Datasheet AD842 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Wideband, High Output Current, Fast Settling Op Amp |
| Pages / Page | 16 / 10 — AD842. Data Sheet. GROUNDING AND BYPASSING. USING A HEAT SINK. 5mV. 2µs. … |
| Revision | F |
| File Format / Size | PDF / 362 Kb |
| Document Language | English |
AD842. Data Sheet. GROUNDING AND BYPASSING. USING A HEAT SINK. 5mV. 2µs. 100%. 90%. OUTPUT: 5V/DIV. OUTPUT. TERMINATED LINE DRIVER
Model Line for this Datasheet
Text Version of Document
link to page 10
AD842 Data Sheet GROUNDING AND BYPASSING
the dynamic performance of the device, although instability In designing practical circuits with the AD842, the user must does not occur unless the load exceeds 100 pF. take some special precautions whenever high frequencies are
USING A HEAT SINK
involved. The AD842 draws less quiescent power than most precision
5mV 2µs
high speed amplifiers and is specified for operation without a
100%
heat sink. However, when driving low impedance loads, the
90% OUTPUT: 5V/DIV
current to the load can be 10 times the quiescent current. This creates a noticeable temperature rise. Use of a small heat sink improves performance.
OUTPUT TERMINATED LINE DRIVER ERROR: 0.01%/DIV
The AD842 is optimized for high speed line driver applications. Figure 32 shows the AD842 driving a doubly terminated cable in a gain-of-2 follower configuration. The AD842 maintains a
10% 0%
typical slew rate of 375 V/μs, which means it can drive a ±10 V, 031 6.0 MHz signal, or a ±3 V, 19.9 MHz signal. 09477- Figure 31. AD842 Settling Demonstrating No Settling Tails The termination resistor, RT, minimizes reflections from the far Circuits must be built with short interconnect leads. Use large end of the cable when equal to the characteristic impedance of ground planes whenever possible to provide a low resistance, the cable. A back-termination resistor (RBT, also equal to the low inductance circuit path; this also minimizes the effects of characteristic impedance of the cable) can be placed between high frequency coupling. Avoid sockets because the increased the AD842 output and the cable to damp any stray signals interlead capacitance can degrade bandwidth. caused by a mismatch between RT and the characteristic impedance of the cable. This configuration results in a cleaner Use feedback resistors of low enough value to ensure that the signal. With this circuit, the voltage on the line equals VIN time constant formed with the circuit capacitances does not because one half of VOUT is dropped across RBT. limit the amplifier performance. Resistor values of less than 5 kΩ are recommended. If a larger resistor must be used, a The AD842 has a 100 mA minimum output current and, small (<10 pF) feedback capacitor connected in parallel with therefore, can drive ±5 V into a 50 Ω cable. the feedback resistor, RF, can be used to compensate for these Choose the feedback resistors, R1 and R2, carefully. Large value stray capacitances and to optimize the dynamic performance of resistors are desirable to limit the amount of current drawn the amplifier in the particular application. from the amplifier output. Large resistors can cause amplifier Bypass power supply leads to ground as close as possible to the instability because the parallel resistance of R1||R2 combines amplifier pins. A 2.2 μF capacitor in parallel with a 0.1 μF with the input capacitance (typically 2 pF to 5 pF) to create an ceramic disk capacitor is recommended. additional pole. The voltage noise of the AD842 is equivalent to a 5 kΩ resistor; these large resistors can significantly increase
CAPACITIVE LOAD DRIVING ABILITY
the system noise. Resistor values of 1 kΩ or 2 kΩ are Like all wideband amplifiers, the AD842 is sensitive to recommended. capacitive loading. The AD842 is designed to drive capacitive If termination is not used, cables appear as capacitive loads and loads of up to 20 pF without degradation of its rated can be decoupled from the AD842 by a resistor in series with performance. Capacitive loads of greater than 20 pF decrease the output.
