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AD668KQ bảng dữ liệu(PDF) 9 Page - Analog Devices |
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AD668KQ bảng dữ liệu(HTML) 9 Page - Analog Devices |
9 / 16 page AD668 REV. A –9– Figure 8. 1.25 V REFIN/ ±500 mV Unbuffered Bipolar Output 5 V REFIN, 2 V BIPOLAR, UNBUFFERED VOLTAGE OUTPUT Figure 9 demonstrates how a larger unbuffered voltage output swing can be realized. RLOAD (Pin 19) is tied to the DAC output (Pin 20) to produce an output resistance of roughly 200 Ω. Figure 9. 5 V REFIN/ ±1 V Unbuffered Bipolar Output It should be noted that this impedance is not trimmed, and may vary by as much as 20%, but this can be compensated by adjust- ing the reference voltage. It is also important to note that limita- tions in the DAC output compliance would prohibit use of a 2 V unipolar output voltage swing. 1 V REFIN, –10 V UNIPOLAR, BUFFERED VOLTAGE OUTPUT Figure 10 shows the implementation of the 1 V full scale for the reference input by tying REFIN1 and REFIN2 together and driving them both with the input voltage. This generates a high input impedance, and some care should be taken to insure that the driving impedance at this node is finite at all times to avoid saturating the reference amplifier. This is typically accomplished by a using a low impedance voltage source to drive the refer- ence, but if the topology calls for this source to be switched out, a high impedance (10 k Ω) termination resistor should be used on the REFIN node. Figure 10. 1 V REFIN/–10 V Unipolar Buffered Output For full-scale output ranges greater than 2 V, some type of ex- ternal buffer amplifier is needed. The AD840 fills this require- ment perfectly, settling to within 0.025% from a 10 V full-scale step in less than 100 ns. As shown in Figure 10, the amplifier establishes a summing node at ground for the DAC output. The output voltage is determined by the amplifier’s feedback resistor (10.24 V for a 1k resistor). Note that since the DAC generates a positive current to ground, the voltage at the amplifier output will be negative. A series resistor between the noninverting am- plifier input and ground minimizes the offset effects of op amp input bias currents. The optimal DAC output impedance in buffered output appli- cations depends on the buffer amplifier being used. The AD840 is stable at a gain of 10, so a lower DAC output impedance (higher noise gain) is desired for stability reasons, and RLOAD should be grounded. The 100 Ω DAC output impedance pro- duces a noise gain of 11 with the 1k feedback resistor. If the gain-of-two stable AD842 is used as a buffer, a 200 Ω DAC out- put impedance will produce a stable configuration with lower noise gain to the output; hence, RLOAD should be connected to the DAC output. As noted earlier, these four examples are part of an array of possible configurations available. Table II provides a quick reference chart for the more straightforward applications, but many other input and output signals are possible with some modifications. The next three circuits provide examples of different analog in- put drives, including a fixed dc reference, a capacitively coupled ac reference, and a DAC driving the reference channel. Note that the entire spectrum of input and output range configura- tions are available regardless of the type of reference drive being used. DC REFERENCE: THE AD586 DRIVING THE AD668 Figure 11 illustrates one of the more obvious analog input sources: a fixed reference. The AD586 produces a temperature stable 5 V analog output to drive the AD668 in the 5 V input |
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