60 MHz 2000 V/μs Monolithic Op Amp AD844 FUNCTIONAL BLOCK DIAGRAMS Wide bandwidth 60 MHz at gain of −1 33 MHz at gain of −10 Slew rate: 2000 V/μs 20 MHz full power bandwidth, 20 V p-p, RL = 500 Ω Fast settling: 100 ns to 0.1% (10 V step) Differential gain error: 0.03% at 4.4 MHz Differential phase error: 0.16° at 4.4 MHz Low offset voltage: 150 μV maximum (B Grade) Low quiescent current: 6.5 mA Available in tape and reel in accordance with EIA-481-A standard NULL 1 +VS +IN 3 6 OUTPUT –VS 4 5 TZ NC 1 16 NC OFFSETNULL 2 15 OFFSETNULL –IN 3 14 V+ NC 4 13 NC +IN 5 12 OUTPUT V– 7 AD844 11 TZ 10 NC TOP VIEW NC 8 (Not to Scale) 9 NC NC = NO CONNECT 00897-002 NC 6 Figure 2. 16-Lead SOIC (R) Package The AD844A and AD844B are specified for the industrial temperature range of −40°C to +85°C and are available in the CERDIP (Q) package. The AD844A is also available in an 8-lead PDIP (N). The AD844S is specified over the military temperature range of −55°C to +125°C. It is available in the 8-lead CERDIP (Q) package. A and S grade chips and devices processed to MIL-STD-883B, Rev. C are also available. PRODUCT HIGHLIGHTS The AD844 can be used in place of traditional op amps, but its current feedback architecture results in much better ac performance, high linearity, and an exceptionally clean pulse response. 1. This type of op amp provides a closed-loop bandwidth that is determined primarily by the feedback resistor and is almost independent of the closed-loop gain. The AD844 is free from the slew rate limitations inherent in traditional op amps and other current-feedback op amps. Peak output rate of change can be over 2000 V/μs for a full 20 V output step. Settling time is typically 100 ns to 0.1%, and essentially independent of gain. The AD844 can drive 50 Ω loads to ±2.5 V with low distortion and is short-circuit protected to 80 mA. 3. The AD844 is available in four performance grades and three package options. In the 16-lead SOIC (RW) package, the AD844J is specified for the commercial temperature range of 0°C to 70°C. NULL 7 Figure 1. 8-Lead PDIP (N) and 8-Lead CERDIP (Q) Packages Flash ADC input amplifiers High speed current DAC interfaces Video buffers and cable drivers Pulse amplifiers The AD844 is a high speed monolithic operational amplifier fabricated using the Analog Devices, Inc., junction isolated complementary bipolar (CB) process. It combines high bandwidth and very fast large signal response with excellent dc performance. Although optimized for use in current-to-voltage applications and as an inverting mode amplifier, it is also suitable for use in many noninverting applications. 8 TOP VIEW (Not to Scale) APPLICATIONS GENERAL DESCRIPTION AD844 –IN 2 00897-001 FEATURES 2. 4. 5. 6. The AD844 is a versatile, low cost component providing an excellent combination of ac and dc performance. It is essentially free from slew rate limitations. Rise and fall times are essentially independent of output level. The AD844 can be operated from ±4.5 V to ±18 V power supplies and is capable of driving loads down to 50 Ω, as well as driving very large capacitive loads using an external network. The offset voltage and input bias currents of the AD844 are laser trimmed to minimize dc errors; VOS drift is typically 1 μV/°C and bias current drift is typically 9 nA/°C. The AD844 exhibits excellent differential gain and differential phase characteristics, making it suitable for a variety of video applications with bandwidths up to 60 MHz. The AD844 combines low distortion, low noise, and low drift with wide bandwidth, making it outstanding as an input amplifier for flash analog-to-digital converters (ADCs). Rev. F Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©1989–2009 Analog Devices, Inc. All rights reserved. AD844 TABLE OF CONTENTS Features .............................................................................................. 1 Response as an Inverting Amplifier ......................................... 12 Applications ....................................................................................... 1 Response as an I-V Converter .................................................. 13 Functional Block Diagrams ............................................................. 1 Circuit Description of the AD844 ............................................ 13 General Description ......................................................................... 1 Response as a Noninverting Amplifier.................................... 14 Product Highlights ........................................................................... 1 Noninverting Gain of 100 ......................................................... 14 Revision History ............................................................................... 2 Using the AD844 ............................................................................ 15 Specifications..................................................................................... 3 Board Layout ............................................................................... 