0.1µF +VS 2.2µF 50Ω OR 75Ω 11 V 5 CABLE IN RBT TERMINATION AD842 10 RESISTOR FOR 4 0.1µF 6 INPUT SIGNAL RT 2.2µF R1 –VS R2
032
RT = RBT = CABLE CHARACTERISTIC IMPEDANCE
09477- Figure 32. Line Driver Configuration (PDIP) Rev. F | Page 10 of 16 Document Outline Features Applications Connection Diagrams General Description Product Highlights Table of Contents Revision History Specifications Electrical Characteristics—±15 V Operation Absolute Maximum Ratings Thermal Characteristics ESD Caution Metalization Photograph Typical Performance Characteristics Theory of Operation Offset Nulling Settling Time Grounding and Bypassing Capacitive Load Driving Ability Using a Heat Sink Terminated Line Driver Overdrive Recovery Outline Dimensions Ordering Guide
AD842 Data Sheet GROUNDING AND BYPASSING
the dynamic performance of the device, although instability In designing practical circuits with the AD842, the user must does not occur unless the load exceeds 100 pF. take some special precautions whenever high frequencies are
USING A HEAT SINK
involved. The AD842 draws less quiescent power than most precision
5mV 2µs
high speed amplifiers and is specified for operation without a
100%
heat sink. However, when driving low impedance loads, the
90% OUTPUT: 5V/DIV
current to the load can be 10 times the quiescent current. This creates a noticeable temperature rise. Use of a small heat sink improves performance.
OUTPUT TERMINATED LINE DRIVER ERROR: 0.01%/DIV
The AD842 is optimized for high speed line driver applications. Figure 32 shows the AD842 driving a doubly terminated cable in a gain-of-2 follower configuration. The AD842 maintains a
10% 0%
typical slew rate of 375 V/μs, which means it can drive a ±10 V, 031 6.0 MHz signal, or a ±3 V, 19.9 MHz signal. 09477- Figure 31. AD842 Settling Demonstrating No Settling Tails The termination resistor, RT, minimizes reflections from the far Circuits must be built with short interconnect leads. Use large end of the cable when equal to the characteristic impedance of ground planes whenever possible to provide a low resistance, the cable. A back-termination resistor (RBT, also equal to the low inductance circuit path; this also minimizes the effects of characteristic impedance of the cable) can be placed between high frequency coupling. Avoid sockets because the increased the AD842 output and the cable to damp any stray signals interlead capacitance can degrade bandwidth. caused by a mismatch between RT and the characteristic impedance of the cable. This configuration results in a cleaner Use feedback resistors of low enough value to ensure that the signal. With this circuit, the voltage on the line equals VIN time constant formed with the circuit capacitances does not because one half of VOUT is dropped across RBT. limit the amplifier performance. Resistor values of less than 5 kΩ are recommended. If a larger resistor must be used, a The AD842 has a 100 mA minimum output current and, small (<10 pF) feedback capacitor connected in parallel with therefore, can drive ±5 V into a 50 Ω cable. the feedback resistor, RF, can be used to compensate for these Choose the feedback resistors, R1 and R2, carefully. Large value stray capacitances and to optimize the dynamic performance of resistors are desirable to limit the amount of current drawn the amplifier in the particular application. from the amplifier output. Large resistors can cause amplifier Bypass power supply leads to ground as close as possible to the instability because the parallel resistance of R1||R2 combines amplifier pins. A 2.2 μF capacitor in parallel with a 0.1 μF with the input capacitance (typically 2 pF to 5 pF) to create an ceramic disk capacitor is recommended. additional pole. The voltage noise of the AD842 is equivalent to a 5 kΩ resistor; these large resistors can significantly increase
CAPACITIVE LOAD DRIVING ABILITY
the system noise. Resistor values of 1 kΩ or 2 kΩ are Like all wideband amplifiers, the AD842 is sensitive to recommended. capacitive loading. The AD842 is designed to drive capacitive If termination is not used, cables appear as capacitive loads and loads of up to 20 pF without degradation of its rated can be decoupled from the AD842 by a resistor in series with performance. Capacitive loads of greater than 20 pF decrease the output.
0.1µF +VS 2.2µF 50Ω OR 75Ω 11 V 5 CABLE IN RBT TERMINATION AD842 10 RESISTOR FOR 4 0.1µF 6 INPUT SIGNAL RT 2.2µF R1 –VS R2
032
RT = RBT = CABLE CHARACTERISTIC IMPEDANCE
09477- Figure 32. Line Driver Configuration (PDIP) Rev. F | Page 10 of 16 Document Outline Features Applications Connection Diagrams General Description Product Highlights Table of Contents Revision History Specifications Electrical Characteristics—±15 V Operation Absolute Maximum Ratings Thermal Characteristics ESD Caution Metalization Photograph Typical Performance Characteristics Theory of Operation Offset Nulling Settling Time Grounding and Bypassing Capacitive Load Driving Ability Using a Heat Sink Terminated Line Driver Overdrive Recovery Outline Dimensions Ordering Guide
