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AD688 Arkusz danych(PDF) 4 Page - Analog Devices |
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AD688 Arkusz danych(HTML) 4 Page - Analog Devices |
4 / 8 page AD688 REV. A –4– APPLYING THE AD688 The AD688 can be configured to provide ±10 V reference out- puts as shown in Figure 2a. The architecture of the AD688 pro- vides ground sense and uncommitted output buffer amplifiers which offer the user a great deal of functional flexibility. The AD688 is specified and tested in the configuration shown in Figure 2a. The user may choose to take advantage of other configuration options available with the AD688; however performance in these configurations is not guaranteed to meet the stringent data sheet specifications. Unbuffered outputs are available at Pins 6 and 8. Loading of these unbuffered outputs will impair circuit performance. Amplifiers A3 and A4 can be used interchangeably. However, the AD688 is tested (and the specifications are guaranteed) with the amplifiers connected as indicated in Figure 2a. When either A3 or A4 is unused, its output force and sense pins should be connected and the input tied to ground. Two outputs of the same voltage polarity may be obtained by connecting both A3 and A4 to the appropriate unbuffered out- put on Pin 6 or 8. Performance in these dual output configura- tions will typically meet data sheet specifications. Figure 2a. +10 V and –10 V Outputs CALIBRATION Generally, the AD688 will meet the requirements of a precision system without additional adjustment. Initial output voltage error of 2 mV and output noise specs of 6 µV p-p allow for accuracies of 12-16 bits. However, in applications where an even greater level of accuracy is required, additional calibration may be called for. The provision for trimming has been made through the use of the GAIN ADJUST and BALANCE ADJUST pins (Pins 5 and 12, respectively). The AD688 provides a precision 20 V span with a center tap which is used with the buffer and ground sense amplifiers to achieve the ±10 V output configuration. The GAIN ADJUST and BALANCE ADJUST can be used to trim the magnitude of the 20 V span voltage and the position of the center tap within the span. The GAIN ADJUST should be performed first. Al- though the trims are not interactive within the device, the GAIN trim will move the BALANCE trim point as it changes the magnitude of the span. Figure 2b shows the GAIN and BALANCE trims of the AD688. A 100 k Ω 20-turn potentiometer is used for each trim. The potentiometer for the GAIN trim is connected between Pins 6 (VHIGH) and 8 (VLOW) with the wiper connected to Pin 5 (GAIN ADJ). The potentiometer is adjusted to produce exactly 20 V between Pins 1 and 15, the amplifier outputs. The BAL- ANCE potentiometer, also connected between Pins 6 and 8 with the wiper to Pin 12 (BAL ADJ), is then adjusted to center the span from +10 V to –10 V. Input impedance on both the GAIN ADJUST and the BAL- ANCE ADJUST pins is approximately 150 k Ω. The GAIN ADJUST trim network effectively attenuates the 20 V across the trim potentiometer by a factor of about 1150 to provide a trim range of –5.8 mV to + 12.0 mV with a resolution of approxi- mately 900 µV/turn (20 turn potentiometer). The BALANCE ADJUST trim network attenuates the trim voltage by a factor of about 1250, providing a trim range of ±8 mV with a resolution of 800 µV/turn. Trimming the AD688 introduces no additional errors over temperature, so precision potentiometers are not required. In cases when BALANCE ADJUST is not necessary, Pin 12 should be left floating. If GAIN ADJUST is not required, Pin 5 should also be left floating. Figure 2b. Gain and Balance Adjust with Noise Reduction NOISE PERFORMANCE AND REDUCTION The noise generated by the AD688 is typically less than 6 µV p-p over the 0.1 Hz to 10 Hz band. Noise in a 1 MHz bandwidth is approximately 840 µV p-p. The dominant source of this noise is the buried Zener which contributes approxi- mately 140 nV/ √Hz. In comparison, the op amp’s contribution is negligible. Figure 3 shows the 0.1 Hz to 10 Hz noise of a typical AD688. Figure 3. 0.1 Hz to 10 Hz Noise |
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Podobny opis - AD688 |
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