15 Absolute Maximum Ratings............................................................ 5 Input Impedance ........................................................................ 15 Metallization Photograph ............................................................ 5 Driving Large Capacitive Loads ............................................... 15 ESD Caution .................................................................................. 5 Settling Time ............................................................................... 15 Typical Performance Characteristics ............................................. 6 DC Error Calculation ................................................................ 16 Inverting Gain-of-1 AC Characteristics .................................... 8 Noise ............................................................................................ 16 Inverting Gain-of-10 AC Characteristics .................................. 9 Video Cable Driver Using ±5 V Supplies ................................ 16 Inverting Gain-of-10 Pulse Response ...................................... 10 High Speed DAC Buffer ............................................................ 17 Noninverting Gain-of-10 AC Characteristics ........................ 11 20 MHz Variable Gain Amplifier ............................................. 17 Understanding the AD844 ............................................................ 12 Outline Dimensions ....................................................................... 19 Open-Loop Behavior ................................................................. 12 Ordering Guide .......................................................................... 20 REVISION HISTORY 2/09—Rev. E to Rev F Updated Format .................................................................. Universal Changes to Features Section............................................................ 1 Changes to Differential Phase Error Parameter, Table 1 ............. 3 Changes to Figure 13 ........................................................................ 8 Changes to Figure 18 ........................................................................ 9 Changes to Figure 23 and Figure 24 ............................................. 11 Changes to Figure 42 and High Speed DAC Buffer Section ..... 17 Updated Outline Dimensions ....................................................... 19 Changes to Ordering Guide .......................................................... 20 1/03 Data Sheet changed from REV. D to REV. E. Updated Features ...............................................................................1 Edit to TPC 18 ...................................................................................7 Edits to Figure 13 and Figure 14................................................... 13 Updated Outline Dimensions ....................................................... 15 11/01 Data Sheet changed from REV. C to REV. D. Edits to Specifications ......................................................................2 Edits to Absolute Maximum Ratings ..............................................3 Edits to Ordering Guide ...................................................................3 Rev. F | Page 2 of 20 AD844 SPECIFICATIONS TA = 25°C and VS = ±15 V dc, unless otherwise noted. Table 1. Parameter INPUT OFFSET VOLTAGE 1 TMIN to TMAX vs. Temperature vs. Supply Initial TMIN to TMAX vs. Common Mode Initial TMIN to TMAX INPUT BIAS CURRENT Negative Input Bias Current1 TMIN to TMAX vs. Temperature vs. Supply Initial TMIN to TMAX vs. Common Mode Initial TMIN to TMAX Positive Input Bias Current1 TMIN to TMAX vs. Temperature vs. Supply Initial TMIN to TMAX vs. Common Mode Initial TMIN to TMAX INPUT CHARACTERISTICS Input Resistance Negative Input Positive Input Input Capacitance Negative Input Positive Input Input Common-Mode Voltage Range INPUT VOLTAGE NOISE INPUT CURRENT NOISE Negative Input Positive Input OPEN-LOOP TRANSRESISTANCE TMIN to TMAX Transcapacitance DIFFERENTIAL GAIN ERROR 2 DIFFERENTIAL PHASE ERROR2 Conditions AD844J/AD844A Min Typ Max 50 300 75 500 1 Min AD844B Typ 50 75 1 Max 150 200 5 Min AD844S Typ 50 125 1 Max 300 500 5 Unit μV μV μV/°C 5 V to 18 V 4 4 20 4 4 10 10 4 4 20 20 μV/V μV/V 10 10 35 10 10 20 20 10 10 35 35 μV/V μV/V 200 800 9 450 1500 150 750 9 250 1100 15 200 1900 20 450 2500 30 nA nA nA/°C 175 220 250 175 220 200 240 175 220 250 300 nA/V nA/V 90 110 150 350 3 160 90 110 100 300 3 110 150 200 500 7 90 120 100 800 7 160 200 400 1300 15 nA/V nA/V nA nA nA/°C VCM = ±10 V 5 V to 18 V VCM = ±10 V 400 700 5 V to 18 V 80 100 150 80 100 100 120 80 120 150 200 nA/V nA/V 90 130 150 90 130 120 190 90 140 150 200 nA/V nA/V 50 10 65 50 10 65 50 10 65 Ω MΩ VCM = ±10 V 7 7 2 2 ±10 7 2 2 ±10 2 2 pF pF V ±10 f ≥ 1 kHz 2 2 2 nV/√Hz f ≥ 1 kHz f ≥ 1 kHz VOUT = ±10 V RL = 500 Ω 10 12 10 12 10 12 pV/√Hz pV/√Hz 3.0 1.6 4.5 0.03 0.16 MΩ MΩ pF % Degree f = 4.4 MHz f = 4.4 MHz 2.2 1.3 3.0 2.0 4.5 0.03 0.16 Rev. F | Page 