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ADM1021 Scheda tecnica(PDF) 6 Page - Analog Devices |
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ADM1021 Scheda tecnica(HTML) 6 Page - Analog Devices |
6 / 12 page ADM1021 –6– REV. 0 Figure 12 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 ∆V be, 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, 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 ∆V be. 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. 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 1 °C, so the ADC can theoretically measure from –128 °C to +127°C, although the practical lowest value is limited to –65 °C due to device maxi- mum ratings. The temperature data format is shown in Table I. 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. C1* D+ D– REMOTE SENSING TRANSISTOR IN I IBIAS VDD VOUT+ TO ADC VOUT– BIAS DIODE LOWPASS FILTER fC = 65kHz CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 2.2nF TYPICAL, 3nF MAX. * Figure 12. Input Signal Conditioning Table I. Temperature Data Format Temperature Digital Output –128 °C 1 000 0000 –125 °C 1 000 0011 –100 °C 1 001 1100 –75 °C 1 011 0101 –50 °C 1 100 1110 –25 °C 1 110 0111 –1 °C 1 111 1111 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 REGISTERS The ADM1021 contains nine registers that are used to store the results of remote and local temperature measurements, high and low temperature limits, and to configure and control the device. A description of these registers follows, and further details are given in Tables II to IV. It should be noted that the ADM1021’s registers are dual port, and have different addresses for read and write operations. Attempting to write to a read address, or to read from a write address, will produce an invalid result. Regis- ter addresses above 0Fh are reserved for future use or used for factory test purposes and should not be written to. Address Pointer Register The Address Pointer Register itself does not have, nor does it require, an address, as it is the register to which the first data byte of every Write operation is written automatically. This data byte is an address pointer that sets up one of the other registers for the second byte of the Write operation, or for a subsequent read operation. |
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