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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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ADS7811U Arkusz danych(PDF) 9 Page - Texas Instruments |
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ADS7811U Arkusz danych(HTML) 9 Page - Texas Instruments |
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9 / 15 page  ADS7811 9 SBOS041A www.ti.com LAYOUT The layout of the ADS7811 and accompanying components will be critical for optimum performance. Use of an analog ground plane is essential. Use of +5V and –5V power planes is not critical as long as the supplies are well bypassed, and the traces connecting +5V and –5V to the power connector are not too long or too thin. The two +VS power pins of the ADS7811 must be tied together. The voltage source for these pins should also power the input buffer and the 74HC00 shown in Figure 1. This supply should separate from the positive +5V supply for the system’s digital logic Three ground pins are present on the ADS7811: pin 2, pin 5, and pin 14. These should all be tied to the analog ground plane. The analog ground plane should extend underneath all analog signal conditioning components and up to the 74HC574s (or equivalent components) shown in Figure 1. The 74HC574s should not be located more than several inches from the ADS7811. The ground for the 74HC574s should be connected to the digital ground. The analog ground plane should extend up to the 74HC574s but should be kept at least 1/4" (6mm) distant from the digital ground plane (if present). The analog and digital grounds planes should not overlap at any point. INTERMEDIATE LATCHES The 74HC574s shown in Figure 1 isolate the ADS7811 from digital signals on a microprocessor, digital signal processor (DSP), or microcontroller bus. This is necessary because of the precision needed within the ADS7811. The weight of a single LSB in the ADS7811 is 76 µV, and the comparator must be able to resolve differences in voltage to this level. External digital signals which transition during the conver- sion can easily couple onto the substrate and produce volt- ages larger than this. In place of the 74HC574s, it might be possible to use a FIFO or similar type of memory device. For many systems, it may be difficult to go directly from the ADS7811 into a micro- controller or DSP even if the ADS7811 is not connected to shared bus. The reason for this is that the outputs are active only during the acquisition period. SIGNAL CONDITIONING The ADS7811 input essentially consists of a switch and a capacitor. In the acquisition or sample mode, the switch is closed and the input signal drives the capacitor directly. When a conversion is started, the switch is opened capturing the input signal at that moment. This voltage is held on the capacitor for the remainder of the conversion. While this provides for a wide bandwidth sample and hold function and results in excellent AC performance, this archi- tecture requires a high bandwidth, precision op amp to drive the analog input. The op amp and configuration shown in Figure 1 is highly recommended. The amplifier should be placed within 1 to 2 inches (25 to 50mm) of the ADS7811, and the layout guidelines in the OPA628 data sheet should be strictly followed. |
Podobny numer części - ADS7811U |
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Podobny opis - ADS7811U |
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