3 of 20 2.8 1.6 3.0 2.0 4.5 0.03 0.16 2.2 1.3 AD844 Parameter FREQUENCY RESPONSE Small Signal Bandwidth 3, 4 Gain = −1 Gain = −10 TOTAL HARMONIC DISTORTION SETTLING TIME 10 V Output Step Gain = −1, to 0.1%5 Gain = −10, to 0.1% 6 2 V Output Step Gain = −1, to 0.1%5 Gain = −10, to 0.1%6 OUTPUT SLEW RATE FULL POWER BANDWIDTH VOUT = 20 V p-p5 VOUT = 2 V p-p5 OUTPUT CHARACTERISTICS Voltage Short-Circuit Current TMIN to T MAX Output Resistance POWER SUPPLY Operating Range Quiescent Current TMIN to TMAX Conditions AD844J/AD844A Min Typ Max f = 100 kHz, 2 V rms 5 Min AD844B Typ Max Min AD844S Typ Max Unit 60 33 0.005 60 33 0.005 60 33 0.005 MHz MHz % 100 100 100 100 100 100 ns ns 110 100 2000 110 100 2000 110 100 2000 ns ns V/μs 20 20 MHz MHz ±11 80 60 15 V mA mA Ω ±15 V supplies ±5 V supplies Overdriven input THD = 3% VS = ±15 V VS = ±5 V 1200 RL = 500 Ω ±10 1200 20 20 Open loop 20 20 ±11 80 60 15 ±4.5 6.5 7.5 1200 ±10 ±18 7.5 8.5 1 ±4.5 Rated performance after a 5 minute warm-up at TA = 25°C. Input signal 285 mV p-p carrier (40 IRE) riding on 0 mV to 642 mV (90 IRE) ramp. RL = 100 Ω; R1, R2 = 300 Ω. 3 For gain = −1, input signal = 0 dBm, CL = 10 pF, RL = 500 Ω, R1 = 500 Ω, and R2 = 500 Ω in Figure 29. 4 For gain = −10, input signal = 0 dBm, CL =10 pF, RL = 500 Ω, R1 = 500 Ω, and R2 = 50 Ω in Figure 29. 5 CL = 10 pF, RL = 500 Ω, R1 = 1 kΩ, R2 = 1 kΩ in Figure 29. 6 CL = 10 pF, RL = 500 Ω, R1 = 500 Ω, R2 = 50 Ω in Figure 29. 2 Rev. F | Page 4 of 20 ±11 80 60 15 6.5 7.5 ±10 ±18 7.5 8.5 ±4.5 6.5 7.5 ±18 7.5 8.5 V mA mA AD844 ABSOLUTE MAXIMUM RATINGS METALLIZATION PHOTOGRAPH Table 2. Parameter Supply Voltage Power Dissipation1 Output Short-Circuit Duration Input Common-Mode Voltage Differential Input Voltage Inverting Input Current Continuous Transient Storage Temperature Range (Q) Storage Temperature Range (N, RW) Lead Temperature (Soldering, 60 sec) ESD Rating Dimensions shown in inches and (millimeters). –IN 5 mA 10 mA −65°C to +150°C −65°C to +125°C 300°C 1000 V NULL NULL +VS 0.076 (1.9) 28-lead PDIP package: θJA = 90°C/W. 8-lead CERDIP package: θJA = 110°C/W. 16-lead SOIC package: θJA = 100°C/W. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. +IN –VS TZ 0.095 (2.4) SUBSTRATE CONNECTED TO +VS Figure 3. Die Photograph ESD CAUTION Rev. F | Page 5 of 20 OUTPUT 00897-003 1 Contact factory for latest dimensions. Ratings ±18 V 1.1 W Indefinite ±VS 6V AD844 TYPICAL PERFORMANCE CHARACTERISTICS 20 60 15 INPUT VOLTAGE (V) 70 50 40 0 5 10 15 20 SUPPLY VOLTAGE (±V) 0 0 5 10 15 20 SUPPLY VOLTAGE (±V) Figure 4. −3 dB Bandwidth vs. Supply Voltage, R1 = R2 = 500 Ω –60 10 00897-007 30 TA = 25°C 5 00897-004 –3dB BANDWIDTH (MHz) TA = 25°C and VS = ±15 V, unless otherwise noted. Figure 7. Noninverting Input Voltage Swing vs. Supply Voltage 20 1V rms RL = 500Ω TA = 25°C HARMONIC DISTORTION (dB) –70 15 OUTPUT VOLTAGE (V) –80 –90 –100 –110 SECOND HARMONIC 10 5 10k 100k INPUT FREQUENCY (Hz) 0 00897-005 1k 0 5 10 15 20 00897-008 THIRD HARMONIC –130 100 140 00897-009 –120 SUPPLY VOLTAGE (±V) Figure 5. Harmonic Distortion vs. Input Frequency, R1 = R2 = 1 kΩ Figure 8. Output Voltage Swing vs. Supply Voltage 10 5 RL = ∞ 9 SUPPLY CURRENT (mA) RL = 500Ω 3 2 RL = 50Ω 0 –50 8 7 VS = ±15V 6 VS = ±5V 1 5 0 50 100 TEMPERATURE (°C) Figure 6. Transresistance vs. Temperature 150 4 –60 00897-006 TRANSRESISTANCE (MΩ) 4 –40 –20 0 20 40 60 TEMPERATURE (-°C) 80 100 120 Figure 9. Quiescent Supply Current vs. Temperature and Supply Voltage Rev. F | Page 6 of 20 AD844 40 2 VS = ±15V 1 –3dB BANDWIDTH (MHz) IBP 0 –1 IBN 0 50 100 150 TEMPERATURE (°C) Figure 10. Inverting Input Bias Current (IBN) and Noninverting Input Bias Current (IBP) vs. Temperature 0.1 10M 100M 00897-011 OUTPUT IMPEDANCE (Ω) ±5V SUPPLIES 1M –20 0 20 40 60 80 100 120 140 Figure 12. –3 dB Bandwidth vs. Temperature, Gain = −1, R1 = R2 = 1 kΩ 1 FREQUENCY (Hz) –40 TEMPERATURE (-°C) 10 100k VS = ±5V 20 10 –60 100 0.01 10k 25 15 00897-010 –2 –50 30 00897-012 INPUT BIAS CURRENT (µA) 35 Figure 11. Output Impedance vs. Frequency, Gain = −1, R1 = R2 = 1 kΩ Rev. F | Page 7 of 20 AD844 INVERTING GAIN-OF-1 AC CHARACTERISTICS +VS 0.22µF 5V 4.7Ω 100 R1 –IN R2 90 – AD844 OUTPUT + RL CL 10 0.22µF 00897-013 –VS 20ns 00897-016 0 4.7Ω Figure 16. Large Signal Pulse Response, Gain = −1, R1 = R2 = 1 kΩ Figure 13. Inverting Amplifier, Gain of −1 (R1 = R2) 6 500nV R1 = R2 = 500Ω 100 0 90 GAIN (dB) R1 = R2 = 1kΩ –6 –12 10 –18 1M 10M 100M FREQUENCY (Hz) 00897-014 20ns –24 100k Figure 14. Gain vs. Frequency for Gain = −1, RL = 500 Ω, CL = 0 pF –180 R1 = R2 = 500Ω –240 –270 R1 = R2 = 1kΩ –300 –330 0 25 50 FREQUENCY (MHz) 00897-015 PHASE (Degrees) –210 Figure 15. Phase vs. Frequency for Gain = −1, RL = 500 Ω, CL = 0 pF Rev. F | Page 8 of 20 00897-017 0 Figure 17. Small Signal Pulse Response, Gain = −1, R1 = R2 = 1 kΩ AD844 INVERTING GAIN-OF-10 AC CHARACTERISTICS –180 +VS –210 500Ω PHASE (Degrees) 50Ω RL = 50Ω – –IN AD844 OUTPUT + RL CL RL = 500Ω –240 –270 –300 0.22µF 00897-018 4.7Ω –VS –330 0 25 FREQUENCY (MHz) Figure 18. Gain of −10 Amplifier Figure 20. Phase vs. Frequency, Gain = −10 26 RL = 500Ω RL = 50Ω 14 8 2 –4 100k 1M 10M FREQUENCY (Hz) 100M 00897-019 GAIN (dB) 20 Figure 19. Gain vs. Frequency, Gain = −10 Rev. F | Page 9 of 20 50 00897-020 0.22µF 4.7Ω AD844 INVERTING GAIN-OF-10 PULSE RESPONSE 500nV 100 90 90 10 10 0 0 20ns 00897-021 100 20ns Figure 21. Large Signal Pulse Response, Gain = –10, RL = 500 Ω 00897-022 5V Figure 22. Small Signal Pulse Response, Gain = −10, RL = 500 Ω Rev. F | Page 10 of 20 AD844 NONINVERTING GAIN-OF-10 AC CHARACTERISTICS +VS 4.7Ω 2V 0.22µF 100ns 100 450Ω 50Ω 90 – OUTPUT AD844 + –IN 0.22µF RL CL 00897-023 4.7Ω 10 0 00897-026 –VS Figure 26. Noninverting Amplifier Large Signal Pulse Response, Gain = +10, RL = 500 Ω Figure 23. Noninverting Gain of +10 Amplifier 26 200nV 20 GAIN (dB) RL = 50Ω 50ns 100 RL = 500Ω 90 14 8 2 10 1M 10M 100M FREQUENCY (Hz) Figure 24. Gain vs. Frequency, Gain = +10 Figure 27. Small Signal Pulse Response, Gain = +10, RL = 500 Ω –180 –210 RL = 500Ω –240 –270 –300 –330 0 25 FREQUENCY (MHz) 50 00897-025 PHASE (Degrees) RL = 50Ω 00897-027 –4 100k 00897-024 0 Figure 25. Phase vs. Frequency, Gain = +10 Rev. F | Page 11 of 20 AD844 UNDERSTANDING THE AD844 RESPONSE AS AN INVERTING AMPLIFIER The AD844 can be used in ways similar to a conventional op amp while providing performance advantages in wideband applications. However, there are important differences in the internal structure that need to be understood to optimize the performance of the AD844 op amp. Figure 29 shows the connections for an inverting amplifier. Unlike a conventional amplifier, the transient response and the small signal bandwidth are determined primarily by the value of the external feedback resistor, R1, rather than by the ratio of R1/R2 as is customarily the case in an op amp application. This is a direct result of the low impedance at the inverting input. As with conventional op amps, the closed-loop gain is −R1/R2. OPEN-LOOP BEHAVIOR Figure 28 shows a current feedback amplifier reduced to essentials. Sources of fixed dc errors, such as the inverting node bias current and the offset voltage, are excluded from this model. The most important parameter limiting the dc gain is the transresistance, Rt, which is ideally infinite. A finite value of Rt is analogous to the finite open-loop voltage gain in a conventional op amp. The closed-loop transresistance is the parallel sum of R1 and Rt. Because R1 is generally in the range of 500 Ω to 2 kΩ and Rt is about 3 MΩ, the closed-loop transresistance is only 0.02% to 0.07% lower than R1. This small error is often less than the resistor tolerance. +1 IIN Ct +1 IIN 00897-028 RIN Rt Figure 28. Equivalent Schematic The important parameters defining ac behavior are the transcapacitance, Ct, and the external feedback resistor (not shown). The time constant formed by these components is analogous to the dominant pole of a conventional op amp and thus cannot be reduced below a critical value if the closed-loop system is to be stable. In practice, Ct is held to as low a value as possible (typically 4.5 pF) so that the feedback resistor can be maximized while maintaining a fast response. The finite RIN also affects the closed-loop response in some applications. When R1 is fairly large (above 5 kΩ) but still much less than Rt, the closed-loop HF response is dominated by the time constant R1 Ct. Under such conditions, the AD844 is overdamped and provides only a fraction of its bandwidth potential. Because of the absence of slew rate limitations under these conditions, the circuit exhibits a simple single-pole response even under large signal conditions. In Figure 29, R3 is used to properly terminate the input if desired. R3 in parallel with R2 gives the terminated resistance. As R1 is lowered, the signal bandwidth increases, but the time constant R1 Ct becomes comparable to higher order poles in the closedloop response. Therefore, the closed-loop response becomes complex, and the pulse response shows overshoot. When R2 is much larger than the input resistance, RIN, at Pin 2, most of the feedback current in R1 is delivered to this input, but as R2 becomes comparable to RIN, less of the feedback is absorbed at Pin 2, resulting in a more heavily damped response. Consequently, for low values of R2, it is possible to lower R1 without causing instability in the closed-loop response. Table 3 lists combinations of R1 and R2 and the resulting frequency response for the circuit of Figure 29. Figure 16 shows the very clean and fast ±10 V pulse response of the AD844. The open-loop ac gain is also best understood in terms of the transimpedance rather than as an open-loop voltage gain. The open-loop pole is formed by Rt in parallel with Ct. Because Ct is typically 4.5 pF, the open-loop corner frequency occurs at about 12 kHz. However, this parameter is of little value in determining the closed-loop response. Rev. F | Page 12 of 20 R1 VIN R3 OPTIONAL R2 AD844 VOUT RL CL 00897-029 The current applied to the inverting input node is replicated by the current conveyor to flow in Resistor Rt. The