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ADT7518 Datasheet(PDF) 25 Page - Analog Devices |
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ADT7518 Datasheet(HTML) 25 Page - Analog Devices |
25 / 40 page ADT7518 Rev. A | Page 25 of 40 5. Place 0.1 µF bypass and 2,200 pF input filter capacitors close to the ADT7518. 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 long distances (up to 100 feet), use shielded twisted- pair cable, such as Belden #8451 microphone cable. Connect the twisted pair to D+ and D– and the shield to GND close to the ADT7518. 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. Series resistance of 1 Ω introduces about 0.5°C error. Temperature Value Format One LSB of the ADC corresponds to 0.25°C. The ADC can theoretically measure a temperature span of 255°C. The internal temperature sensor is guaranteed to a low value limit of –40°C. It is possible to measure the full temperature span using the external temperature sensor. The temperature data format is shown in Table 9. The result of the internal or external temperature measure- ments is stored in the temperature value registers, and is com- pared with limits programmed into the internal or external high and low registers. Table 9. Temperature Data Format (Internal and External Temperature) Temperature Digital Output –40°C 11 0110 0000 –25°C 11 1001 1100 –10°C 11 1101 1000 –0.25°C 11 1111 1111 0°C 00 0000 0000 +0.25°C 00 0000 0001 +10°C 00 0010 1000 +25°C 00 0110 0100 +50°C 00 1100 1000 +75°C 01 0010 1100 +100°C 01 1001 0000 +105°C 01 1010 0100 +125°C 01 1111 0100 Temperature Conversion Formula: Positive Temperature = ADC Code/4 Negative Temperature = (ADC Code* – 512)/4 * where DB9 is removed from the ADC code. Interrupts The measured results from the internal temperature sensor, external temperature sensor, VDD pin, and AIN inputs are compared with the THIGH/VHIGH (greater than comparison) and TLOW/VLOW (less than or equal to comparison) limits. An inter- rupt occurs if the measurement exceeds or equals the limit registers. These limits are stored in on-chip registers. Note that the limit registers are 8 bits long while the conversion results are 10 bits long. If the limits are not masked, any out-of-limit com- parisons generate flags that are stored in the Interrupt Status 1 register (Address 00h) and Interrupt Status 2 register (Address 01h). One or more out-of-limit results will cause the INT/INT output to pull either high or low depending on the output polarity setting. It is good design practice to mask out interrupts for channels that are of no concern to the application. Figure 49 shows the interrupt structure for the ADT7518. It gives a block diagram representation of how the various measurement channels affect the INT/INT pin. ADT7518 REGISTERS The ADT7518 contains registers that are used to store the results of external and internal temperature measurements, VDD value measurements, analog input measurements, high and low temperature limits, supply voltage and analog input limits, to set output DAC voltage levels, to configure multipurpose pins, and generally to control the device. A description of these registers follows. The register map is divided into registers of 8 bits. Each register has its own individual address, but some consist of data that is linked to other registers. These registers hold the 10-bit conver- sion results of measurements taken on the temperature, VDD, and AIN channels. For example, the eight MSBs of the VDD measurement are stored in Register Address 06h, while the two LSBs are stored in Register Address 03h. These types of registers are linked such that when the LSB register is read first, the MSB registers associated with that LSB register are locked to prevent any updates. To unlock these MSB registers, the user has only to read any one of them, which will have the effect of unlocking all previously locked MSB registers. So, for the preceding example, if Register 03h was read first, MSB Registers 06h and 07h would be locked to prevent any updates to them. If Register 06h were read, this register and Register 07h would be subsequently unlocked. LOCK ASSOCIATED MSB REGISTERS FIRST READ COMMAND LSB REGISTER OUTPUT DATA Figure 51. Phase 1 of 10-Bit Read |
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