ADM1023
Rev. 8 | Page 9 of 18 | www.onsemi.com
MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, or the base emitter
voltage of a transistor, operating at constant current. Thus, the
temperature may be obtained from a direct measurement of V
BE
where
()
S
C
BE
I
I
n
q
nKT
V 1×=
(1)
This technique, however, requires calibration to nullify the effect
of the absolute value of V
BE
, which varies from device to device.
The technique used in the ADM1023 is to measure the change in
V
BE
when the device is operated at two different collector currents.
This is given by
()
Nn
q
nKT
V
BE
1×=Δ
(2)
where:
K is Boltzmann’s constant.
q is the charge on the electron (1.6 × 10
–19
Coulombs).
T is the absolute temperature in Kelvins.
N is the ratio of the two collector currents.
n is the ideality factor of the thermal diode (TD).
To measu re ΔV
BE
, the sensor is switched between operating
currents of I and NI. The resulting waveform is passed through a
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 ΔV
BE
. This voltage is measured by the ADC, which gives a
temperature output in binary format. To further reduce the effects
of noise, digital filtering is performed by averaging the results of 16
measurement cycles. Signal conditioning and measurement of the
internal temperature sensor are performed in a similar manner.
Figure 14 shows the input signal conditioning used to measure the
output of an external temperature sensor. This figure shows the
external sensor as a substrate PNP transistor, provided for
temperature monitoring on some microprocessors, but it could
equally well be a discrete transistor. If a discrete transistor is used,
the collector is not grounded and should be connected 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 a noisy environment, C1 may
optionally be added as a noise filter. Its value is 1000 pF maximum.
See the Layout Considerations section for more information on
C1.
SOURCES OF ERRORS ON THERMAL TRANSISTORS
MEASUREMENT METHOD
The Effect Of Ideality Factor (n)
The effects of ideality factor (n) and beta (β) of the temperature
measured by a thermal transistor are described in this section.
For a thermal transistor implemented on a submicron process,
such as the substrate PNP used on a Pentium III processor, the
temperature errors due to the combined effect of the ideality
factor and beta are shown to be less than 3°C. Equation 2 is
optimized for a substrate PNP transistor (used as a thermal
diode) usually found on CPUs designed on submicron CMOS
processes such as the Pentium III processor. There is a thermal
diode on board each of these processors. The n in Equation 2
represents the ideality factor of this thermal diode. This ideality
factor is a measure of the deviation of the thermal diode from
ideal behavior.
According to Pentium III processor manufacturing specifications,
measured values of n at 100°C are
0125.1008.10057.1 =<=<=
MAXTYPICALMIN
nnn
The ADM1023 takes this ideality factor into consideration
when calculating temperature T
TD
of the thermal diode. The
ADM1023 is optimized for n
TYPICAL
= 1.008; any deviation
on n from this typical value causes a temperature error that
is calculated below for the n
MIN
and n
MAX
of a Pentium III
processor at T
TD
= 100°C.
()
C85.010015.273
008.1
008.10057.1
oo
−=+×
−
=Δ CKelvinT
MIN
()
C67.110015.273
008.1
008.10125.1
oo
+=+×
−
=Δ CKelvinT
MAX
Thus, the temperature error due to variation on n of the
thermal diode for a Pentium III processor is about 2.5°C.
In general, this additional temperature error of the thermal
diode measurement due to deviations on n from its typical
value is given by
()
TD
TKelvin
n
T +×
−
=Δ 15.273
008.1
008.1
where T
TD
is in °C.
Beta of Thermal Transistor (β)
In Figure 14, the thermal diode is a substrate PNP transistor where
the emitter current is forced into the device. The derivation of
Equation 2 assumed that the collector currents were scaled by N as
the emitter currents were also scaled by N. Thus, this assumes that
beta (β) of the transistor is constant for various collector currents.
Figure 15 shows typical β variation vs. collector current for
Pentium III processors at 100°C. The maximum β is 4.5 and varies
less than 1% over the collector current range from 7 μA to 300 μA.