voltage developed across Rt is buffered by the unity gain voltage follower. Voltage gain is the ratio Rt/RIN. With typical values of Rt = 3 MΩ and RIN = 50 Ω, the voltage gain is about 60,000. The open-loop current gain, another measure of gain that is determined by the beta product of the transistors in the voltage follower stage (see Figure 31), is typically 40,000. Figure 29. Inverting Amplifier AD844 R1 Table 3. Gain vs. Bandwidth R1 1 kΩ 500 Ω 2 kΩ 1 kΩ 5 kΩ 500 Ω 1 kΩ 500 Ω 1 kΩ 5 kΩ R2 1 kΩ 500 Ω 1 kΩ 500 Ω 1 kΩ 100 Ω 100 Ω 50 Ω 50 Ω 50 Ω BW (MHz) 35 60 15 30 5.2 49 23 33 21 3.2 GBW (MHz) 35 60 30 60 26 245 230 330 420 320 RESPONSE AS AN I-V CONVERTER The AD844 works well as the active element in an operational current-to-voltage converter, used in conjunction with an external scaling resistor, R1, in Figure 30. This analysis includes the stray capacitance, CS, of the current source, which may be a high speed DAC. Using a conventional op amp, this capacitance forms a nuisance pole with R1 that destabilizes the closed-loop response of the system. Most op amps are internally compensated for the fastest response at unity gain, so the pole due to R1 and CS reduces the already narrow phase margin of the system. For example, if R1 is 2.5 kΩ, a CS of 15 pF places this pole at a frequency of about 4 MHz, well within the response range of even a medium speed operational amplifier. In a current feedback amp, this nuisance pole is no longer determined by R1 but by the input resistance, RIN. Because this is about 50 Ω for the AD844, the same 15 pF forms a pole at 212 MHz and causes little trouble. It can be shown that the response of this system is: VOUT = I sig K R1 (1 + sTd ) (1 + sTn ) Rt Rt + R1 VOUT RL CL Figure 30. Current-to-Voltage Converter CIRCUIT DESCRIPTION OF THE AD844 A simplified schematic is shown in Figure 31. The AD844 differs from a conventional op amp in that the signal inputs have radically different impedance. The noninverting input (Pin 3) presents the usual high impedance. The voltage on this input is transferred to the inverting input (Pin 2) with a low offset voltage, ensured by the close matching of like polarity transistors operating under essentially identical bias conditions. Laser trimming nulls the residual offset voltage, down to a few tens of microvolts. The inverting input is the common emitter node of a complementary pair of grounded base stages and behaves as a current summing node. In an ideal current feedback op amp, the input resistance is zero. In the AD844, it is about 50 Ω. A current applied to the inverting input is transferred to a complementary pair of unity-gain current mirrors that deliver the same current to an internal node (Pin 5) at which the full output voltage is generated. The unity-gain complementary voltage follower then buffers this voltage and provides the load driving power. This buffer is designed to drive low impedance loads, such as terminated cables, and can deliver ±50 mA into a 50 Ω load while maintaining low distortion, even when operating at supply voltages of only ±6 V. Current limiting (not shown) ensures safe operation under short-circuited conditions. 7 +VS where: K is a factor very close to unity and represents the finite dc gain of the amplifier. Td is the dominant pole. Tn is the nuisance pole. K= AD844 CS 00897-030 Gain −1 −1 −2 −2 −5 −5 −10 −10 −20 −100 ISIG IB +IN 3 2 –IN TZ 5 6 OUTPUT Td = KR1Ct Using typical values of R1 = 1 kΩ and Rt = 3 MΩ, K = 0.9997; in other words, the gain error is only 0.03%. This is much less than the scaling error of virtually all DACs and can be absorbed, if necessary, by the trim needed in a precise system. In the AD844, Rt is fairly stable with temperature and supply voltages, and consequently the effect of finite gain is negligible unless high value feedback resistors are used. Because that results in slower response times than are possible, the relatively low value of Rt in the AD844 is rarely a significant source of error. Rev. F | Page 13 of 20 4 –VS Figure 31. Simplified Schematic 00897-031 IB Tn = RINCS (assuming RIN << R1) AD844 It is important to understand that the low input impedance at the inverting input is locally generated and does not depend on feedback. This is very different from the virtual ground of a conventional operational amplifier used in the current summing mode, which is essentially an open circuit until the loop settles. In the AD844, transient current at the input does not cause voltage spikes at the summing node while the amplifier is settling. Furthermore, all of the transient current is delivered to the slewing (TZ) node (Pin 5) via a short signal path (the grounded base stages and the wideband current mirrors). The current available to charge the capacitance (about 4.5 pF) at the TZ node is always proportional to the input error current, and the slew rate limitations associated with the large signal response of the op amps do not occur. For this reason, the rise and fall times are almost independent of signal level. In practice, the