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AD7740YRTZ-REEL7 데이터시트(PDF) 10 Page - Analog Devices |
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AD7740YRTZ-REEL7 데이터시트(HTML) 10 Page - Analog Devices |
10 / 11 page –10– Temperature Sensor Application The AD7740 can be used with an AD22100S temperature sensor to give a digital measure of ambient temperature. The output voltage of the AD22100S is proportional to the tempera- ture times the supply voltage. It uses a single 5 V supply, and its output swings from 0.25 V at –50 °C to 4.75 V at +150°C. By feeding its output through the AD7740, the value of ambient temperature is converted into a digital pulse train. See Figure 12. REFIN AD7740 AD22100S V+ CLKIN CLKOUT C1 C2 FOUT GND BUF 0.1 F 10 F 0.1 F VIN VDD 5V 10 F Figure 12. Using the AD7740 with a Temperature Sensor Due to its ratiometric nature this application provides an extremely cost-effective solution. The need for an external pre- cision reference is eliminated since the 5 V power-supply is used as a reference to both the VFC and the AD22100S. 32 kHz Operation The AD7740 oscillator circuit will not operate at 32 kHz. If the user wishes to use a 32 kHz watch crystal, some additional external circuitry is required. The circuit in Figure 13 is for a crystal with a required drive of 1 µW. Resistors R1 and R2 reduce the power to this level. R3 1M CLKIN R2 100k R1 220k 32kHz 40106 40106 Figure 13. 32 kHz Watch Crystal Circuit Power Supply Bypassing and Grounding In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. The printed circuit board housing the AD7740 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. To minimize capacitive coupling between them, digital and analog ground planes should only be joined in one place, close to the AD7740, and should not overlap. Avoid running digital lines under the device, as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7740 to avoid noise coupling. The power supply lines to the AD7740 should use as large a trace as pos- sible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other parts of the board, and clock signals should never be run near analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This reduces the effect of feedthrough through the board. A microstrip technique is by far the best but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to the ground plane while the signal traces are placed on the solder side. Good decoupling is also important. All analog supplies should be decoupled to GND with surface mount capacitors, 10 µF in parallel with 0.1 µF located as close to the package as possible, ideally right up against the device. The lead lengths on the by- pass capacitor should be as short as possible. It is essential that these capacitors be placed physically close to the AD7740 to minimize the inductance of the PCB trace between the capacitor and the supply pin. The 10 µF are the tantalum bead type and are located in the vicinity of the VFC to reduce low-frequency ripple. The 0.1 µF capacitors should have low Effective Series Resistance (ESR) and Effective Series Inductance (ESI), such as the common ceramic types, which provide a low imped- ance path to ground at high frequencies to handle transient currents due to internal logic switching. Additionally, it is ben- eficial to have large capacitors (> 47 µF) located at the point where the power connects to the PCB. AD7740 REV. A |
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