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AD768 датащи(PDF) 7 Page - Analog Devices |
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AD768 датащи(HTML) 7 Page - Analog Devices |
7 / 20 page AD768 REV. B –7– REFERENCE OUTPUT The internal 2.5 V bandgap reference is provided for generation of the IREFIN current, and must be compensated externally with a capacitor of 0.1 µF or greater from REFOUT to REFCOM. If an external reference is used, REFOUT should be tied directly to the positive supply voltage, VDD. This effectively turns off the internal reference, eliminating the need for the external capaci- tor at REFOUT. The reference is specified to drive a nominal load of 5 mA with a maximum of 15 mA. Operation with a heavier load will result in degradation of supply rejection and reference voltage accuracy. Therefore, the reference output should be buffered with an amplifier when additional load cur- rent is required. A properly sized pull-up resistor can also be used to source additional current to the load. The resistors value should be selected such that REFOUT will always source a minimum of 5 mA to IREFIN and the additional load. AD768 IREFIN REFOUT REFCOM CREFCOMP 1µF 500 Ω 5 3 6 Figure 6. Typical Reference Hookup TEMPERATURE CONSIDERATIONS Note that the reference plays a key role in the overall tempera- ture performance of the AD768. Any drift of IREFIN shows up directly in IOUT. When the output is taken as a current, the drift of IREFIN (which depends on both VREF and RREF) must be mini- mized. This can be done by using the internal temperature com- pensated reference for VREF and a low temperature coefficient resistor for RREF. If the output is taken as a voltage, it is a func- tion of a resistor ratio, not an absolute resistor value. By select- ing resistors with matched temperature coefficients for RREF and RLOAD, the drift in the resistor values will cancel, providing optimal drift performance. REFERENCE NOISE REDUCTION AND MULTIPLYING BANDWIDTH For application flexibility and multiplying capabilities, the refer- ence amplifier is designed to offer adjustable bandwidth that can be reduced by connecting an external capacitor from the NR node to the negative supply pin, VEE. This capacitor limits the bandwidth and acts as a filter to reduce the noise contribution from the reference amplifier. The noise reduction capacitor, CNR, is not required for stability and does not affect the settling time of the DAC output. With- out this capacitor, the IREFIN bandwidth is 15 MHz allowing high frequency modulation of the DAC full-scale range through the reference input node. Figure 7 shows the relationship be- tween the external noise reduction capacitor and the –3 dB bandwidth of the reference amplifier. NOISE REDUCTION CAPACITOR – F 100M 1k 10p 1µ 100p 1n 10n 100n 10M 1M 100k 10k Figure 7. External Noise Reduction Capacitor vs. –3 dB Bandwidth The sensitivity of the NR node requires that care be taken in capacitor placement. The capacitor should be located as physi- cally close to the package pins as possible and lead lengths should be minimized. For this purpose, the use of a chip capacitor is recommended. For applications that do not require high frequency modulation at IREFIN, it is recommended that a capacitor on the order of 1 µF be connected from NR to V EE. If the reference input is purely dc, noise may be minimized with multiple capacitors, such as 1 µF and 0.1 µF, to more effectively filter both high and low frequency disturbances. ANALOG OUTPUTS The AD768 offers two analog outputs; IOUTA is trimmed for optimal INL and DNL performance and has a full-scale output when all bits are high. For applications that require the specified dc accuracy, IOUTA should be used. IOUTB is the comple- mentary output with full-scale output when all bits are low. Both IOUTA and IOUTB provide similar dynamic perfor- mance. Refer to Figures 8 and 9 for typical INL and DNL per- formance curves. The outputs can also be used differentially. Refer to the section “Applying the AD768” for examples of vari- ous output configurations. DIGITAL INPUT CODE – k 8 –8 065 10 20 30 40 4 –2 –4 –6 6 0 2 50 60 5 152535 4555 Figure 8. Typical INL Performance |
Аналогичный номер детали - AD768_15 |
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Аналогичное описание - AD768_15 |
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