|Home Analog Devices Feedback Subscribe Archives 简体中文 日本語|
Dual Difference Amplifier with On-Chip Resistors Implements Precision ADC Driver
The versatile AD8270 dual difference amplifier, shown in Figure 1, overcomes these limitations, providing a complete, inexpensive, high-performance solution in the smallest available package. Each channel, which includes a low distortion amplifier and seven trimmed resistors, can be configured to implement a wide variety of high-performance amplifiers with various gains. All precision resistors are integrated on chip, so resistance matching and temperature tracking are excellent. Operating on a single 5-V to 36-V supply, or dual ±2.5-V to ±18-V supplies, and drawing a maximum supply current of only 2.5 mA per amplifier, the AD8270 is useful for driving high-performance ADCs.
This article shows two pin-strapped circuits that provide 0.1% gain accuracy with less than 10 ppm/°C gain driftusing no external resistors.
Figure 1. AD8270 functional block diagram.
G = VOUT/VIN = ½ (½) = 1.
The output common-mode voltage, (OUT+ + OUT)/2, is equal to VOCM.
When driving ADCs, the gain should be chosen such that the signal swing is close to the full-scale input range of the ADC. The impedance at the inverting and noninverting inputs of the amplifiers should be equal to eliminate the effect of bias currents and to maximize the common-mode rejection. The AD8603 unity-gain follower sets the common-mode output voltage of the differential amplifier to VOCM, centering the signal in the middle of the ADC's input range. This pin can be tied to ground when the circuit is operated with dual supplies, to VS/2 for single-supply operation, oras shownto the ADC's reference pin when driving single-supply ADCs, allowing ratiometric operation. The AD8603 can be eliminated if VOCM is a low-impedance source.
Figure 2. Differential amplifier drives ADC.
at Gains Less Than 1 (Differential to Single-Ended)
Figure 3. Connection for gains less than 1.
Pin strapping configures Amplifier A for a gain of +½. Amplifier B, configured for a gain of ½, attenuates the signal again, so the total gain for this connection is equal to 0.25.
Copyright 1995- Analog Devices, Inc. All rights reserved.