AD844

a FEATURES Wide Bandwidth: 60 MHz at Gain of –1 Wide Bandwidth: 33 MHz at Gain of –10 Very High Output Slew Rate: Up to 2000 V/ s 20 MHz Full Power Bandwidth, 20 V p-p, RL = 500 Fast Settling: 100 ns to 0.1% (10 V Step) Differential Gain Error: 0.03% at 4.4 MHz Differential Phase Error: 0.158 at 4.4 MHz High Output Drive: 650 mA into 50 Load Low Offset Voltage: 150 mV Max (B Grade) Low Quiescent Current: 6.5 mA Available in Tape and Reel in Accordance with EIA-481A Standard APPLICATIONS Flash ADC Input Amplifiers High-Speed Current DAC Interfaces Video Buffers and Cable Drivers Pulse Amplifiers PRODUCT DESCRIPTION NULL 1 –IN 2 +IN 3 –VS 4 60 MHz, 2000 V/ s Monolithic Op Amp AD844 CONNECTION DIAGRAMS 8-Lead Plastic (N), and Cerdip (Q) Packages 16-Lead SOIC (R) Package NC 1 OFFSETNULL 2 –IN 3 NC 4 +IN 5 NC 6 V– 7 NC 8 AD844 8 7 6 NULL +VS OUTPUT AD844 16 NC 15 OFFSETNULL 14 V+ 13 NC 12 OUTPUT 11 TZ TOP VIEW 5 TZ (Not to Scale) 10 NC TOP VIEW 9 NC (Not to Scale) NC = NO CONNECT The AD844 is a high-speed monolithic operational amplifier fabricated using Analog Devices’ 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. 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. This type of op amp provides a closed-loop bandwidth which 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. The AD844 is available in four performance grades and three package options. In the 16-lead SOIC (R) package, the AD844J is specified for the commercial temperature range of 0°C to 70°C. 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 plastic mini-DIP (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 1. The AD844 is a versatile, low cost component providing an excellent combination of ac and dc performance. 2. It is essentially free from slew rate limitations. Rise and fall times are essentially independent of output level. 3. 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. 4. The offset voltage and input bias currents of the AD844 are laser trimmed to minimize dc errors; VOS d rift is typically 1 µV/°C and bias current drift is typically 9 nA/°C. 5. 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. 6. The AD844 combines low distortion, low noise and low drift with wide bandwidth, making it outstanding as an input amplifier for flash A/D converters. R EV. D 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. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 2001 AD844–SPECIFICATIONS (@ T = 25 C and V = A S 15 V dc, unless otherwise noted) Min AD844B Typ 50 75 1 4 4 10 10 150 750 9 175 220 90 110 100 300 3 80 100 90 130 Max 150 200 5 10 10 20 20 250 1100 15 200 240 110 150 200 500 7 100 120 120 190 AD844S Min Typ Max 50 125 1 4 4 10 10 200 1900 20 175 220 90 120 100 800 7 80 120 90 140 300 500 5 20 20 35 35 450 2500 30 250 300 160 200 400 1300 15 150 200 150 200 Unit µV µV µV/°C µV/V µV/V µV/V µV/V nA nA nA/°C nA/V nA/V nA/V nA/V nA nA nA/°C nA/V nA/V nA/V nA/V Model INPUT OFFSET VOLTAGE1 TMIN–TMAX vs. Temperature vs. Supply Initial TMIN–TMAX vs. Common Mode Initial TMIN–TMAX INPUT BIAS CURRENT –Input Bias Current1 TMIN–TMAX vs. Temperature vs. Supply Initial TMIN–TMAX vs. Common Mode Initial TMIN–TMAX +Input Bias Current1 TMIN–TMAX vs. Temperature vs. Supply Initial TMIN–TMAX vs. Common Mode Initial TMIN–TMAX INPUT CHARACTERISTICS Input Resistance –Input +Input