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ADM1025A Datasheet(PDF) 13 Page - ON Semiconductor |
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ADM1025A Datasheet(HTML) 13 Page - ON Semiconductor |
13 / 21 page ![]() Preliminary Technical Data ADM1025/ADM1025A Rev. P5 | Page 13 of 21| www.onsemi.com TEMPERATURE MEASUREMENT SYSTEM INTERNAL TEMPERATURE MEASUREMENT The ADM1025/ADM1025A contains an on-chip band gap temperature sensor whose output is digitized by the on-chip ADC. The temperature data is stored in the Local Temperature Value Register (Address 27h). As both positive and negative temperatures can be measured, the temperature data is stored in twos complement format, as shown in Table 6. Theoretically, the temperature sensor and ADC can measure temperatures from −128°C to +127°C with a resolution of 1°C, although temperatures below 0°C and above +100°C are outside the operating temperature range of the device. EXTERNAL TEMPERATURE MEASUREMENT The ADM1025/ADM1025A can measure temperature using an external diode sensor or diode-connected transistor connected to Pins 9 and 10. The forward voltage of a diode or diode-connected transistor, operated at a constant current, exhibits a negative temperature coefficient of about −2 mV/°C. Unfortunately, the absolute value of VBE, varies from device to device, and individual calibration is required to null this out, so the technique is unsuitable for mass production. The technique used in the ADM1025/ADM1025A is to measure the change in VBE when the device is operated at two different currents. This is given by: ) ( / N In q KT VBE × = Δ where: K is Boltzmann’s constant. q is the charge on the carrier. T is the absolute temperature in Kelvins. N is the ratio of the two currents. Figure 15 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 provided for temperature monitoring on some microprocessors, but it could equally well be a discrete transistor. If a discrete transistor is used, the collector will not be grounded and should be linked to the base. If a PNP transistor is used, the base is connected to the D− input and the emitter to the D+ input. If an NPN transistor is used, the emitter is connected to the D− input and the base to the D+ input. Bit 6 of Status Register 2 (42h) is set if a remote diode fault is detected. The ADM1025/ADM1025A detects shorts from D+ to GND or supply, as well as shorts/opens between D+/D−. Figure 15. Signal Conditioning for External Diode Temperature Sensors Table 6. Temperature Data Format Temperature Digital Output −128°C 1000 0000 −125°C 1000 0011 −100°C 1001 1100 −75°C 1011 0101 −50°C 1100 1110 −25°C 1110 0111 0°C 0000 0000 +10°C 0000 1010 +25°C 0001 1001 +50°C 0011 0010 +75°C 0100 1011 +100°C 0110 0100 +125°C 0111 1101 +127°C 0111 1111 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. If the sensor is used in a very noisy environment, a capacitor of value up to 1 nF may be placed between the D+ and D– inputs to filter the noise. To measure ΔVBE, the sensor is switched between operating currents of I and N × I. The resulting waveform is passed through a 65 kHz low-pass filter to remove noise, then to a chopperstabilized amplifier that performs the functions of amplification and rectification of the waveform to produce a dc voltage proportional to ΔVBE. This voltage is measured by the ADC to give a temperature output in 8-bit twos complement format. To further reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles. An external temperature measurement takes nominally 34.8 ms. |
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