Datasheet ADT7461A (ON Semiconductor) - 7

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ADT7461A Differences between the ADT7461A and the ADT7461 Series Resistance Cancellation
Although the ADT7461A is pin- and register-compatible Parasitic resistance to the D+ and D− inputs to the with the ADT7461, there are some specification differences ADT7461A, seen in series with the remote diode, is caused between the two devices. A summary of these differences is by a variety of factors, including PCB track resistance and shown below in Table 6. track length. This series resistance appears as a temperature offset in the remote sensor’s temperature measurement. This
Table 6. DIFFERENCES BETWEEN THE ADT7461A
error typically causes a 0.5°C offset per ohm of parasitic
AND THE ADT7461
resistance in series with the remote diode.
Specification ADT7461A ADT7461 Unit
The ADT7461A automatically cancels the effect of this Supply Voltage 3.0 to 3.6 3.0 to 5.5 V series resistance on the temperature reading, giving a more accurate result, without the need for user characterization of Maximum Local Sensor 1.0 3.0 °C Accuracy this resistance. The ADT7461A is designed to automatically cancel typically up to 1.5 kW of resistance. By using an Maximum Series 1.5 3.0 kW Resistance Cancellation advanced temperature measurement method, this process is transparent to the user. This feature permits resistances to be Average Operating mA Supply Current added to the sensor path to produce a filter, allowing the part 16 Conversions/Sec 240 170 to be used in noisy environments. See the section on Noise Standby Mode 5.0 5.5 Filtering for more details. Max Conversion Time ms One Shot, Averaging On 52 114.6
Temperature Measurement Method
One Shot, Averaging Off 8.0 12.56 A simple method of measuring temperature is to exploit Remote Sensor Current mA the negative temperature coefficient of a diode, measuring Levels the base emitter voltage (V High 220 96 BE) of a transistor operated at Mid 82 36 constant current. However, this technique requires Low 13.5 6.0 calibration to null the effect of the absolute value of VBE, which varies from device to device.
Theory of Operation
The technique used in the ADT7461A measures the The ADT7461A is a local and remote temperature sensor change in VBE when the device operates at three different and over/under temperature alarm, with the added ability to currents. Previous devices used only two operating currents, automatically cancel the effect of 1.5 kW (typical) of but it is the use of a third current that allows automatic resistance in series with the temperature monitoring diode. cancellation of resistances in series with the external When the ADT7461A is operating normally, the on-board temperature sensor. ADC operates in a free running mode. The analog input Figure 14 shows the input signal conditioning used to multiplexer alternately selects either the on-chip measure the output of an external temperature sensor. This temperature sensor to measure its local temperature or the figure shows the external sensor as a substrate transistor, but remote temperature sensor. The ADC digitizes these signals it can equally be a discrete transistor. If a discrete transistor and the results are stored in the local and remote temperature is used, the collector is not grounded but is linked to the base. value registers. To prevent ground noise interfering with the measurement, The local and remote measurement results are compared the more negative terminal of the sensor is not referenced to with the corresponding high, low, and THERM temperature ground, but is biased above ground by an internal diode at limits, stored in eight on-chip registers. Out-of-limit the D− input. C1 may be added as a noise filter comparisons generate flags that are stored in the status (a recommended maximum value of 1000 pF). However, a register. A result that exceeds the high temperature limit or better option in noisy environments is to add a filter, as the low temperature limit causes the ALERT output to described in the Noise Filtering section. See the Layout assert. The ALERT output also asserts if an external diode Considerations section for more information on C1. fault is detected. Exceeding the THERM temperature limits To measure DVBE, the operating current through the causes the THERM output to assert low. The ALERT output sensor is switched among three related currents. As shown can be reprogrammed as a second THERM output. in Figure 14, N1 × I and N2 × I are different multiples of the The limit registers are programmed and the device current, I. The currents through the temperature diode are controlled and configured via the serial SMBus. The switched between I and N1 × I, giving DVBE1; and then contents of any register are also read back via the SMBus. between I and N2 × I, giving DVBE2. The temperature is Control and configuration functions consist of switching then calculated using the two DVBE measurements. This the device between normal operation and standby mode, method also cancels the effect of any series resistance on the selecting the temperature measurement range, masking or temperature measurement. enabling the ALERT output, switching Pin 6 between ALERT and THERM2, and selecting the conversion rate.
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