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AD587 Datasheet(PDF) 6 Page - Analog Devices |
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AD587 Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page ![]() AD587 Rev. H | Page 6 of 12 THEORY OF OPERATION The AD587 consists of a proprietary buried Zener diode reference, an amplifier to buffer the output, and several high stability thin-film resistors, as shown in Figure 3. This design results in a high precision monolithic 10 V output reference with initial offset of 5 mV or less. The temperature-compensa- tion circuitry provides the device with a temperature coefficient of less than 5 ppm/°C. 8 2 6 4 5 RT RI AD587 A1 +VIN NOISE REDUCTION VOUT TRIM GND NOTE PIN 1, PIN 3, AND PIN 7 ARE INTERNAL TEST POINTS. NO CONNECTIONS TO THESE POINTS. RF RS Figure 3. Functional Block Diagram A capacitor can be added at the NOISE REDUCTION pin (Pin 8) to form a low-pass filter with RS to reduce the noise contribution of the Zener to the circuit. APPLYING THE AD587 The AD587 is simple to use in virtually all precision reference applications. When power is applied to Pin 2 and Pin 4 is grounded, Pin 6 provides a 10 V output. No external compo- nents are required; the degree of desired absolute accuracy is achieved simply by selecting the required device grade. The AD587 requires less than 4 mA quiescent current from an operating supply of 15 V. Fine trimming may be desired to set the output level to exactly 10.000 V (calibrated to a main system reference). System cali- bration may also require a reference voltage that is slightly different from 10.000 V, for example, 10.24 V for binary applications. In either case, the optional fine-trimming circuit shown in Figure 4 can offset the output by as much as 300 mV with minimal effect on other device characteristics. 8 2 6 4 GND 5 AD587 NOISE REDUCTION TRIM 10kΩ OUTPUT OPTIONAL NOISE- REDUCTION CAPACITOR CN 1µF +VIN VOUT VIN Figure 4. Optional Fine-Trimming Configuration NOISE PERFORMANCE AND REDUCTION Noise generated by the AD587 is typically less than 4 μV p-p over the 0.1 Hz to 10 Hz band. Noise in a 1 MHz bandwidth is approximately 200 μV p-p. The dominant source of this noise is the buried Zener, contributing approximately 100 nV/√Hz. By comparison, the contribution of the op amp is negligible. Figure 5 shows the 0.1 Hz to 10 Hz noise of a typical AD587. The noise measurement is made with a band-pass filter made of a 1-pole high-pass filter with a corner frequency at 0.1 Hz and a 2-pole low-pass filter with a corner frequency at 12.6 Hz to create a filter with a 9.922 Hz bandwidth. 1µV 5s 100 90 10 0% 1µV Figure 5. 0.1 Hz to 10 Hz Noise If further noise reduction is desired, an external capacitor can be added between the NOISE REDUCTION pin and ground, as shown in Figure 4. This capacitor, combined with the 4 kΩ RS and the Zener resistances, forms a low-pass filter on the output of the Zener cell. A 1 μF capacitor has a 3 dB point at 40 Hz and reduces the high frequency (up to 1 MHz) noise to about 160 μV p-p. Figure 6 shows the 1 MHz noise of a typical AD587, both with and without a 1 μF capacitor. 100 0% 200µV 50µs 90 10 NO CN CN – 1µF Figure 6. Effect of 1 μF Noise-Reduction Capacitor on Broadband Noise |
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