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ADM1024ARUZ Datasheet(PDF) 13 Page - ON Semiconductor

Part No. ADM1024ARUZ
Description  System Hardware Monitor with Remote Diode Thermal Sensing
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADM1024ARUZ Datasheet(HTML) 13 Page - ON Semiconductor

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ADM1024
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13
positive and negative temperatures can be measured, the
temperature data is stored in twos complement format, as
shown in Table 4. Theoretically, the temperature sensor and
ADC can measure temperatures from −128
°C to +127°C
with a resolution of 1
°C, although temperatures below
−40
°C and above +125°C are outside the operating
temperature range of the device.
External Temperature Measurement
The ADM1024 can measure the temperature of two
external diode sensors or diode−connected transistors,
connected to Pins 13 and 14 or 17 and 18.
Pins 13 and 14 are a dedicated temperature input channel.
Pins 17 and 18 can be configured to measure a diode sensor
by setting Bit 2 of the Channel Mode Register to 1.
The forward voltage of a diode or diode−connected
transistor, operated at a constant current, exhibits a negative
temperature coefficient of about –2 mV/
°C. Unfortunately,
the absolute value of VBE varies from device to device, and
individual calibration is required to null this out, so the
technique is unsuitable for mass production.
The technique used in the ADM1024 is to measure the
change in VBE when the device is operated at two different
currents.
This is given by:
(eq. 5)
DVbe + KT q ln(N)
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 17 shows the input signal conditioning used to
measure the output of an external temperature sensor. This
figure shows the external sensor as a substrate transistor
provided
for
temperature
monitoring
on
some
microprocessors, but it could equally well be a discrete
transistor.
I
D+
LPF
N y I
IBIAS
VDD
BIAS
DIODE
D–
LOW−PASS
FILTER
fC = 65kHz
VOUT+
VOUT–
TO
ADC
REMOTE
SENSING
TRANSISTOR
Figure 17. Signal Conditioning for External Diode
Temperature Sensors
If a discrete transistor is used, the collector will not be
grounded and should be linked to the base. If a PNP
transistor is used, the base is connected to the D− input and
the emitter to the D+ input. If an NPN transistor is used, the
emitter is connected to the D− input and the base to the D+
input.
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. As the sensor is operating in
a noisy environment, C1 is provided as a noise filter. See the
Layout Considerations section for more information on C1.
To measure
DVBE, the sensor is switched between
operating currents of I and N
× I. The resulting waveform is
passed through a 65 kHz low−pass filter to remove noise,
then to a chopper−stabilized amplifier that performs the
functions of amplification and rectification of the waveform
to produce a dc voltage proportional to
DVBE. This voltage
is measured by the ADC to give a temperature output in
8−bit twos complement format. To further reduce the effects
of noise, digital filtering is performed by averaging the
results of 16 measurement cycles. An external temperature
measurement takes nominally 9.6 ms.
The results of external temperature measurements are
stored in 8−bit, twos complement format, as illustrated in
Table 4.
Table 4. Temperature Data Format
Temperature
Digital Output
−128°C
1000 0000
−125°C
1000 0011
−100°C
1001 1100
−75°C
1011 0101
−50°C
1100 1110
−25°C
1110 0111
0°C
0000 0000
+0.5°C
0000 0000
+10°C
0000 1010
+25°C
0001 1001
+50°C
0011 0010
+75°C
0100 1011
+100°C
0110 0100
+125°C
0111 1101
+127°C
0111 1111
Layout Considerations
Digital boards can be electrically noisy environments, and
care must be taken to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor. The following precautions should be
taken:
1. Place the ADM1024 as close as possible to the
remote sensing diode. Provided that the worst
noise sources such as clock generators,
data/address buses, and CRTs are avoided, this
distance can be 4 inches to 8 inches.
2. Route the D+ and D− tracks close together, in
parallel, with grounded guard tracks on each side.
Provide a ground plane under the tracks if
possible.


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