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AD7818ARM bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD7818ARM bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 20 page Data Sheet AD7817/AD7818 Rev. D | Page 11 of 20 CONTROL BYTE The AD7817/AD7818 contain two on-chip registers, the address register and the overtemperature register. These registers can be accessed by carrying out an 8-bit serial write operation to the devices. The 8-bit word or control byte written to the AD7817/AD7818 is transferred to one of the two on-chip registers as follows. Address Register If the five MSBs of the control byte are logic zero, the three LSBs of the control byte are transferred to the address register (see Figure 6). The address register is a 3-bit-wide register used to select the analog input channel on which to carry out a conversion. It is also used to select the temperature sensor, which has the 000 address. Table 6 shows the channel selection. The internal reference selection connects the input of the ADC to a band gap reference. When this selection is made and a conversion is initiated, the ADC output must be approximately midscale. After power-up, the default channel selection is DB2 = DB1 = DB0 = 0 (temperature sensor). Table 6. Channel Selection DB2 DB1 DB0 Channel Selection Device 0 0 0 Temperature sensor All 0 0 1 Channel 1 All 0 1 0 Channel 2 AD7817 0 1 1 Channel 3 AD7817 1 0 0 Channel 4 AD7817 1 1 1 Internal reference (1.23 V) All Overtemperature Register If any of the five MSBs of the control byte are logic one, the entire eight bits of the control byte are transferred to the overtemperature register (see Figure 6). At the end of a temperature conversion, a digital comparison is carried out between the 8 MSBs of the temperature conversion result (10 bits) and the contents of the overtemperature register (8 bits). If the result of the temperature conversion is greater than the contents of the overtemperature register (OTR), the overtemperature indicator (OTI) goes logic low. The resolution of the OTR is 1°C. The lowest temperature that can be written to the OTR is −95°C and the highest is +152°C (see Figure 7). However, the usable temperature range of the temperature sensor is −55°C to +125°C. Figure 7 shows the OTR and how to set TALARM (the temperature at which the OTI goes low). OTR (Dec) = TALARM (°C) + 103°C For example, to set TALARM to 50°C, OTR = 50 + 103 = 153 Dec or 10011001 bin. If the result of a temperature conversion exceeds 50°C, OTI goes logic low. The OTI logic output is reset high at the end of a serial read operation or if a new temperature measurement is lower than TALARM. The default power on TALARM is 50°C. DB2 DB1 DB0 ADDRESS REGISTER LSB MSB DB0 CONTROL BYTE DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0 OVERTEMPERATURE REGISTER (OTR) DB7 DB6 DB5 DB4 DB3 DB2 DB1 IF ANY BIT DB7 TO DB3 ARE LOGIC 0 THEN DB2 TO DB0 ARE WRITTEN TO THE ADDRESS REGISTER IF ANY BIT DB7 TO DB3 IS SET TO A LOGIC 1, THEN THE FULL 8 BITS OF THE CONTROL WORD ARE WRITTEN TO THE OVERTEMPERATURE REGISTER Figure 6. Address and Overtemperature Register Selection DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0 0 0 0 0 1 0 0 0 1 1 1 1 1 1 1 1 MINIMUM TEMPERATURE = –95°C MAXIMUM TEMPERATURE = +152°C MSB LSB OVERTEMPERATURE REGISTER OVERTEMPERATURE REGISTER (DEC) = TALARM + 103°C TALARM RESOLUTION = 18/LSB Figure 7. The Overtemperature Register (OTR) |
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