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ADM1020 Fiches technique(PDF) 11 Page - Analog Devices |
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ADM1020 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 12 page REV. 0 ADM1020 –11– LAYOUT CONSIDERATIONS Digital boards can be electrically noisy environments, and the ADM1020 is measuring very small voltages from the remote sensor, so care must be taken to minimize noise induced at the sensor inputs. The following precautions should be taken: 1. Place the ADM1020 as close as possible to the remote sens- ing diode. Provided that the worst noise sources such as clock generators, data/address buses and CRTs are avoided, this distance can be 4 to 8 inches. 2. Route the D+ and D– tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under the tracks if possible. 3. Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and spacing is recommended. 10mil GND D+ D– GND 10mil 10mil 10mil 10mil 10mil 10mil Figure 14. Arrangement of Signal Tracks 4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D+ and D– path and at the same temperature. Thermocouple effects should not be should not be a major problem as 1 °C corresponds to about 200 mV, and thermo- couple voltages are about 3 mV/ °C of temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 mV. 5. Place a 0.1 µF bypass capacitor close to the V DD pin and 2200 pF input filter capacitors across D+, D– close to the ADM1020. 6. If the distance to the remote sensor is more than 8 inches, the use of twisted pair cable is recommended. This will work up to about 6 to 12 feet. 7. For really long distances (up to 100 feet) use shielded twisted pair such as Belden #8451 microphone cable. Connect the twisted pair to D+ and D– and the shield to GND close to the ADM1020. Leave the remote end of the shield uncon- nected to avoid ground loops. Because the measurement technique uses switched current sources, excessive cable and/or filter capacitance can affect the measurement. When using long cables, the filter capacitor may be reduced or removed. Cable resistance can also introduce errors. 1 Ω series resistance introduces about 0.5 °C error. APPLICATION CIRCUITS Figure 15 shows a typical application circuit for the ADM1020, using a discrete sensor transistor connected via a shielded, twisted pair cable. The pull-ups on SCLK, SDATA and ALERT are required only if they are not already provided elsewhere in the system. SCLK SDATA ALERT ADD GND D+ D– 0.1 F 10k +3.3V SET TO REQUIRED ADDRESS C1* SHIELD 2N3904 *C1 IS OPTIONAL ADM1020 VDD 10k 10k TO PIIX4 CHIP IN OUT I/O Figure 15. Typical ADM1020 Application Circuit |
Numéro de pièce similaire - ADM1020 |
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Description similaire - ADM1020 |
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