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ADM1024 Datasheet(PDF) 13 Page - Analog Devices |
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ADM1024 Datasheet(HTML) 13 Page - Analog Devices |
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13 / 28 page  ADM1024 –13– REV. 0 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL GND D+ D– GND Figure 15. 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 a major problem as 1 °C corresponds to about 240 µV, and thermocouple voltages are about 3 µV/oC 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 0.1 µF bypass and 2200 pF input filter capacitors close to the ADM1024. 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 feet 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 ADM1024. Leave the remote end of the shield unconnected 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. One Ω series resis- tance introduces about 0.5 °C error. LIMIT VALUES Limit values for analog measurements are stored in the appro- priate limit registers. In the case of voltage measurements, high and low limits can be stored so that an interrupt request will be generated if the measured value goes above or below acceptable values. In the case of temperature, a Hot Temperature or High Limit can be programmed, and a Hot Temperature Hyster- esis or Low Limit, which will usually be some degrees lower. This can be useful as it allows the system to be shut down when the hot limit is exceeded, and restarted automatically when it has cooled down to a safe temperature. MONITORING CYCLE TIME The monitoring cycle begins when a one is written to the Start Bit (Bit 0), and a zero to the INT_Clear Bit (Bit 3) of the Configuration Register. INT_Enable (Bit 1) should be set to one to enable the INT output. The ADC measures each analog input in turn, as each measurement is completed the result is automatically stored in the appropriate value register. This “round- robin” monitoring cycle continues until it is disabled by writing a 0 to Bit 0 of the Configuration Register. As the ADC will normally be left to free-run in this manner, the time taken to monitor all the analog inputs will normally not be of interest, as the most recently measured value of any input can be read out at any time. For applications where the monitoring cycle time is important, it can be calculated as follows: m × t1 × n × t2 where: m is the number of inputs configured as analog inputs, plus the internal VCC measurement and internal temperature sensor. t1 is the time taken for an analog input conversion, nominally 755 µs. n is the number of inputs configured as external temperature inputs. t2 is the time taken for a temperature conversion, nominally 33.24 ms. This rapid sampling of the analog inputs ensures a quick response in the event of any input going out of limits, unlike other moni- toring chips that employ slower ADCs. FAN MONITORING CYCLE TIME When a monitoring cycle is started, monitoring of the fan speed inputs begins at the same time as monitoring of the analog inputs. However, the two monitoring cycles are not synchronized in any way. The monitoring cycle time for the fan inputs is dependent on fan speed and is much slower than for the analog inputs. For more details see Fan Speed Measurement section. INPUT SAFETY Scaling of the analog inputs is performed on chip, so external attenuators are normally not required. However, since the power supply voltages will appear directly at the pins, its is advisable to add small external resistors in series with the supply traces to the chip to prevent damaging the traces or power supplies should a accidental short such as a probe connect two power supplies together. As the resistors will form part of the input attenuators, they will affect the accuracy of the analog measurement if their value is too high. The analog input channels are calibrated assuming an external series resistor of 500 Ω, and the accuracy will remain within specification for any value from zero to 1 k Ω, so a stan- dard 510 Ω resistor is suitable. The worst such accident would be connecting –12 V to +12 V— a total of 24 V difference, with the series resistors this would draw a maximum current of approximately 24 mA. ANALOG OUTPUT The ADM1024 has a single analog output from a unsigned 8-bit DAC which produces 0 V–2.5 V. The analog output register defaults to FF during power-on reset, which produces maximum fan speed. The analog output may be amplified and buffered with external circuitry such as an op amp and transistor to provide fan speed control. |
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