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AD779SD 数据表(PDF) 10 Page - Analog Devices |
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AD779SD 数据表(HTML) 10 Page - Analog Devices |
10 / 12 page AD779 –10– REV. B An effort should be made to minimize the trace length between the capacitor leads and the respective converter power supply and common pins. The circuit layout should attempt to locate the AD779, associated analog input circuitry and interconnec- tions as far as possible from logic circuitry. A solid analog ground plane around the AD779 will isolate large switching ground currents. For these reasons, the use of wire wrap circuit construction is not recommended; careful printed circuit con- struction is preferred. GROUNDING If a single AD779 is used with separate analog and digital ground planes, connect the analog ground plane to AGND and the digital ground plane to DGND keeping lead lengths as short as possible. Then connect AGND and DGND together at the AD779. If multiple AD779s are used or the AD779 shares analog supplies with other components, connect the analog and digital returns together once at the power supplies rather than at each chip. This prevents large ground loops which inductively couple noise and allow digital currents to flow through the analog system. USE OF EXTERNAL VOLTAGE REFERENCE The AD779 features an on-chip voltage reference. For improved gain accuracy over temperature, a high performance external voltage reference may be used in place of the on-chip reference. The AD586 and AD588 are popular references appropriate for use with high resolution converters. The AD586 is a low cost reference which utilizes a buried Zener architecture to provide low noise and drift. The AD588 is a higher performance refer- ence which uses a proprietary ion-implanted buried Zener diode in conjunction with laser-trimmed thin-film resistors for low off- set and low drift. Figure 7 shows the use of the AD586 with the AD779 in a bipo- lar input mode. Over the 0 °C to +70°C range, the AD586 L-grade exhibits less than a 2.25 mV output change from its initial value at 25 °C. REF IN, (Pin 9) scales its input by a factor of two; thus, this change becomes effectively 4.5 mV. When applied to the AD779, this results in a total gain drift of 0.09% FSR which is an improvement over the on-chip reference performance of 0.11% FSR. A noise-reduction capacitor, CN, has been shown. This capacitor reduces the broadband noise of the AD586 out- put, thereby optimizing the overall ac and dc performance of the AD779. Figure 7. Bipolar Input with Gain and Offset Trims Figure 8 shows the AD779 in unipolar input mode with the AD588 reference. The AD588 output is accurate to 0.65 mV from its value at 25 °C over the 0°C to 70°C range. This results in a 0.06% FSR total gain drift for the AD779, which is a sub- stantial improvement over the on-chip reference performance of 0.11% FSR. A noise-reduction network on Pins 4, 6 and 7 has been shown. The 1 µF capacitors form low pass filters with the internal resistance of the AD588 Zener and amplifier cells and external resistance. This reduces the high frequency noise of the AD588, providing optimum ac and dc performance of the AD779. Figure 8. Unipolar Input with Gain and Offset Trims INTERFACING THE AD779 TO MICROPROCESSORS The I/O capabilities of the AD779 allow direct interfacing to general purpose and DSP microprocessor buses. The asynchro- nous conversion control feature allows complete flexibility and control with minimal external hardware. The following examples illustrate typical AD779 interface configurations. AD779 TO TMS320C25 In Figure 9 the AD779 is mapped into the TMS320C25 I/O space. AD779 conversions are initiated by issuing an OUT instruction to Port 1. EOC status and the conversion result are read in with an IN instruction to Port 1. A single wait state is inserted by generating the processor READY input from IS, Port 1 and MSC. This configuration supports processor clock speeds of 20 MHz and is capable of supporting processor clock speeds of 40 MHz if a NOP instruction follows each AD779 read instruction. Figure 9. AD779 to TMS320C25 Interface |
类似零件编号 - AD779SD |
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类似说明 - AD779SD |
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