Keywords: DS1852, temperature and voltage, temperature measurement, voltage measurement
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APPLICATION NOTE 217

Abstract: The DS1852 is a digital potentiometer with five analog inputs that can be used in conjunction with the potentiometers to gather feedback information about an analog system, as well as provide analog control functions. Its intended application is to provide a high level of control and instrumentation in a laser transceiver, but it could be used in any sort of control system where this kind of instrumentation and control is required. This application brief discusses the interpretation of the analog input registers in the DS1852. It is intended as a supplement to the data sheet for a software/firmware developer writing code to control the DS1852.

60h (Temperature MSB) | 61h (Temperature LSB) | ||||||||||||||

S | 2^{6} |
2^{5} |
2^{4} |
2^{3} |
2^{2} |
2^{1} |
2^{0} |
2^{-1} |
2^{-2} |
2^{-3} |
2^{-4} |
2^{-5} |
2^{-6} |
2^{-7} |
2^{-8} |

S | 2^{6} |
2^{5} |
2^{4} |
2^{3} |
2^{2} |
2^{1} |
2^{0} |
2^{-1} |
2^{-2} |
2^{-3} |
2^{-4} |
0 | 0 | 0 | 0 |

The temperature is stored in the 12 MSBs of the binary data. The MSB is the sign bit and indicates whether or not the 2's complement binary number is positive or negative. If the MSB is 0, then the temperature is positive. If the MSB is 1, then the temperature is negative. The next 11 bits contain the value of the temperature and are translated into decimal differently for positive and negative values.

HEXIDECIMAL DATA | BINARY DATA | MASKED BINARY DATA |

3C50h | 0011 1100 0101 0000 | 0011 1100 0101 0000 |

Since the MSB sign bit is 0, the temperature is positive and requires no 2's complement transformation. Therefore, the value can be translated directly into a decimal number using a calculator. The binary number 0011 1100 0101 0000 is equal to the decimal value of 15440. The decimal value must be divided by 256 (or multiplied by 2

HEXIDECIMAL DATA | BINARY DATA | MASKED BINARY DATA |

D360h | 1101 0011 0110 0000 | 1101 0011 0110 0000 |

The next step is to take the 2's complement of the masked binary value. This is done by first inverting the masked binary data and then adding 1 to the LSB. The binary number 1101 0011 0110 0000 converts to 0010 1100 1001 1111. Adding 1 to the LSB results in the binary number 0010 1100 1010 0000. This binary number is equal to the decimal number 11424. Like in the positive temperature example, the translated decimal value is divided by 256, but is also made negative because the temperature is negative as indicated by the MSB. So, the decimal value 11424 is divided by -256, resulting in a decimal temperature value of -44.625°C.

62h (V_{CC} MSB) |
63h (V_{CC} LSB) |
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2^{15} |
2^{14} |
2^{13} |
2^{12} |
2^{11} |
2^{10} |
2^{9} |
2^{8} |
2^{7} |
2^{6} |
2^{5} |
2^{4} |
2^{3} |
2^{2} |
2^{1} |
2^{0} |

2^{15} |
2^{14} |
2^{13} |
2^{12} |
2^{11} |
2^{10} |
2^{9} |
2^{8} |
2^{7} |
2^{6} |
2^{5} |
2^{4} |
0 | 0 | 0 | 0 |

As an example, the values read from 62h and 63h are found to be D7A0h. Translating this number directly to a decimal value results in a value of 55200, which translates to 5.52V when multiplied by the 100µV scaling factor.

64h (B_{in} MSB) |
65h (B_{in} LSB) |
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66h (P_{in} MSB) |
67h (P_{in} LSB) |
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68h (R_{in} MSB) |
69h (R_{in} LSB) |
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2^{15} |
2^{14} |
2^{13} |
2^{12} |
2^{11} |
2^{10} |
2^{9} |
2^{8} |
2^{7} |
2^{6} |
2^{5} |
2^{4} |
2^{3} |
2^{2} |
2^{1} |
2^{0} |

2^{15} |
2^{14} |
2^{13} |
2^{12} |
2^{11} |
2^{10} |
2^{9} |
2^{8} |
2^{7} |
2^{6} |
2^{5} |
2^{4} |
0 | 0 | 0 | 0 |

As an example, the hex value of 9E70h is read from the two address locations. After masking the 4 lower bits with 0s, the binary value is determined to be 1001 1110 0111 0000. The decimal equivalent is 40560. Multiplying this value by the scaling factor of 38.147µV results in the scaled voltage value of 1.547V.

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APP 217: APPLICATION NOTE 217,AN217, AN 217, APP217, Appnote217, Appnote 217 |