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ADT7482 Datasheet(PDF) 9 Page - Analog Devices |
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ADT7482 Datasheet(HTML) 9 Page - Analog Devices |
9 / 24 page ADT7482 Rev. 0 | Page 9 of 24 THEORY OF OPERATION The ADT7482 is a local and 2× remote temperature sensor and overtemperature/undertemperature alarm. When the ADT7482 is operating normally, the on-board ADC operates in a free- running mode. The analog input multiplexer alternately selects either the on-chip temperature sensor to measure its local temperature or either of the remote temperature sensors. The ADC digitizes these signals and the results are stored in the local, Remote 1, and Remote 2 temperature value registers. The local and remote measurement results are compared with the corresponding high, low, and THERM temperature limits, stored in on-chip registers. Out-of-limit comparisons generate flags that are stored in the status register. A result that exceeds the high temperature limit, the low temperature limit, or a remote diode open circuit causes the ALERT output to assert low. Exceeding THERM temperature limits causes the THERM output to assert low. The ALERT output can be reprogrammed as a second THERM output. The limit registers can be programmed, and the device controlled and configured, via the serial SMBus. The contents of any register can also be read back via the SMBus. Control and configuration functions consist of switching the device between normal operation and standby mode, selecting the temperature measurement scale, masking or enabling the ALERT output, switching Pin 8 between ALERT and THERM2, and selecting the conversion rate. SERIES RESISTANCE CANCELLATION Parasitic resistance to the D+ and D− inputs to the ADT7482, seen in series with the remote diode, is caused by a variety of factors, including PCB track resistance and track length. This series resistance appears as a temperature offset in the remote sensor temperature measurement. This error typically causes a 0.5°C offset per ohm of parasitic resistance in series with the remote diode. The ADT7482 automatically cancels out the effect of this series resistance on the temperature reading, providing a more accurate result, without the need for user characterization of this resistance. The ADT7482 is designed to automatically cancel typically up to 1.5 kΩ of resistance. By using an advanced temperature measurement method, this is transparent to the user. This feature allows resistances to be added to the sensor path to produce a filter, allowing the part to be used in noisy environments. See the Noise Filtering section for more details. TEMPERATURE MEASUREMENT METHOD A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, measuring the base-emitter voltage (VBE) of a transistor operated at constant current. However, this technique requires calibration to null out the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADT7482 is to measure the change in VBE when the device is operated at three different currents. Previous devices have used only two operating currents. The use of a third current allows automatic cancellation of resistances in series with the external temperature sensor. Figure 16 shows the input signal conditioning used to measure the output of an external temperature sensor. This figure shows the external sensor as a substrate transistor, but it could equally be a discrete transistor. If a discrete transistor is used, the collec- tor is not grounded and should be linked to the base. To prevent ground noise from interfering with the measurement, the more negative terminal of the sensor is not referenced to ground, but is biased above ground by an internal diode at the D− input. Capacitor C1 can be added as a noise filter (a recommended maximum value of 1000 pF). However, a better option in noisy environments is to add a filter, as described in the Noise Filtering section. See the Layout Considerations section for more information. To measure ΔVBE, the operating current through the sensor is switched among three related currents. Shown in Figure 16, N1 × I and N2 × I are different multiples of the current, I. The currents through the temperature diode are switched between I and N1 × I, giving ΔVBE1, and then between I and N2 × I, giving ΔVBE2. The temperature can then be calculated using the two ΔVBE measurements. This method can also be shown to cancel the effect of any series resistance on the temperature measurement. The resulting ΔVBE waveforms are passed through a 65 kHz low-pass filter to remove noise and then to a chopper-stabilized amplifier. This amplifies and rectifies the waveform to produce a dc voltage proportional to ΔVBE. The ADC digitizes this vol- tage and a temperature measurement is produced. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. At rates of 16, 32, and 64 conversions/second, no digital averaging takes place. Signal conditioning and measurement of the internal tempera- ture sensor are performed in the same manner. |
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