ADT7484A/ADT7486A
http://onsemi.com
9
Temperature Measurement
The ADT7484A/ADT7486A each have 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 or GPU.
The ADT7484A monitors one local and one remote
temperature channel, whereas the ADT7486A monitors one
local and two remote temperature channels. Monitoring of
each of the channels is done in a round-robin sequence. The
monitoring sequence is in the order shown in Table 11.
Table 11. TEMPERATURE MONITORING SEQUENCE
Channel
Number
Measurement
Conversion
Time (ms)
0
Local Temperature
12
1
Remote Temperature 1
38
2
Remote Temperature 2
(ADT7486A Only)
38
Temperature Measurement Method
A simple method for 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 the effect of the absolute value of V
BE
,
which varies from device to device.
The technique used in the ADT7484A/ADT7486A
measures the change in V
BE
when the device is operated at
three different currents.
Figure 14 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,
which is 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 D1 input.
If the sensor is operating in an extremely noisy environment,
C1 can be added as a noise filter. Its value should not exceed
1,000 pF.
To measure DV
BE
, the operating current through the
sensor is switched between three related currents. Figure 14
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 DV
BE1
, and then between I and
N2 ?I, giving DV
BE2
. The temperature can then be
calculated using the two DV
BE
measurements. This method
can also cancel the effect of series resistance on the
temperature measurement. The resulting DV
BE
waveforms
are passed through a 65 kHz low-pass filter to remove noise
and then through a chopper-stabilized amplifier to amplify
and rectify the waveform, producing a dc voltage
proportional to DV
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.
Figure 14. Signal Conditioning for Remote Diode Temperature Sensors
LOW-PASS FILTER
f
C
= 65 kHz
REMOTE
SENSING
TRANSISTOR
BIAS
DIODE
D+
D
V
CC
I
BIAS
I
N2 ?I
V
OUT+
V
OUT
To ADC
C1*
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
N1 ?I
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