Datasheet AD825 (Analog Devices) - 10

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AD825 Data Sheet DRIVING CAPACITIVE LOADS
The internal compensation of the AD825, together with its high
VPOS
output current drive, permits excellent large signal performance while driving extremely high capacitive loads.
1k
Ω
NEG +VS 10
µ
F POS 0.01
µ
F V 7 C HP PULSE IN F VOUT 2 TEKTRONIX GENERATOR TEKTRONIX RIN AD825 6 P6204 FET 7A24 1k
Ω
VOUT 3 PROBE PREAMP 4 50
Ω
0.01
µ
F CL –VS 10
µ
F
00876-E-028 Figure 30. Inverting Amplifier Driving a Capacitive Load
VNEG
00876-E-030 Figure 32. Simplified Schematic
5V 500ns INPUT
The capacitor, CF, in the output stage, enables the AD825 to drive heavy capacitive loads. For light loads, the gain of the output buffer is close to unity, CF is bootstrapped, and not much happens. As the capacitive load is increased, the gain of the output buffer is decreased and the bandwidth of the amplifier is reduced through a portion of CF adding to the dominant pole. As the capacitive load is further increased, the amplifier’s bandwidth continues to drop, maintaining the stability of the AD825.
OUTPUT 5V INPUT CONSIDERATION
00876-E-029 The AD825 with its unique input stage ensures no phase Figure 31. Inverting Amplifier Pulse Response While Driving a 400 pF Capacitive Load reversal for signals as large as or even larger than the supply voltages. Also, layout considerations of the input transistors
THEORY OF OPERATION
ensure functionality even with a large differential signal. The AD825 is a low cost, wideband, high performance FET The need for a low noise input stage cal s for a larger FET input operational amplifier. With its unique input stage design, transistor. One should consider the additional capacitance that the AD825 ensures no phase reversal, even for inputs that is added to ensure stability. When filters are designed with the exceed the power supply voltages, and its output stage is AD825, one needs to consider the input capacitance (5 pF to designed to drive heavy capacitive or resistive loads with small 6 pF) of the AD825 as part of the passive network. changes relative to no load conditions.
GROUNDING AND BYPASSING
The AD825 (Figure 32) consists of common-drain, common- The AD825 is a low input bias current FET amplifier. Its high base FET input stage driving a cascoded, common-base frequency response makes it useful in applications, such as matched NPN gain stage. The output buffer stage uses emitter photodiode interfaces, filters, and audio circuits. When followers in a Class AB amplifier that can deliver large current designing high frequency circuits, some special precautions are to the load while maintaining low levels of distortion. in order. Circuits must be built with short interconnects, and resistances should have low inductive paths to ground. Power supply leads should be bypassed to common as close as possible to the amplifier pins. Ceramic capacitors of 0.1 µF are recommended. Rev. G | Page 10 of 12 Document Outline Features Applications Connection Diagrams General Description Table of Contents Revision History Specifications Absolute Maximum Ratings Pin Configurations ESD Caution Typical Performance Characteristics Driving Capacitive Loads Theory of Operation Input Consideration Grounding and Bypassing Second-Order Low-Pass Filter Outline Dimensions Ordering Guide