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ADT7475
because differential inputs, by their very nature, have a high
immunity to noise.
ADT7475
D+
2N3904
NPN
D ?
Figure 21. Measuring Temperature by Using
an NPN Transistor
ADT7475
D+
2N3906
PNP
D ?
Figure 22. Measuring Temperature by Using
a PNP Transistor
algebraic sum of these offsets must be programmed to
the offset register.
If a discrete transistor is used with the ADT7475, the best
accuracy is obtained by choosing devices according to the
following criteria:
? Base-emitter voltage greater than 0.25 V at 11 m A, at
the highest operating temperature.
? Base-emitter voltage less than 0.95 V at 180 m A, at the
lowest operating temperature.
? Base resistance less than 100 W .
? Small variation in h FE (approximately 50 to 150) that
indicates tight control of V BE characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in
SOT ? 23 packages, are suitable devices to use.
Table 11. TWOS COMPLEMENT TEMPERATURE DATA
FORMAT
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7475 is designed to work with either substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode
(base-shorted to the collector). If an NPN transistor is used,
the collector and base are connected to D+ and the emitter
to D ? . If a PNP transistor is used, the collector and base are
connected to D ? and the emitter is connected to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
Temperature
–128 ° C
–63 ° C
–50 ° C
–25 ° C
–10 ° C
0 ° C
10.25 ° C
25.5 ° C
50.75 ° C
75 ° C
100 ° C
Digital Output (10-bit) (Note 1)
1000 0000 00 (Diode Fault)
1100 0001 00
1100 1110 00
1110 0111 00
1111 0110 00
0000 0000 00
0000 1010 01
0001 1001 10
0011 0010 11
0100 1011 00
0110 0100 00
consideration:
? The ideality factor, n f , of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7475 is trimmed for an n f value of 1.008. Use
the following equation to calculate the error introduced
at a temperature, T ( ° C), when using a transistor whose
n f does not equal 1.008. See the processor data sheet for
the n f values.
D T + n f * 1.008 273.15 k ) T (eq. 2)
125 ° C 0111 1101 00
127 ° C 0111 1111 00
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution Register 2 (0x77) with 0.25 ° C resolution.
Table 12. EXTENDED RANGE, TEMPERATURE DATA
FORMAT
Temperature Digital Output (10-bit) (Note 1)
–64 ° C 0000 0000 00 (Diode Fault)
–63 ° C 0000 0001 00
?
To factor this in, the user can write the D T value to the
offset register. The ADT7475 automatically adds it
to or subtracts it from the temperature measurement.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7475, I HIGH , is 180 m A and the
low level current, I LOW , is 11 m A. If the ADT7475
current levels do not match the current levels specified
by the CPU manufacturer, it might be necessary to
remove an offset. The CPU’s data sheet advises
–1 ° C
0 ° C
1 ° C
10 ° C
25 ° C
50 ° C
75 ° C
100 ° C
125 ° C
0011 1111 00
0100 0000 00
0100 0001 00
0100 1010 00
0101 1001 00
0111 0010 00
1000 1001 00
1010 0100 00
1011 1101 00
whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. If more than one offset must be considered, the
191 ° C 1111 1111 00
1. Bold numbers denote 2 LSBs of measurement in the Extended
Resolution Register 2 (0x77) with 0.25 ° C resolution.
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