Datasheet AD845 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Precision, 16 MHz CBFET Op Amp |
| Pages / Page | 13 / 9 — AD845. DRIVING THE ANALOG INPUT OF AN A/D CONVERTER |
| Revision | E |
| File Format / Size | PDF / 223 Kb |
| Document Language | English |
AD845. DRIVING THE ANALOG INPUT OF AN A/D CONVERTER
Model Line for this Datasheet
AD845
Text Version of Document
AD845
Figure 5. Settling Time of the 3-Op Amp Instrumentation Figure 6. Settling Time of the Three Op Amp Instru- Amplifier. Horizontal Scale is 200 ns/Div, Vertical Scale, mentation Amplifier. Horizontal Scale: 200 ns/Div; Vertical Positive Pulse Input is 5 V/Div and Output Settling is Scale, Negative Pulse Input: 5 V/ Div; Output Settling: 1 mV/Div. 1 mV/Div.
DRIVING THE ANALOG INPUT OF AN A/D CONVERTER
AD845 is ideally suited to drive high resolution A/D converters An op amp driving the analog input of an A/D converter, such with 5 ms or longer conversion times since it offers both wide as that shown in Figure 7, must be capable of maintaining a bandwidth and high open-loop gain. constant output voltage under dynamically changing load condi- tions. In successive approximation converters, the input current is compared to a series of switched trial currents. The compari- son point is diode clamped but may deviate several hundred millivolts, resulting in high frequency modulation of A/D input current. The output impedance of a feedback amplifier is made artificially low by the loop gain. At high frequencies, where the loop gain is low, the amplifier output impedance can approach its open-loop value. Most IC amplifiers exhibit a minimum open-loop output impedance of 25 W due to current limiting resistors. A few hundred microamps reflected from the change in converter loading can introduce errors in instantaneous input voltage. If the A/D conversion speed is not excessive and the bandwidth of the amplifier is sufficient, the amplifier’s output will return to the nominal value before the converter makes its comparison. However, many amplifiers have relatively narrow bandwidth, yielding slow recovery from output transients. The Figure 7. AD845 As ADC Unity Gain Buffer –8– REV. E Document Outline FEATURES AC PERFORMANCE: DC PERFORMANCE: CONNECTION DIAGRAMS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS METALIZATION PHOTOGRAPH ORDERING GUIDE Typical Performance Characteristics MEASURING AD845 SETTLING TIME A HIGH SPEED INSTRUMENTATION AMP DRIVING THE ANALOG INPUT OF AN A/D CONVERTER OUTLINE DIMENSIONS Revision History
Figure 5. Settling Time of the 3-Op Amp Instrumentation Figure 6. Settling Time of the Three Op Amp Instru- Amplifier. Horizontal Scale is 200 ns/Div, Vertical Scale, mentation Amplifier. Horizontal Scale: 200 ns/Div; Vertical Positive Pulse Input is 5 V/Div and Output Settling is Scale, Negative Pulse Input: 5 V/ Div; Output Settling: 1 mV/Div. 1 mV/Div.
DRIVING THE ANALOG INPUT OF AN A/D CONVERTER
AD845 is ideally suited to drive high resolution A/D converters An op amp driving the analog input of an A/D converter, such with 5 ms or longer conversion times since it offers both wide as that shown in Figure 7, must be capable of maintaining a bandwidth and high open-loop gain. constant output voltage under dynamically changing load condi- tions. In successive approximation converters, the input current is compared to a series of switched trial currents. The compari- son point is diode clamped but may deviate several hundred millivolts, resulting in high frequency modulation of A/D input current. The output impedance of a feedback amplifier is made artificially low by the loop gain. At high frequencies, where the loop gain is low, the amplifier output impedance can approach its open-loop value. Most IC amplifiers exhibit a minimum open-loop output impedance of 25 W due to current limiting resistors. A few hundred microamps reflected from the change in converter loading can introduce errors in instantaneous input voltage. If the A/D conversion speed is not excessive and the bandwidth of the amplifier is sufficient, the amplifier’s output will return to the nominal value before the converter makes its comparison. However, many amplifiers have relatively narrow bandwidth, yielding slow recovery from output transients. The Figure 7. AD845 As ADC Unity Gain Buffer –8– REV. E Document Outline FEATURES AC PERFORMANCE: DC PERFORMANCE: CONNECTION DIAGRAMS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS METALIZATION PHOTOGRAPH ORDERING GUIDE Typical Performance Characteristics MEASURING AD845 SETTLING TIME A HIGH SPEED INSTRUMENTATION AMP DRIVING THE ANALOG INPUT OF AN A/D CONVERTER OUTLINE DIMENSIONS Revision History