input current eventually causes the mirrors to saturate. When using ±15 V supplies, this occurs at about 10 mA (or ±2200 V/μs). Because signal currents are rarely this large, classical slew rate limitations are absent. NONINVERTING GAIN OF 100 The AD844 provides very clean pulse response at high noninverting gains. Figure 32 shows a typical configuration providing a gain of 100 with high input resistance. The feedback resistor is kept as low as practicable to maximize bandwidth, and a peaking capacitor (CPK) can optionally be added to further extend the bandwidth. Figure 33 shows the small signal response with CPK = 3 nF, RL = 500 Ω, and supply voltages of either ±5 V or ±15 V. Gain bandwidth products of up to 900 MHz can be achieved in this way. The offset voltage of the AD844 is laser trimmed to the 50 μV level and exhibits very low drift. In practice, there is an additional offset term due to the bias current at the inverting input (IBN), which flows in the feedback resistor (R1). This can optionally be nulled by the trimming potentiometer shown in Figure 32. +VS 4.7Ω OFFSET TRIM This inherent advantage is lost if the voltage follower used to buffer the output has slew rate limitations. The AD844 is designed to avoid this problem, and as a result, the output buffer exhibits a clean large signal transient response, free from anomalous effects arising from internal saturation. CPK 3nF R1 499Ω 20Ω 1 RESPONSE AS A NONINVERTING AMPLIFIER VIN 7 AD844 6 3 RL 4 0.22µF 4.7Ω 00897-032 Because current feedback amplifiers are asymmetrical with regard to their two inputs, performance differs markedly in noninverting and inverting modes. In noninverting modes, the large signal high speed behavior of the AD844 deteriorates at low gains because the biasing circuitry for the input system (not shown in Figure 31) is not designed to provide high input voltage slew rates. 0.22µF 8 2 R2 4.99Ω –VS Figure 32. Noninverting Amplifier Gain = 100, Optional Offset Trim Is Shown 46 VS = ±15V 40 GAIN (dB) However, good results can be obtained with some care. The noninverting input does not tolerate a large transient input; it must be kept below ±1 V for best results. Consequently, this mode is better suited to high gain applications (greater than ×10). Figure 23 shows a noninverting amplifier with a gain of 10 and a bandwidth of 30 MHz. The transient response is shown in Figure 26 and Figure 27. To increase the bandwidth at higher gains, a capacitor can be added across R2 whose value is approximately (R1/R2) × Ct. VS = ±5V 34 28 16 100k 1M FREQUENCY (Hz) 10M 20M 00897-040 22 Figure 33. AC Response for Gain = 100, Configuration Shown in Figure 32 Rev. F | Page 14 of 20 AD844 USING THE AD844 BOARD LAYOUT As with all high frequency circuits considerable care must be used in the layout of the components surrounding the AD844. A ground plane, to which the power supply decoupling capacitors are connected by the shortest possible leads, is essential to achieving clean pulse response. Even a continuous ground plane exhibits finite voltage drops between points on the plane, and this must be kept in mind when selecting the grounding points. In general, decoupling capacitors should be taken to a point close to the load (or output connector) because the load currents flow in these capacitors at high frequencies. The +IN and −IN circuits (for example, a termination resistor and Pin 3) must be taken to a common point on the ground plane close to the amplifier package. AD844 VOUT 6 CL 750Ω 22pF 00897-034 5 Figure 34. Feedforward Network for Large Capacitive Loads 5V 100 90 Use low impedance 0.22 μF capacitors (AVX SR305C224KAA or equivalent) wherever ac coupling is required. Include either ferrite beads and/or a small series resistance (approximately 4.7 Ω) in each supply line. 10 00897-035 0 500ns INPUT IMPEDANCE Figure 35. Driving 1000 pF CL with Feedforward Network of Figure 34 SETTLING TIME Settling time is measured with the circuit of Figure 36. This circuit employs a false summing node, clamped by the two Schottky diodes, to create the error signal and limit the input signal to the oscilloscope. For measuring settling time, the ratio of R6/R5 is equal to R1/R2. For unity gain, R6 = R5 = 1 kΩ, and RL = 500 Ω. For the gain of −10, R5 = 50 Ω, R6 = 500 Ω, and RL was not used because the summing network loads the output with approximately 275 Ω. Using this network in a unity-gain configuration, settling time is 100 ns to 0.1% for a –5 V to +5 V step with CL = 10 pF. TO SCOPE (TEK 7A11 FET PROBE) DRIVING LARGE CAPACITIVE LOADS R5 Capacitive drive capability is 100 pF without an external network. With the addition of the network shown in Figure 34, the capacitive drive can be extended to over 10,000 pF, limited by internal power dissipation. With capacitive loads, the output speed becomes a function of the overdriven output current limit. Because this is roughly ±100 mA, under these conditions, the maximum slew rate into a 1000 pF load is ±100 V/μs. Figure 35 shows the transient response of an inverting