Input Capacitance –Input +Input Input Voltage Range Common Mode INPUT VOLTAGE NOISE INPUT CURRENT NOISE –Input +Input OPEN LOOP TRANSRESISTANCE TMIN–TMAX Transcapacitance DIFFERENTIAL GAIN ERROR2 DIFFERENTIAL PHASE ERROR2 FREQUENCY RESPONSE Small Signal Bandwidth Gain = –13 Gain = –104 TOTAL HARMOMIC 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 Conditions Min AD844J/A Typ Max 50 75 1 300 500 5 V–18 V 4 4 VCM = ± 10 V 10 10 200 800 9 5 V–18 V 175 220 VCM = ± 10 V 90 110 150 350 3 5 V–18 V 80 100 VCM = ± 10 V 90 130 150 150 160 400 700 250 35 20 450 1500 7 50 10 2 2 65 7 50 10 2 2 65 7 50 10 2 2 65 Ω MΩ pF pF V ± 10 f ≥ 1 kHz f ≥ 1 kHz f ≥ 1 kHz VOUT = ± 10 V RLOAD = 500 Ω 2.2 1.3 2 10 12 3.0 2.0 4.5 0.03 0.15 ± 10 2 10 12 2.8 1.6 3.0 2.0 4.5 0.03 0.15 ± 10 2 10 12 2.2 1.3 3.0 1.6 4.5 0.03 0.15 nV/√Hz pA/√Hz pA/√Hz MΩ MΩ pF % Degree f = 4.4 MHz f = 4.4 MHz 60 33 f = 100 kHz, 2 V rms5 ± 15 V Supplies 100 100 ± 5 V Supplies 110 100 0.005 60 33 0.005 60 33 0.005 MHz MHz % 100 100 110 100 100 100 110 100 ns ns ns ns –2– REV. D AD844 Model OUTPUT SLEW RATE FULL POWER BANDWIDTH VOUT = 20 V p-p5 VOUT = 2 V p-p5 OUTPUT CHARACTERISTICS Voltage Short Circuit Current TMIN–TMAX Output Resistance POWER SUPPLY Operating Range Quiescent Current TMIN–TMAX Conditions Overdriven Input VS = ± 15 V VS = ± 5 V THD = 3% RLOAD = 500 Ω Open Loop ± 4.5 6.5 7.5 10 Min AD844J/A Typ Max 2000 20 20 Min 1200 AD844B Typ 2000 20 20 Max AD844S Min Typ Max 1200 2000 20 20 Unit V/µs MHz MHz 1200 11 80 60 15 ± 18 7.5 8.5 10 11 80 60 15 ± 18 7.5 8.5 10 11 80 60 15 ± 18 7.5 9.5 ±V mA mA Ω V mA mA ± 4.5 6.5 7.5 +4.5 6.5 8.5 NOTES 1 Rated performance after a 5 minute warmup at T A = 25°C. 2 Input signal 285 mV p-p carrier (40 IRE) riding on 0 mV to 642 mV (90 IRE) ramp. R L= 100 Ω; R1, R2 = 300 Ω. 3 Input signal 0 dBm, C L = 10 pF, RL = 500 Ω, R1 = 500 Ω, R2 = 500 Ω in Figure 2. 4 Input signal 0 dBm, C L =10 pF, R L = 500 Ω, R1 = 500 Ω, R2 = 50 Ω in Figure 2. 5 CL = 10 pF, RL = 500 Ω, R1 = 1 kΩ, R2 = 1 kΩ in Figure 2. 6 CL = 10 pF, RL = 500 Ω, R1 = 500 Ω, R2 = 50 Ω in Figure 2. Specifications subject to change without notice. All min and max specifications are guaranteed. ABSOLUTE MAXIMUM RATINGS 1 Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V Power Dissipation2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 W Output Short Circuit Duration . . . . . . . . . . . . . . . . . Indefinite Common-Mode Input Voltage . . . . . . . . . . . . . . . . . . . . . ± VS Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . 6 V Inverting Input Current Continuous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 mA Transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 mA Storage Temperature Range (Q) . . . . . . . . . –65°C to +150°C Storage Temperature Range (N, R) . . . . . . . –65°C to +125°C Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 V ORDERING GUIDE NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 8-Lead Plastic Package: θJA = 90°C/W 8-Lead Cerdip Package: θJA = 110°C/W 16-Lead SOIC Package: θJA = 100°C/W METALIZATION PHOTOGRAPH Contact factory for latest dimensions. Dimension shown in inches and (mm). Model AD844AN AD844ACHIPS AD844AQ AD844BQ AD844JR-16 AD844JR-16-REEL AD844JR-16-REEL7 AD844SCHIPS AD844SQ AD844SQ/883B 5962-8964401PA *N Temperature Range –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 –55°C to +125°C –55°C to +125°C –55°C to +125°C –55°C to +125°C Package Option* N-8 Die Q-8 Q-8 R-16 13" Tape and Reel 7" Tape and Reel Die Q-8 Q-8 Q-8 = Plastic DIP; Q = Cerdip; R = Small Outline IC (SOIC). REV. D –3– AD844–Typical Characteristics (T = 25 C and V = A S 70 –60 –70 15 V, unless otherwise noted) 5 RL = HARMONIC DISTORTION – dB –3dB BANDWIDTH – MHz 4 –80 1V rms –90 –100 –110 2ND HARMONIC –120 TRANSRESISTANCE – M 60 RL = 500 3 50 2 RL = 50 1 40 30 0 10 15 5 SUPPLY VOLTAGE – V 20 –130 100 3RD HARMONIC 10k 1k INPUT FREQUENCY – Hz 100k 0 –50 0 50 100 TEMPERATURE – C 150 TPC 1. –3 dB Bandwidth vs. Supply Voltage R1 = R2 = 500 Ω MAGNITUDE OF THE OUTPUT VOLTAGE – V TA = 25 C MAGNITUDE OF THE OUTPUT VOLTAGE – V 20 TPC 2. Harmonic Distortion vs. Frequency, R1 = R2 = 1 kΩ 20 RL = 500 TA = 25 C 15 10 TPC 3. Transresistance vs. Temperature 9 15 SUPPLY CURRENT – mA 8 10 10 7 VS = 6 VS = 5 5V 15V 5 5 0 0 10 15 5 SUPPLY VOLTAGE – V 20 0 0 10 15 5 SUPPLY VOLTAGE – V 20 4 –60 –40 –20 0 20 40 60 80 100 120 140 TEMPERATURE – C TPC 4. Noninverting Input Voltage Swing vs. Supply Voltage TPC 5. Output Voltage Swing vs. Supply Voltage TPC 6. Quiescent Supply Current vs. Temperature and Supply Voltage 40 VS = 15V 2 100 INPUT BIAS CURRENT – A 35 OUTPUT IMPEDANCE – 1 10 5V SUPPLIES 1 –3dB BANDWIDTH – MHz 30 0 IBP 25 VS = 5V 20 –1 IBN 0.1 15 0.01 10k –2 –50 0 50 100 TEMPERATURE – C 150 100k 1M 10M 100M 10 –60 –40 –20 0 20 40 60 80 100 120 140 FREQUENCY – Hz TEMPERATURE – C TPC 7. Inverting Input Bias Current (IBN) and Noninverting Input Bias Current (IBP) vs. Temperature TPC 8. Output Impedance vs. Frequency, Gain = –1, R1 = R2 = 1 kΩ TPC 9. –3 dB Bandwidth vs. Temperature, Gain = –1, R1 = R2 = 1 kΩ –4– REV. 0 AD844 Inverting Gain-of-1 AC Characteristics +VS 0.22 F 4.7 R1 GAIN – dB 0 PHASE – Degrees R1 = R2 = 1k 6 R1 = R2 = 500 –180 –210 R1 = R2 = 500 –240 R2 VIN – –6 AD844 + RL 0.22 F 4.7 –VS VOUT –12 –270 R1 = R2 = 1k –300 CL –18 –24 100k –330 1M 10M FREQUENCY – Hz 100M 0 25 FREQUENCY – MHz 50 TPC 10. Inverting Amplifier, Gain of –1 (R1 = R2) TPC 11. Gain vs. Frequency for Gain = –1, RL = 500 Ω, CL = 0 pF TPC 12. Phase vs. Frequency Gain = –1, RL = 500 Ω, CL = 0 pF TPC 13. Large Signal Pulse Response, Gain = –1, R1 = R2 = 1 kΩ TPC 14. Small Signal Pulse Response, Gain = –1, R1 = R2 = 1 kΩ Inverting Gain-of-10 AC Characteristics 26 –180 RL = 500 +VS 0.22 F 4.7 500 20 –210 RL = 50 14 PHASE – Degrees GAIN – dB –240 RL = 500 50 VIN – 8 –270 RL = 50 –300 AD844 + VOUT 2 RL 0.22 F 4.7 –VS CL –4 100k 1M 10M 100M –330 0 25 FREQUENCY – MHz 50 FREQUENCY – Hz TPC 15. Gain of –10 Amplifier TPC 16. Gain vs. Frequency, Gain = –10 TPC 17. Phase vs. Frequency, Gain = –10 REV. D –5– AD844 Inverting Gain-of-10 Pulse Response TPC 18. Large Signal Pulse Response, Gain = –10, RL = 500 Ω TPC 19. Small Signal Pulse Response, Gain = –10, RL = 500 Ω Noninverting Gain-of-10 AC Characteristics 26 RL = 500 4.7 0.22 F 20 PHASE – Degrees –180 +VS –210 GAIN – dB 450 – 50 VIN VOUT 14 RL = 50 –240 RL = 50k RL = 500 AD844 + 0.22 F RL 8 –270 CL 2 –300 4.7 –VS –4 100k 1M 10M FREQUENCY – Hz 100M –330 0 25 FREQUENCY – MHz 50 TPC 20. Noninverting Gain of +10 Amplifier TPC 21. Gain vs. Frequency, Gain = +10 TPC 22. Phase vs. Frequency, Gain = +10 TPC 23. Noninverting Amplifier Large Signal Pulse Response, Gain = +10, RL = 500 Ω TPC 24. Small Signal Pulse Response, Gain = +10, RL = 500 Ω –6– REV. D 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 which need to be understood in order to optimize the performance of the AD844 op amp. Open Loop Behavior Figure 1 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 and are discussed later. 