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ADT7483AARQZ-RL Datasheet(PDF) 7 Page - ON Semiconductor |
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ADT7483AARQZ-RL Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 20 page ADT7483A http://onsemi.com 7 Theory of Operation The ADT7483A is a local and 2 remote temperature sensor and over/under temperature alarm. When the ADT7483A is operating normally, the on-board ADC operates in a freerunning mode. The analog input multiplexer alternately selects either the on-chip temperature sensor or one of the remote temperature sensors to measure its local temperature. 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. Likewise, 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 13 between ALERT and THERM2 Selecting the Conversion Rate Temperature Measurement Method A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, measuring the baseemitter voltage (VBE) of a transistor, operated at constant current. Unfortunately, this technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADT7483A is to measure the change in VBE when the device is operated at two different currents. Figure 14 shows the input signal conditioning used to measure the output of a remote temperature sensor. This figure shows the remote sensor as a substrate transistor, but it could equally be a discrete transistor. If a discrete transistor is used, the collector is not grounded and should be linked to the base. To prevent ground noise 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. C1 can be optionally added as a noise filter (recommended maximum value 1,000 pF). To measure DVBE, the operating current through the sensor is switched among two related currents, I and N I. The currents through the temperature diode are switched between I and N I, giving DVBE. The temperature is then calculated using the DVBE measurement. The resulting DVBE waveforms pass 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 DVBE. The ADC digitizes this voltage and produces a temperature measurement. 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 local temperature sensor is performed in the same manner. Figure 14. Input Signal Conditioning LOW-PASS FILTER fC = 65 kHz REMOTE SENSING TRANSISTOR BIAS DIODE D+ D− VDD IBIAS IN I VOUT+ VOUT− To ADC C1* *CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1,000 pF MAX Temperature Measurement Results The results of the local and remote temperature measurements are stored in the local and remote temperature value registers and are compared with limits programmed into the local and remote high and low limit registers. The local temperature measurement is an 8-bit measurement with 1C resolution. The remote temperature measurements are 10-bit measurements, with eight MSBs stored in one register and two LSBs stored in another register. Table 6 lists the temperature measurement registers. |
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