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AD667JN bảng dữ liệu(PDF) 6 Page - Analog Devices |
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6 / 8 page AD667 REV. A –6– Small resistors may be added to the feedback resistors in order to accomplish small modifications in the scaling. For example, if a 10.24 V full scale is desired, a 140 Ω 1% low TC metal-film resistor can be added in series with the internal (nominal) 5k feedback resistor, and the gain trim potentiometer (between Pins 6 and 7) should be increased to 200 Ω. In the bipolar mode, increase the value of the bipolar offset trim potentiometer also to 200 Ω. GROUNDING RULES The AD667 brings out separate analog and power grounds to allow optimum connections for low noise and high speed perfor- mance. These grounds should be tied together at one point, usually the device power ground. The separate ground returns are provided to minimize current flow in low level signal paths. The analog ground at Pin 5 is the ground point for the output amplifier and is thus the “high quality” ground for the AD667; it should be connected directly to the analog reference point of the system. The power ground at Pin 16 can be connected to the most convenient ground point; analog power return is preferred. If power ground contains high frequency noise be- yond 200 mV, this noise may feed through the converter, thus some caution will be required in applying these grounds. It is also important to apply decoupling capacitors properly on the power supplies for the AD667 and the output amplifier. The correct method for decoupling is to connect a capacitor from each power supply pin of the AD667 to the analog ground pin of the AD667. Any load driven by the output amplifier should also be referred to the analog ground pin. OPTIMIZING SETTLING TIME The dynamic performance of the AD667’s output amplifier can be optimized by adding a small (20 pF) capacitor across the feedback resistor. Figure 4 shows the improvement in both large-signal and small-signal settling for the 10 V range. In Fig- ure 4a, the top trace shows the data inputs (DB11–DB0 tied to- gether), the second trace shows the CS pulse (A3–A0 tied low), and the lower two traces show the analog outputs for CF = 0 pF and 20 pF respectively. Figures 4b and 4c show the settling time for the transition from all bits on to all bits off. Note that the settling time to ±1/2 LSB for the 10 V step is improved from 2.4 microseconds to 1.6 mi- croseconds by the addition of the 20 pF capacitor. Figures 4d and 4e show the settling time for the transition from all bits off to all bits on. The improvement in settling time gained by adding CC = 20 pF is similar. a. Large Scale Settling b. Fine-Scale Settling, CF = 0 pF c. Fine-Scale Settling, CF = 20 pF d. Fine-Scale Settling, CF = 0 pF e. Fine-Scale Settling, CF = 20 pF Figure 4. Settling Time Performance DIGITAL CIRCUIT DETAILS The bus interface logic of the AD667 consists of four indepen- dently addressable registers in two ranks. The first rank consists of three four-bit registers which can be loaded directly from a 4-, 8-, 12-, or 16-bit microprocessor bus. Once the complete 12-bit data word has been assembled in the first rank, it can be loaded into the 12-bit register of the second rank. This double-buffered organization avoids the generation of spurious analog output values. Figure 5 shows the block diagram of the AD667 logic section. The latches are controlled by the address inputs, A0–A3, and the CS input. All control inputs are active low, consistent with general practice in microprocessor systems. The four address lines each enable one of the four latches, as indicated in Table II. All latches in the AD667 are level-triggered. This means that data present during the time when the control signals are valid will enter the latch. When any one of the control signals returns high, the data is latched. |
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