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ADM1025ARQ Folha de dados(PDF) 11 Page - Analog Devices |
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ADM1025ARQ Folha de dados(HTML) 11 Page - Analog Devices |
11 / 16 page REV. A ADM1025/ADM1025A –11– 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 µV. 5. Place 0.1 µF bypass and 1 nF input filter capacitors close to the ADM1025/ADM1025A. 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 ADM1025/ADM1025A. 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. 1 Ω series resistance introduces about 0.5 °C error. LIMIT VALUES High and low limit values for each measurement channel are stored in the appropriate limit registers. As each channel is measured, the measured value is stored and compared with the programmed limit. STATUS REGISTERS The results of limit comparisons are stored in Status Registers 1 and 2. The Status Register bit for a particular measurement channel reflects the status of the last measurement and limit comparison on that channel. If a measurement is within limits the corresponding Status Register bit will be cleared to “0.” If the measurement is out of limits the corresponding status regis- ter bit will be set to “1.” The state of the various measurement channels may be polled by reading the Status Registers over the serial bus. Reading the Status Registers does not affect their contents. Out-of-limit temperature/voltage events may also be used to generate an interrupt, so that remedial action such as turning on a cooling fan may be taken immediately. This is described in the section on RST and INT. MONITORING CYCLE TIME The monitoring cycle begins when a one is written to the Start Bit (Bit 0) of the Configuration Register. The ADC measures each analog input in turn and as each measurement is com- pleted 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. 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 an accidental short such as a probe connect two power sup- plies 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 0 V to 12 V—a total of 12 V difference, with the series resistors this would draw a maximum current of approximately 12 mA. LAYOUT AND GROUNDING Analog inputs will provide best accuracy when referred to a clean ground. A separate, low impedance ground plane for analog ground, which provides a ground point for the voltage dividers and analog components, will provide best performance but is not mandatory. The power supply bypass, the parallel combination of 10 µF (electrolytic or tantalum) and 0.1 µF (ceramic) bypass capacitors connected between Pin 9 and ground, should also be located as close as possible to the ADM1025/ADM1025A. RST/INT OUTPUT As previously mentioned, Pin 16 is a multifunction pin. Its state after power-on is latched to set the lowest two bits of the serial bus address. During NAND tree board-level connectivity testing it functions as the output of the NAND tree. It may also be used as a reset output, or as an interrupt output for out-of-limit tem- perature/voltage events. Pin 16 is programmed as a reset output by clearing bit 0 of the Test Register and setting Bit 7 of the VID Register. A low going, 20 ms, reset output pulse can then be generated by setting Bit 4 of the Configuration Register. If Bit 7 of the VID Register is cleared, Pin 16 can be programmed as an interrupt output for out-of-limit temperature/voltage events ( INT). Desired interrupt operation is achieved by changing the values of Bits 1 and 0 of the Test Register as shown in Table IV. Note, however, that Bits 2 to 7 of the Test Register must be zeros (not don’t cares). If, for example, INT is programmed for thermal and voltage interrupts, then if any temperature or volt- age measurement goes outside its respective high or low limit, the INT output will go low. It will remain low until Status Reg- ister 1 is read, when it will be cleared. If the temperature or voltage remains out of limit, INT will be reasserted on the next monitoring cycle. INT can also be cleared by issuing an Alert Response Address Call. |
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