amplifier (R1 = R2 = 1 kΩ) using the feedforward network shown in Figure 34, driving a load of 1000 pF. R6 D1 D2 R1 VIN R2 AD844 VOUT R3 RL NOTES 1. D1, D2 IN6263 OR EQUIVALENT SCHOTTKY DIODE. Figure 36. Settling Time Test Fixture Rev. F | Page 15 of 20 CL 00897-036 At low frequencies, negative feedback keeps the resistance at the inverting input close to zero. As the frequency increases, the impedance looking into this input increases from near zero to the open-loop input resistance, due to bandwidth limitations, making the input seem inductive. If it is desired to keep the input impedance flatter, a series RC network can be inserted across the input. The resistor is chosen so that the parallel sum of it and R2 equals the desired termination resistance. The capacitance is set so that the pole determined by this RC network is about half the bandwidth of the op amp. This network is not important if the input resistor is much larger than the termination used, or if frequencies are relatively low. In some cases, the small peaking that occurs without the network can be of use in extending the −3 dB bandwidth. AD844 +5V DC ERROR CALCULATION 2.2µF Figure 37 shows a model of the dc error and noise sources for the AD844. The inverting input bias current, IBN, flows in the feedback resistor. IBP, the noninverting input bias current, flows in the resistance at Pin 3 (RP), and the resulting voltage (plus any offset voltage) appears at the inverting input. The total error, VO, at the output is: 3 VIN 50Ω 50Ω 6 4 ZO = 50Ω VOUT RL 50Ω 2.2µF 300Ω –5V 00897-038 300Ω R1 ⎞ VO = (I BP R P + VOS + I BN R IN )⎛⎜1 + ⎟ + I BN R1 R2 ⎝ ⎠ Figure 38. The AD844 as a Cable Driver Because IBN and IBP are unrelated both in sign and magnitude, inserting a resistor in series with the noninverting input does not necessarily reduce dc error and may actually increase it. HP8753A NETWORK ANALYZER RF OUT IN OUT RF IN EXT TRIG SYNC OUT R1 OUT HP11850C SPLITTER 50Ω (TERMINATOR) VIN VIN CIRCUIT UNDER TEST VOUT OUT 470Ω RIN INN IBN 00897-039 HP3314A OUT STAIRCASE GENERATOR VN R2 7 2 Figure 39. Differential Gain/Phase Test Setup VOS 0.3 AD844 00897-037 RP IBP Figure 37. Offset Voltage and Noise Model for the AD844 NOISE Noise sources can be modeled in a manner similar to the dc bias currents, but the noise sources are INN, INP, VN, and the amplifier induced noise at the output, VON, is: VON = 0.2 0.1 0 –0.1 –0.2 –0.3 0 18 ((I NP R P )2 + VN 2 )⎛⎜1 + R1 ⎞⎟ + (I NN R1)2 ⎝ 36 54 72 90 VOUT (IRE) 2 00897-040 INP DIFFERENTIAL PHASE (Degrees) IRE = 7.14mV Figure 40. Differential Phase for the Circuit of Figure 38 R2 ⎠ 0.06 The AD844 can be used to drive low impedance cables. Using ±5 V supplies, a 100 Ω load can be driven to ±2.5 V with low distortion. Figure 38 shows an illustrative application that provides a noninverting gain of +2, allowing the cable to be reverse-terminated while delivering an overall gain of +1 to the load. The −3 dB bandwidth of this circuit is typically 30 MHz. Figure 39 shows a differential gain and phase test setup. In video applications, differential-phase and differential-gain characteristics are often important. Figure 40 shows the variation in phase as the load voltage varies. Figure 41 shows the gain variation. 0.04 Rev. F | Page 16 of 20 0.02 0 –0.02 –0.04 –0.06 0 18 36 54 72 VOUT (IRE) Figure 41. Differential Gain for the Circuit of Figure 38 90 00897-041 VIDEO CABLE DRIVER USING ±5 V SUPPLIES IRE = 7.14mV DIFFERENTIAL GAIN (%) Overall noise can be reduced by keeping all resistor values to a minimum. With typical numbers, R1 = R2 = 1 kΩ, RP = 0 Ω, VN = 2 nV/√Hz, INP = 10 pA/√Hz, INN = 12 pA/√Hz, and VON calculates to 12 nV/√Hz. The current noise is dominant in this case, because it is in most low gain applications. AD844 +15V 1 MSB +15V (VCC) 24 DIGITAL INPUTS 2 REFCOM 23 3 –15V (VEE) 22 4 IBPO 21 5 IOUT 20 AD568 RL 6 0.22µF* 0.22µF* 0.22µF* AD844 ACOM 18 8 LCOM 17 9 SPAN 16 10 SPAN 15 11 THCOM 14 3 –15V 7 2 19 7 12 LSB 0.22µF* VOUT 6 4 RI ANALOG SUPPLY GROUND GROUND DIGITAL SUPPLY 100pF VTH 13 –5V 00897-042 TOP VIEW (Not to Scale) *POWER SUPPLY BYPASS CAPACITORS. Figure 42. High Speed DAC Amplifier HIGH SPEED DAC BUFFER 20 MHZ VARIABLE GAIN AMPLIFIER The AD844 performs very well in applications requiring currentto-voltage conversion. Figure 42 shows connections for use with the AD568 current output DAC. In this application, the bipolar offset is used so that the full-scale current is ±5.12 mA, which generates an output of ±5.12 V using the 1 kΩ application resistor on the AD568. Figure 43 shows the full-scale transient response. Care is needed in power supply decoupling and grounding techniques to achieve the full 12-bit accuracy and realize the fast settling capabilities of the system. The AD568 data sheet should be consulted for more complete details about its use. The AD844 is an excellent choice as an output amplifier for the AD539 multiplier, in all of its connection modes. (See the AD539 data sheet for full details.) Figure 44 shows a simple multiplier providing the