and series output resistor, if any, as close as possible to the output pin. Other network components, such as non-inverting input termination resistors, should also be placed close to the package. Where double-side component mounting is allowed, place the feedback resistor directly under the package on the other side of the board between the output and inverting input pins. Even with a low parasitic capacitance shunting the external resistors, excessively high resistor values can create significant time constants that can degrade performance. Good axial metal film or surface-mount resistors have approximately 0.2pF in shunt with the resistor. For resistor values >1.5kΩ, this parasitic capacitance can add a pole and/or zero below 500MHz that can effect circuit operation. Keep resistor values as low as possible consistent with load driving considerations. The 402Ω feedback used in the typical performance specifications is a good starting point for design. Note that a 25Ω feedback resistor, rather than a direct short, is suggested for the unity gain follower application. This effectively isolates the inverting input capacitance from the output pin that would otherwise cause additional peaking in the gain of +1 frequency response. Operating junction temperature (TJ) is given by TA + PD•θJA. The total internal power dissipation (PD) is the sum of quiescent power (PDQ) and additional power dissipated in the output stage (PDL) to deliver load power. Quiescent power is simply the specified no-load supply current times the total supply voltage across the part. PDL will depend on the required output signal and load but would, for a grounded resistive load, be at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage (for equal bipolar supplies). Under this condition, PDL = VS2/(4•RL) where RL includes feedback network loading. Note that it is the power in the output stage and not into the load that determines internal power dissipation. As a worst-case example, compute the maximum TJ using an OPA2680U (SO-8 package) in the circuit of Figure 1 operating at the maximum specified ambient temperature of +85°C and with both outputs driving a grounded 20Ω load to +2.5V. PD = 10V • 14.4mA + 2 [52/(4•(20Ω || 804Ω))] = 785mW Maximum TJ = +85°C + (0.79W • 125°C/W) = 184°C. This absolute worst-case condition exceeds the specified maximum junction temperature. Actual PDL will always be less than that considered here. Carefully consider maximum TJ in your application. BOARD LAYOUT GUIDELINES Achieving optimum performance with a high frequency amplifier like the OPA2680 requires careful attention to board layout parasitics and external component types. Recommendations that will optimize performance include: d) Connections to other wideband devices on the board may be made with short direct traces or through on-board transmission lines. For short connections, consider the trace and the input to the next device as a lumped capacitive load. Relatively wide traces (50 to 100mils) should be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set RS from the plot of Recommended RS vs Capacitive Load. Low parasitic capacitive loads (<5pF) may not need an RS since the a) Minimize parasitic capacitance to any AC ground for all of the signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability: on the noninverting input, it can react with the source impedance to cause unintentional bandlimiting. To reduce unwanted capacitance, a window around the signal I/O pins should be ® OPA2680 20 INPUT AND ESD PROTECTION OPA2680 is nominally compensated to operate with a 2pF parasitic load. Higher parasitic capacitive loads without an RS are allowed as the signal gain increases (increasing the unloaded phase margin) If a long trace is required, and the 6dB signal loss intrinsic to a doubly terminated transmission line is acceptable, implement a matched impedance transmission line using microstrip or stripline techniques (consult an ECL design handbook for microstrip and stripline layout techniques). A 50Ω environment is normally not necessary on board, and in fact, a higher impedance environment will improve distortion as shown in the distortion versus load plots. With a characteristic board trace impedance defined (based on board material and trace dimensions), a matching series resistor into the trace from the output of the OPA2680 is used as well as a terminating shunt resistor at the input of the destination device. Remember also that the terminating impedance will be the parallel combination of the shunt resistor and the input impedance of the destination device; this total effective impedance should be set to match the trace impedance. The high output voltage and current capability of the OPA2680 allows multiple destination devices to be handled as separate transmission lines, each with their own series and shunt terminations. If the 6dB attenuation of a doubly terminated transmission line is unacceptable, a long trace can be series-terminated at the source end only. Treat the trace as a capacitive load in this case and set the series resistor value as shown in the plot of Recommended RS vs Capacitive Load. This will not preserve signal integrity as well as a doubly terminated line. If the input impedance of the destination device is low, there will be some signal attenuation due to the voltage divider formed by the series output into the terminating impedance. The OPA2680 is built using a very high speed complementary bipolar process. The internal junction breakdown voltages are relatively low for these very small geometry devices. These breakdowns are reflected in the Absolute Maximum Ratings table. All device pins are protected with internal ESD protection diodes to the power supplies as shown in Figure 16. These diodes provide moderate protection to input overdrive voltages above the supplies as well. The protection diodes can typically support 30mA continuous current. Where higher currents are possible (e.g., in systems with ±15V supply parts driving into the OPA2680), current-limiting series resistors should be added into the two inputs. Keep these resistor values as low as possible since high values degrade both noise performance and frequency response. +V CC External Pin Internal Circuitry –V CC FIGURE 16. Internal ESD Protection. e) Socketing a high speed part like the OPA2680 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network which can make it almost impossible to achieve a smooth, stable frequency response. Best results are obtained by soldering the OPA2680 onto the board. ® 21 OPA2680