ADT7476A
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18
Figure 26. Measuring Temperature by Using
an NPN Transistor
ADT7476A
D+
D−
2N3904
NPN
Figure 27. Measuring Temperature by Using
an PNP Transistor
ADT7476A
D+
D−
2N3906
PNP
To measure
D
V
BE
, the sensor switches between operating
currents of I and N
×
I. The resulting waveform passes
through a 65 kHz low-pass filter to remove noise and
through a chopper-stabilized amplifier. The amplifier
performs the amplification and rectification of the
waveform to produce a dc voltage proportional to
D
V
BE
.
This voltage is measured by the ADC to give a temperature
output in 10-bit, twos complement format. To further reduce
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles.
A remote temperature measurement takes nominally
38 ms. The results of remote temperature measurements are
stored in 10-bit, twos complement format, as illustrated in
Table 22. The extra resolution for the temperature
measurements is held in the Extended Resolution Register 2
(0x77). This gives temperature readings with a resolution of
0.25
°
C.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice placed a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However,
large capacitances affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1,000 pF.
This capacitor reduces the noise but does not eliminate it,
which makes using the sensor difficult in a very noisy
environment. In most cases, a capacitor is not required
because differential inputs by their very nature have a high
immunity to noise.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7476A is designed to work with 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
consideration:
•
The ideality factor, n
f
, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7476A 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’s data sheet for the n
f
values):
D
T
+
(nf
*
1.008)
ǒ
273.15 K
)
T
Ǔ
(eq. 2)
To factor this in, the user can write the
D
T value to the
offset register. The ADT7476A then 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 ADT7476A, I
HIGH
, is 180
m
A, and
the low level current, I
LOW
, is 11
m
A. If the ADT7476A
current levels do not match the current levels specified
by the CPU manufacturer, it could be necessary to
remove an offset. The CPU’s data sheet advises
whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that if more than one
offset must be considered, then the algebraic sum of
these offsets must be programmed to the offset register.
If a discrete transistor is used with the ADT7476A, 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 the current gain, 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.
Nulling Out Temperature Errors
As CPUs run faster, it is more difficult to avoid high
frequency clocks when routing the D+/D– traces around a
system board. Even when recommended layout guidelines
are followed, some temperature errors can still be
attributable to noise coupled onto the D+/D– lines. Constant
high frequency noise usually attenuates, or increases,
temperature measurements by a linear, constant value.
The ADT7476A has temperature offset registers (0x70
and 0x72) for the Remote 1 and Remote 2 temperature
channels. By doing a one-time calibration of the system, the
user can determine the offset caused by system board noise
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