AD844JRZ-16-REEL7

60 MHz, 2000 V/μs, Monolithic Op Amp with Quad Low Noise AD844 Data Sheet 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 8 NULL 7 +VS +IN 3 6 OUTPUT –VS 4 5 TZ TOP VIEW (Not to Scale) Figure 1. 8-Lead PDIP (N) and 8-Lead CERDIP (Q) Packages NC 1 16 NC OFFSETNULL 2 15 OFFSETNULL –IN 3 14 V+ NC 4 13 NC +IN 5 V– 7 12 OUTPUT AD844 11 TZ 10 NC TOP VIEW NC 8 (Not to Scale) 9 NC Flash ADC input amplifiers High speed current DAC interfaces Video buffers and cable drivers Pulse amplifiers NC = NO CONNECT 00897-002 NC 6 APPLICATIONS Figure 2. 16-Lead SOIC (R) Package GENERAL DESCRIPTION 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. 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 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. 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. Rev. G AD844 –IN 2 00897-001 FEATURES 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 1. 2. 3. 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). Document Feedback 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 ©1989-2017 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com AD844 Data Sheet 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 5/2017—Rev. F to Rev. G Change to Figure 32 ....................................................................... 14 2/2009—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 1/2003—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 11/2001—Rev. C to Rev. D Edits to Specifications ......................................................................2 Edits to Absolute Maximum Ratings ..............................................3 Edits to Ordering Guide ...................................................................3 Rev. G | Page 2 of 20 Data Sheet AD844 SPECIFICATIONS TA = 25°C and VS = ±15 V dc, unless otherwise noted. Table 1. Parameter INPUT OFFSET VOLTAGE1 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 ERROR2 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. G | 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 Bandwidth3, 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 Data Sheet Conditions AD844J/AD844A Min Typ Max f = 100 kHz, 2 V rms5 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. G | 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 Data Sheet AD844 ABSOLUTE MAXIMUM RATINGS METALLIZATION PHOTOGRAPH Table 2. 1 Contact factory for latest dimensions. Ratings ±18 V 1.1 W Indefinite ±VS 6V 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 +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. +IN Stresses at or above those listed under Absolute Maximum Ratings may cause permanent damage to the product. This is a stress rating only; functional operation of the product at these or any other conditions above those indicated in the operational section of this specification is not implied. Operation beyond the maximum operating conditions for extended periods may affect product reliability. NULL –VS TZ 0.095 (2.4) SUBSTRATE CONNECTED TO +VS Figure 3. Die Photograph ESD CAUTION Rev. G | Page 5 of 20 OUTPUT 00897-003 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 AD844 Data Sheet 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 8. Output Voltage Swing vs. Supply Voltage Figure 5. Harmonic Distortion vs. Input Frequency, R1 = R2 = 1 kΩ 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. G | Page 6 of 20 Data Sheet 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. G | Page 7 of 20 AD844 Data Sheet INVERTING GAIN-OF-1 AC CHARACTERISTICS +VS 0.22µF 5V 4.7Ω 100 R1 –IN R2 90 – AD844 OUTPUT + CL RL 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. G | Page 8 of 20 00897-017 0 Figure 17. Small Signal Pulse Response, Gain = −1, R1 = R2 = 1 kΩ Data Sheet AD844 INVERTING GAIN-OF-10 AC CHARACTERISTICS +VS RL = 500Ω 20 500Ω – AD844 GAIN (dB) OUTPUT + RL RL = 50Ω 14 8 CL 0.22µF –VS 00897-018 2 4.7Ω –4 100k 1M 10M 100M FREQUENCY (Hz) 00897-019 50Ω Figure 19. Gain vs. Frequency, Gain = −10 Figure 18. Gain of −10 Amplifier –180 –210 RL = 50Ω PHASE (Degrees) –IN RL = 500Ω –240 –270 –300 –330 0 25 FREQUENCY (MHz) Figure 20. Phase vs. Frequency, Gain = −10 Rev. G | Page 9 of 20 50 00897-020 0.22µF 26 4.7Ω AD844 Data Sheet INVERTING GAIN-OF-10 PULSE RESPONSE 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 500nV 5V Figure 22. Small Signal Pulse Response, Gain = −10, RL = 500 Ω Rev. G | Page 10 of 20 Data Sheet 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. G | Page 11 of 20 AD844 Data Sheet UNDERSTANDING THE AD844 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. RESPONSE AS AN INVERTING AMPLIFIER 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 Rt Ct +1 IIN 00897-028 RIN 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. G | 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 Data Sheet 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. G | Page 13 of 20 4 –VS Figure 31. Simplified Schematic 00897-031 IB Tn = RINCS (assuming RIN