Datasheet ADT7461 (ON Semiconductor) - 7

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ADT7461 Functional Description Temperature Measurement Method
The ADT7461 is a local and remote temperature sensor A simple method of measuring temperature is to exploit and over/under temperature alarm, with the added ability to the negative temperature coefficient of a diode by measuring automatically cancel the effect of 3 kW (typical) of the base-emitter voltage (VBE) of a transistor operated at resistance in series with the temperature monitoring diode. constant current. However, this technique requires When the ADT7461 is operating normally, the on-board calibration to null out the effect of the absolute value of VBE, ADC operates in a free-running mode. The analog input which varies from device to device. multiplexer alternately selects either the on-chip The technique used in the ADT7461 is to measure the temperature sensor to measure its local temperature or the change in VBE when the device is operated at three different remote temperature sensor. The ADC digitizes these signals currents. Previous devices have used only two operating and the results are stored in the local and remote temperature currents, but it is the use of a third current that allows value registers. automatic cancellation of resistances in series with the The local and remote measurement results are compared external temperature sensor. with the corresponding high, low, and THERM temperature Figure 15 shows the input signal conditioning used to limits, stored in eight on-chip registers. Out-of-limit measure the output of an external temperature sensor. This comparisons generate flags that are stored in the status register. figure shows the external sensor as a substrate transistor, but A result that exceeds the high temperature limit, the low it could equally be a discrete transistor. If a discrete temperature limit, or an external diode fault causes the ALERT transistor is used, the collector will not be grounded and output to assert low. Exceeding THERM temperature limits should be linked to the base. To prevent ground noise causes the THERM output to assert low. The ALERT output interfering with the measurement, the more negative can be reprogrammed as a second THERM output. terminal of the sensor is not referenced to ground, but is The limit registers can be programmed and the device biased above ground by an internal diode at the D− input. C1 controlled and configured via the serial SMBus. The may be added as a noise filter (a recommended maximum contents of any register can also be read back via the SMBus. value of 1,000 pF). However, a better option in noisy Control and configuration functions consist of switching environments is to add a filter, as described in the Noise the device between normal operation and standby mode, Filtering section. See the Layout Considerations section for selecting the temperature measurement scale, masking or more information on C1. enabling the ALERT output, switching Pin 6 between To measure DVBE, the operating current through the ALERT and THERM2, and selecting the conversion rate. sensor is switched among three related currents. Figure 15 shows N1 × I and N2 × I as different multiples of the
Series Resistance Cancellation
current, I. The currents through the temperature diode are Parasitic resistance to the D+ and D− inputs to the switched between I and N1 × I, giving DV ADT7461, seen in series with the remote diode, is caused by BE1, and then between I and N2 × I, giving DV a variety of factors, including PCB track resistance and track BE2. The temperature may then be calculated using the two DV length. This series resistance appears as a temperature offset BE measurements. This method can also be shown to cancel the effect of any series in the remote sensor’s temperature measurement. This error resistance on the temperature measurement. typically causes a 0.5°C offset per ohm of parasitic resistance The resulting DV in series with the remote diode. BE waveforms are passed through a 65 kHz low-pass filter to remove noise and then to a The ADT7461 automatically cancels out the effect of this chopper-stabilized amplifier. This amplifies and rectifies the series resistance on the temperature reading, giving a more waveform to produce a dc voltage proportional to DV accurate result, without the need for user characterization of BE. The ADC digitizes this voltage and a temperature measurement is this resistance. The ADT7461 is designed to automatically produced. To reduce the effects of noise, digital filtering is cancel typically up to 3 kW of resistance. By using an performed by averaging the results of 16 measurement cycles advanced temperature measurement method, this is for low conversion rates. At rates of 16, 32, and 64 transparent to the user. This feature allows resistances to be conversions per second, no digital averaging takes place. added to the sensor path to produce a filter, allowing the part Signal conditioning and measurement of the internal to be used in noisy environments. See the Noise Filtering temperature sensor is performed in the same manner. section for more details.
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