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 current applied to the inverting input node is replicated by the current conveyor so as 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 is another measure of gain and is determined by the beta product of the transistors in the voltage follower stage (see Figure 4); it is typically 40,000. Figure 2 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. The closed loop transresistance is simply the parallel sum of R1 and Rt. Since R1 will generally be in the range 500 Ω to 2 kΩ and Rt is about 3 MΩ the closed loop transresistance will be only 0.02% to 0.07% lower than R1. This small error will often be less than the resistor tolerance. 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 R1Ct. Under such conditions the AD844 is over-damped and will provide only a fraction of its bandwidth potential. Because of the absence of slew rate limitations under these conditions, the circuit will exhibit a simple single pole response even under large signal conditions. In Figure 2, 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 R1Ct becomes comparable to higher order poles in the closed loop 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 I lists combinations of R1 and R2 and the resulting frequency response for the circuit of Figure 2. TPC 13 shows the very clean and fast ± 10 V pulse response of the AD844. R1 +1 IIN RIN IIN Rt Ct +1 Figure 1. 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 as will be shown. 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. Since 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. VIN R2 R3 OPTIONAL AD844 RL VOUT CL Figure 2. Inverting Amplifier REV. D –7– AD844 Table I. ISIG R1 Gain –1 –1 –2 –2 –5 –5 –10 –10 –20 –100 +100 R1 1 kΩ 500 Ω 2 kΩ 1 kΩ 5 kΩ 500 Ω 1 kΩ 500 Ω 1 kΩ 5 kΩ 5 kΩ R2 1 kΩ 500 Ω 1 kΩ 500 Ω 1 kΩ 100 Ω 100 Ω 50 Ω 50 Ω 50 Ω 50 Ω BW (MHz) 35 60 15 30 5.2 49 23 33 21 3.2 9 GBW (MHz) 35 60 30 60 26 245 230 330 420 320 900 CS AD844 RL VOUT CL Figure 3. Current-to-Voltage Converter Circuit Description of the AD844 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 3. This analysis includes the stray capacitance, CS, of the current source, which might be a high speed DAC. Using a conventional op amp, this capacitance forms a “nuisance pole” with R1 which 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 were 2.5 kΩ a CS of 15 pF would place 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. Since this is about 50 Ω f or the AD844, the same 15 pF forms a pole 212 MHz and causes little trouble. It can be shown that theresponse of this system is: VOUT = – Isig K R1 (1 + sTd )(1 + sTn ) A simplified schematic is shown in Figure 4. 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 would be 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 which 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 and Tn is the nuisance pole: IB K= Rt Rt + R1 +IN 3 2 – IN TZ 5 6 OUT Td = KR1Ct Tn = RINCS (assuming RIN