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DAC16FP Datasheet(PDF) 7 Page - Analog Devices |
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DAC16FP Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page DAC16 REV. B –7– system, such as the DAC16, it is recommended that common ground tie points be provided at each such device. If only one system ground can be connected directly to the DAC16, it is recommended that the analog common be used. If the system’s AGND has suitable low impedance, then the digital signal cur- rents flowing in it should not seriously affect the ground noise. The amount of digital noise introduced by connecting the two grounds together at the device will not adversely affect system performance due to loss of digital noise immunity. Generous bypassing of the DAC’s supplies goes a long way in reducing supply-line induced errors. Even with well-filtered, well-regulated supplies, local bypassing consisting of 10 µF tan- talum electrolytic shunted by a 0.1 µF ceramic is recommended. The decoupling capacitors should be connected between the DAC’s supply pins (Pin 3 for +5 V, Pin 20 for –15 V) and the analog ground (Pin 22). Figure 19 shows how the DGND, AGND, and bypass connections should be made to the DAC16. AGND DGND VEE VCC DB0 – DB15 +5V 0.1 F 10 F 0.1 F 10 F DAC16 TO OTHER ANALOG CIRCUITS TO POWER GROUND –15V IOUT Figure 19. Recommended Grounding and Bypassing Scheme for the DAC16 Using the Right Capacitors Probably the most important external components associated with high speed design are the capacitors used to bypass the power supplies and to provide compensation. Both selection and placement of these capacitors can be critical and, to a large extent, dependent upon the specifics of the system configura- tion. The dominant consideration in selection of bypass and compensation capacitors for the DAC16 is minimization of se- ries resistance and inductance. Many capacitors begin to look inductive at 20 MHz and above—the very frequencies where re- jection of interference is needed. Ceramic and film-type capaci- tors generally feature lower series inductance than tantalum or electrolytic types. A few general rules are of universal use when approaching the issue of compensation or bypassing. Bypass capacitors should be installed on the printed circuit board with the shortest possible leads consistent with reliable construction. This helps to minimize series inductance in the leads. Chip capacitors are optimal in this respect. Where illus- trated in the applications section, large tantalum electrolytic capacitors are shunted by low self-inductance ceramic capaci- tors. This technique reduces the self-resonance of the electrolytic while shifting the resonant frequency of the ceramics out-of-band. Some series inductance between the DAC supply pins and the power supply plane often helps to filter out high frequency power supply noise. This inductance can be generated using a small ferrite bead as shown in Figure 20. +5V 10 F VCC DAC16 AGND 0.1 F FB VEE –15V 10 F 0.1 F FB Figure 20. Using a Ferrite Bead as a High Frequency Filter Reference Amplifier Considerations The reference input current buffer is a high performance ampli- fier optimized for high accuracy and linearity. The design of the reference amplifier ensures fast settling times by tightly control- ling the node common to all the current sources internal to the DAC with an external compensation capacitor (CCOMP). Since the primary design goal of the DAC16 is to achieve 16-bit per- formance, proper operation of the reference amplifier requires a 47 µF tantalum electrolytic capacitor shunted by a 0.1 µF ceramic capacitor, as shown in Figure 21. Increasing the capaci- tance at this node above the recommended values does not fur- ther reduce the analog transition current noise spikes at the output of the reference amplifier. Reducing the value of com- pensation, however, is not recommended as DAC linearity will degrade as a result. In most systems, the VEE supply offers suffi- ciently low impedance to maintain a quiet return point for the reference amplifier. If this is not the case, the AGND point can also be used for the compensation capacitor return, as shown in Figure 21. AGND VEE CCOMP 0.1 F 47 F DAC16 –15V IOUT Figure 21a. Recommended Compensation Scheme to VEE AGND CCOMP 0.1 F 47 F DAC16 IOUT Figure 21b. Recommended Compensation Scheme to AGND In applications where 16-bit multiplying performance is required, the DAC16 might appear to be a viable solution. However, the compensation capacitor network would have to be removed in these applications. The DAC16’s reference am- plifier was specifically designed for low frequency operation, with a compensation capacitor network. In fact, this network serves not only as a charge reservoir for the DAC’s internal current sources but also as a wideband noise filter for the |
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