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ADM1032ARM bảng dữ liệu(PDF) 5 Page - Analog Devices |
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ADM1032ARM bảng dữ liệu(HTML) 5 Page - Analog Devices |
5 / 12 page REV. 0 ADM1032 –5– FUNCTIONAL DESCRIPTION The ADM1032 is a local and remote temperature sensor and over-temperature alarm. When the ADM1032 is operating normally, the on-board A-to-D converter operates in a free- running mode. The analog input multiplexer alternately selects either the on-chip temperature sensor to measure its local tem- perature, or the remote temperature sensor. These signals are digitized by the ADC and the results stored in the Local and Remote Temperature Value Registers. The measurement results are compared with local and remote, high, low and THERM temperature limits, stored in nine on- chip registers. Out-of-limit comparisons generate flags that are stored in the Status Register, and one or more out-of limit results will cause the ALERT output to pull low. Exceeding THERM temperature limits cause the THERM output to assert low. The limit registers can be programmed, and the device con- trolled and configured, via the serial System Management Bus (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. • Masking or enabling the ALERT output. • Selecting the conversion rate. MEASUREMENT METHOD A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, or the base-emitter voltage of a transistor, operated at constant current. Unfortu- nately, 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 ADM1032 is to measure the change in VBE when the device is operated at two different currents. This is given by: where: K is Boltzmann’s constant (1.38 × 10–23). q is charge on the electron (1.6 × 10–19 Coulombs). T is absolute temperature in Kelvins. N is ratio of the two currents. nf is the ideality factor of the thermal diode. The ADM1032 is trimmed for an ideality factor of 1.008. Figure 2 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 tem- perature 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. To prevent ground noise interfering with the measure- ment, 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 operating in a noisy environment, C1 may optionally be added as a noise filter. Its value is typi- cally 2200 pF, but should be no more than 3000 pF. See the section on Layout Considerations for more information on C1. To measure ∆VBE, the sensor is switched between operating cur- rents of I and N × I. The resulting waveform is passed through a 65 kHz low-pass filter to remove noise, thence to a chopper- stabilized amplifier that performs the functions of amplification and rectification of the waveform to produce a dc voltage pro- portional to ∆VBE. This voltage is measured by the ADC to give a temperature output in two’s complement format. To further reduce the effects of noise, digital filtering is performed by aver- aging the results of 16 measurement cycles. Signal conditioning and measurement of the internal tempera- ture sensor is performed in a similar manner. TEMPERATURE DATA FORMAT One LSB of the ADC corresponds to 0.125 °C, so the ADC can measure from 0 °C to 127.875°C. The temperature data format is shown in Tables I and II. 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. Table I. Temperature Data Format (Local Temperature and Remote Temperature High Byte) Temperature Digital Output 0 °C 0 000 0000 1 °C 0 000 0001 10 °C 0 000 1010 25 °C 0 001 1001 50 °C 0 011 0010 75 °C 0 100 1011 100 °C 0 110 0100 125 °C 0 111 1101 127 °C 0 111 1111 C1* D+ D– REMOTE SENSING TRANSISTOR IN I IBIAS VDD VOUT+ TO ADC VOUT– BIAS DIODE LOW-PASS FILTER fC = 65kHz CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. C1 = 2.2nF TYPICAL, 3nF MAX. * Figure 2. Input Signal Conditioning ∆Vn KT q In N BE f = () × () |
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