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AD569KNZ Folha de dados(PDF) 7 Page - Analog Devices |
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AD569KNZ Folha de dados(HTML) 7 Page - Analog Devices |
7 / 12 page AD569 REV. A –7– benefit is that, should a Zener diode fail (a short circuit would be the most likely failure), the supply voltage decreases. This differs from the situation where the diode is used as a series regulator. In that case, a failure would place the unregulated supply voltage on the AD569 terminal. a. Zener Regulates Negative Supply b. Diodes Regulate Both Supplies c. Use of ±15 V and ±5 V Supplies d. AD588 Produces References and Supply Voltages Figure 7. Power Supply Options ANALOG CIRCUIT CONNECTIONS The AD569 is intended for use in applications where high reso- lution and stability are critical. Designed as a multiplying D/A converter, the AD569 may be used with a fixed dc reference or an ac reference. VREF may be any voltage or combination of voltages at +VFORCE and –VFORCE that remain within the bounds set for reference voltages as discussed in the power supply range section. Since the AD569 is a multiplying D/A converter, its output voltage, VOUT, is proportional to the product of the digi- tal input word and the voltage at the reference terminal. The transfer function is VOUT = D·VREF where D is the fractional bi- nary value of the digital word applied to the converter using offset-binary coding. Therefore, the output will range from –VREF for a digital input code of all zeros (0000H) to +VREF for an input code of all ones (FFFFH). For applications where absolute accuracy is not critical, the simple reference connection in Figure 8 can be used. Using only the reference force inputs, this configuration maintains linearity and 16-bit monotonicity, but introduces small, fixed offset and gain errors. These errors are due to the voltage drops across re- sistors RA and RB shown in Figure 9. With a 10 V reference voltage, the gain and offset errors will range from 80 mV to 100 mV. Resistors RA and RB were included in the first resistor string to avoid degraded linearity due to uneven current densi- ties at the string’s endpoints. Similarly, linearity would degrade if the reference voltage were connected across the reference sense terminals. Note that the resistance between the force and sense terminals cannot be measured with an ohmmeter; the lay- out of the thin-film resistor string adds approximately 4 k Ω of resistance (RS) at the sense tap. Figure 8. Simple Reference Connection For those applications in which precision references and high accuracy are critical, buffer amplifiers are used at +VREF and –VREF as shown in Figure 10 to force the voltage across resistors R1 to R256. This insures that any errors induced by currents flowing through the resistances of the package pins, bond wires, aluminum interconnections, as well as RA and RB are mini- mized. Suitable amplifiers are the AD517, AD OP07, AD OP27, or the dual amplifier, the AD712. Errors will arise, however, as the buffer amplifiers’ bias currents flow through RS (4 k Ω). If the bias currents produce such errors, resistance can be inserted at the noninverting terminal (RBC) of the buffer amplifiers to compensate for the errors. |
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