output: VW = − VXVY 2V (1) where VX is the gain control input, a positive voltage from 0 V to 3.2 V (maximum), and VY is the signal voltage, nominally ±2 V full scale but capable of operation up to ±4.2 V. The peak output in this configuration is thus ±6.7 V. Using all four of the internal application resistors provided on the AD539 in parallel results in a feedback resistance of 1.5 kΩ, at which value the bandwidth of the AD844 is about 22 MHz, and is essentially independent of VX. The gain at VX = 3.16 V is 4 dB. 100 90 10Ω 10 50ns Figure 43. DAC Amplifier Full-Scale Transient Response 00897-043 0 INPUTS VX* 0V TO 3V VY* ±2V FS 3nF INPUT GND 0.22µF +VS TYP +6V AT 15µA 10Ω 0.22µF 0.22µF 1 16 2 15 14 3 4 AD539 2 TOP VIEW 5 (Not to Scale) 12 6 11 7 10 8 9 7 AD844 13 3 OUTPUT VW VW = 0.22µF –VXVY 2V 10Ω 10Ω *VX AND VY INPUTS MAY OPTIONALLY BE TERMINATED; TYPICALLY BY USING A 50Ω OR 75Ω RESISTOR TO GROUND. Figure 44. 20 MHz VGA Using the AD539 Rev. F | Page 17 of 20 6 4 –VS TYP –6V AT 15µA 00897-044 2V AD844 4 VX = 3.15V –6 VX = 1.0V –16 GAIN (dB) Figure 45 shows the small signal response for a 50 dB gain control range (VX = 10 mV to 3.16 V). At small values of VX, capacitive feedthrough on the PC board becomes troublesome and very careful layout techniques are needed to minimize this problem. A ground strip between the pins of the AD539 is helpful in this regard. Figure 46 shows the response to a 2 V pulse on VY for VX = 1 V, 2 V, and 3 V. For these results, a load resistor of 500 Ω was used and the supplies were ±9 V. The multiplier operates from supplies between ±4.5 V and ±16.5 V. VX = 0.316V –26 VX = 0.10V –36 VX = 0.032V –56 100k 1M 10M 60M FREQUENCY (Hz) Figure 45. VGA AC Response 1V 1V 50ns 100 90 10 00897-046 0 Figure 46. VGA Transient Response with VX = 1 V, 2 V, and 3 V Rev. F | Page 18 of 20 00897-045 –46 Disconnecting Pin 9 and Pin 16 on the AD539 alters the denominator in Equation 1 to 1 V, and the bandwidth is approximately 10 MHz, with a maximum gain of 10 dB. Using only Pin 9 or Pin 16 results in a denominator of 0.5 V, a bandwidth of 5 MHz, and a maximum gain of 16 dB. AD844 OUTLINE DIMENSIONS 0.400 (10.16) 0.365 (9.27) 0.355 (9.02) 8 5 1 0.280 (7.11) 0.250 (6.35) 0.240 (6.10) 4 0.100 (2.54) BSC 0.325 (8.26) 0.310 (7.87) 0.300 (7.62) 0.060 (1.52) MAX 0.210 (5.33) MAX 0.015 (0.38) MIN 0.150 (3.81) 0.130 (3.30) 0.115 (2.92) SEATING PLANE 0.022 (0.56) 0.018 (0.46) 0.014 (0.36) 0.195 (4.95) 0.130 (3.30) 0.115 (2.92) 0.015 (0.38) GAUGE PLANE 0.430 (10.92) MAX 0.005 (0.13) MIN 0.014 (0.36) 0.010 (0.25) 0.008 (0.20) COMPLIANT TO JEDEC STANDARDS MS-001 CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS. Figure 47. 8-Lead Plastic Dual-in-Line Package [PDIP] (N-8) Dimensions shown in inches and (millimeters) 0.005 (0.13) MIN 8 0.055 (1.40) MAX 5 0.310 (7.87) 0.220 (5.59) 1 4 0.100 (2.54) BSC 0.320 (8.13) 0.290 (7.37) 0.405 (10.29) MAX 0.060 (1.52) 0.015 (0.38) 0.200 (5.08) MAX 0.150 (3.81) MIN 0.200 (5.08) 0.125 (3.18) 0.023 (0.58) 0.014 (0.36) 0.070 (1.78) 0.030 (0.76) SEATING PLANE 15° 0° 0.015 (0.38) 0.008 (0.20) CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 48. 8-Lead Ceramic Dual In-Line Package [CERDIP] (Q-8) Dimensions shown in inches and (millimeters) Rev. F | Page 19 of 20 070606-A 0.070 (1.78) 0.060 (1.52) 0.045 (1.14) AD844 10.50 (0.4134) 10.10 (0.3976) 9 16 7.60 (0.2992) 7.40 (0.2913) 8 1.27 (0.0500) BSC 0.30 (0.0118) 0.10 (0.0039) COPLANARITY 0.10 0.51 (0.0201) 0.31 (0.0122) 10.65 (0.4193) 10.00 (0.3937) 0.75 (0.0295) 0.25 (0.0098) 2.65 (0.1043) 2.35 (0.0925) SEATING PLANE 45° 8° 0° 0.33 (0.0130) 0.20 (0.0079) 1.27 (0.0500) 0.40 (0.0157) COMPLIANT TO JEDEC STANDARDS MS-013- AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. 032707-B 1 Figure 49. 16-Lead Standard Small Outline Package [SOIC_W] Wide Body (RW-16) Dimensions shown in millimeters and (inches) ORDERING GUIDE Model AD844AN AD844ANZ 1 AD844ACHIPS AD844AQ AD844BQ AD844JR-16 AD844JR-16-REEL AD844JR-16-REEL7 AD844JRZ-161 AD844JRZ-16-REEL1 AD844JRZ-16-REEL71 AD844SCHIPS AD844SQ AD844SQ/883B 5962-8964401PA 2 1 2 Temperature Range −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C 0°C to 70°C −55°C to +125°C −55°C to +125°C −55°C to +125°C −55°C to +125°C Package Description 8-Lead Plastic Dual In-Line Package [PDIP] 8-Lead Plastic Dual In-Line Package [PDIP] 8-Lead Ceramic Dual In-Line Package [CERDIP] 8-Lead Ceramic Dual In-Line Package [CERDIP] 16-Lead Standard Small Outline Package [SOIC_W] 16-Lead SOIC_W, 13” Tape and Reel 16-Lead SOIC_W, 7” Tape and Reel 16-Lead Standard Small Outline Package [SOIC_W] 16-Lead SOIC_W, 13” Tape and Reel 16-Lead SOIC_W, 7” Tape and Reel 8-Lead Ceramic Dual In-Line Package [CERDIP] 8-Lead Ceramic Dual In-Line Package [CERDIP] 8-Lead Ceramic Dual In-Line Package [CERDIP] Z = RoHS Compliant Part. Refer to the DESC drawing for tested specifications. ©1989–2009 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D00897-0-2/09(F) Rev. F | Page 20 of 20 Package Option N-8 N-8 Die Q-8 Q-8 RW-16 RW-16 RW-16 RW-16 RW-16 RW-16 Die Q-8 Q-8 Q-8