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參數資料
| 型號： | ADT7466 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | dBCool Remote Thermal Controller and Voltage Monitor |
| 中文描述： | dBCool遠程熱控制器和電壓監視器 |
| 文件頁數： | 17/48頁 |
| 文件大?。?/td> | 1269K |
| 代理商： | ADT7466 |
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ADT7466
TEMPERATURE MEASUREMENT
The ADT7466 has two dedicated temperature measurement
channels, one for measuring the temperature of an on-chip
band gap temperature sensor, and one for measuring the
temperature of a remote diode, usually located in the CPU. In
addition, the analog input channels, AIN1 and AIN2, can be
reconfigured to measure the temperature of a second diode by
setting Bit 7 of Configuration Register 2 (0x01), or to measure
temperature using thermistors by setting Bit 6 and/or Bit 7 of
Configuration Register 3 (0x02).
Rev. 0 | Page 17 of 48
SERIES RESISTANCE CANCELLATION
Parasitic resistance, seen in series with the remote diode
between the D+ and D inputs to the ADT7466, is caused by a
variety of factors including PCB track resistance and track
length. This series resistance appears as a temperature offset in
the sensor’s temperature measurement. This error typically
causes a 1°C offset per ohm of parasitic resistance in series with
the remote diode. The ADT7466 automatically cancels the
effect of this series resistance on the temperature reading, giving
a more accurate result without the need for user characterization
of the resistance. The ADT7466 is designed to automatically
cancel typically 2 k of resistance. This is done transparently to
the user, using an advanced temperature measurement method
described in the following section.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, by measuring the
base emitter voltage (V
BE
) of a transistor operated at constant
current. Unfortunately, this technique requires calibration to
null out the effect of the absolute value of V
BE
, which varies
from device to device.
The technique used in the ADT7466 measures the change in
V
BE
when the device is operated at three different currents.
Previous devices used only two operating currents, but it is the
third current that allows series resistance cancellation.
Figure 24 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the remote sensor as a substrate transistor, provided for
temperature monitoring on some microprocessors, but it could
also be a discrete transistor. If a discrete transistor is used, the
collector is not grounded, and should be linked to the base. To
prevent ground noise from interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground but is biased above ground by an internal diode at the
D– input. If the sensor is operating in an extremely noisy
environment, C1 may optionally be added as a noise filter. Its
value should never exceed 1000 pF. See the Layout
Considerations section for more information on C1.
To measure ΔV
BE
, the operating current through the sensor is
switched between three related currents. Figure 24 shows N1 × I
and N2 × I as different multiples of the current I. The currents
through the temperature diode are switched between I and
N1 × I, giving ΔV
BE1
, and then between I and N2 × I, giving
ΔV
BE2
. The temperature can then be calculated using the two
ΔV
BE
measurements. This method can also cancel the effect of
series resistance on the temperature measurement. The
resulting ΔV
BE
waveforms are passed through a 65 kHz low-pass
filter to remove noise, and then to a chopper-stabilized
amplifier. This amplifies and rectifies the waveform to produce
a dc voltage proportional to ΔV
BE
. The ADC digitizes this
voltage, and a temperature measurement is produced. To reduce
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles for low conversion rates.
Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
USING DISCRETE TRANSISTORS
If a discrete transistor is used, the collector is not grounded and
should be linked to the base. If an NPN transistor is used, the
emitter is connected to the D input and the base to the D+
input. If a PNP transistor is used, the base is connected to the
D input and the emitter to the D+ input. Figure 23 shows how
to connect the ADT7466 to an NPN or PNP transistor for
temperature measurement. To prevent ground noise interfering
with the measurement, the more negative terminal of the sensor
is not referenced to ground, but is biased above ground by an
internal diode at the D input.
0
D+
D–
ADT7466
2N3904
NPN
D+
D–
ADT7466
2N3906
PNP
Figure 23. Connections for NPN and PNP Transistors
0
C1*
D–
D+
DBIAS
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
V
DD
TO ADC
V
OUT+
V
OUT–
TRAREMOTE
I
N1
×
I
N2
×
I
I
BIAS
LOW- = 65kHz
C
Figure 24. Signal Conditioning for Remote Diode Temperature Sensors
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