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ADM1023ARQZ-REEL Datenblatt(PDF) 7 Page - ON Semiconductor |
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ADM1023ARQZ-REEL Datenblatt(HTML) 7 Page - ON Semiconductor |
7 / 16 page ADM1023 http://onsemi.com 7 Theory of Operation Functional Description The ADM1023 contains a two-channel analog-to-digital converter (ADC) with special input-signal conditioning to enable operation with remote and on-chip diode temperature sensors. When the ADM1023 is operating normally, the ADC operates in a free-running mode. The analog input multiplexer alternately selects either the on-chip temperature sensor to measure its local temperature or the remote temperature sensor. These signals are digitized by the ADC, and the results are stored in the local and remote temperature value registers. Only the eight most significant bits (MSBs) of the local temperature value are stored as an 8-bit binary word. The remote temperature value is stored as an 11−bit binary word in two registers. The eight MSBs are stored in the remote temperature value high byte register at Address 0x01. The three least significant bits (LSBs) are stored, left justified, in the remote temperature value low byte register at Address 0x10. Error sources such as PCB track resistance and clock noise can introduce offset errors into measurements on the remote channel. To achieve the specified accuracy on this channel, these offsets must be removed, and two offset registers are provided for this purpose at Address 0x11 and Address 0x12. An offset value may automatically be added to or subtracted from the measurement by writing an 11-bit, twos complement value to Register 0x11 (high byte) and Register 0x12 (low byte, left-justified). The offset registers default to 0 at powerup and have no effect if nothing is written to them. The measurement results are compared with local and remote, high and low temperature limits, stored in six on-chip limit registers. As with the measured value, the local temperature limits are stored as 8-bit values and the remote temperature limits as 11-bit values. Out-of-limit comparisons generate flags that are stored in the status register, and one or more out-of-limit results cause the ALERT output to pull low. Registers can be programmed, and the device controlled 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 On initial powerup, the remote and local temperature values default to −128C. The device normally powers up converting, making a measure of local and remote temperature. These values are then stored before making a comparison with the stored limits. However, if the part is powered up in standby mode (STBY pin pulled low), no new values are written to the register before a comparison is made. As a result, both RLOW and LLOW are tripped in the status register, thus generating an ALERT output. This may be cleared in one of two ways: Change both the local and remote lower limits to –128C and read the status register (which in turn clears the ALERT output). Take the part out of standby and read the status register (which in turn clears the ALERT output). This works only when the measured values are within the limit values. Figure 13. Input Signal Conditioning C11 D+ D– REMOTE SENSING TRANSISTOR I N y I VDD VOUT+ TO ADC VOUT– BIAS DIODE LOW−PASS FILTER fC = 65kHz 1 C1 = 1000pF MAX. IBIAS CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. 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, operating at constant current. Thus, the temperature may be obtained from a direct measurement of VBE where: (eq. 1) VBE + nKT q 1n IC IS This technique, however, requires calibration to nullify the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADM1023 is to measure the change in VBE when the device is operated at two different collector currents. |
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