REV. A
14
ADM1027
TEMPERATURE MEASUREMENT SYSTEM
Local Temperature Measurement
The ADM1027 contains an on-chip band gap temperature
sensor whose output is digitized by the on-chip 10-bit ADC.
The 8-bit MSB temperature data is stored in the local temp
register (Address 0x26). As both positive and negative tempera-
tures can be measured, the temperature data is stored in twos
complement format, as shown in Table III. Theoretically, the
temperature sensor and ADC can measure temperatures from
128
o
C to +127
o
C with a resolution of 0.25
o
C. However, this
exceeds the operating temperature range of the device (0
o
C to
105
o
C), so local temperature measurements outside this range
are not possible. Temperature measurement from 127
o
C to
+127
o
C is possible using a remote sensor.
Remote Temperature Measurement
The ADM1027 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to
Pins 15 and 16, or 17 and 18.
The forward voltage of a diode or diode-connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about 2 mV/
o
C. Unfortunately, the absolute
value of V
be
varies from device to device, and individual calibra-
tion is required to null this out, so the technique is unsuitable
for mass production. The technique used in the ADM1027 is to
measure the change in V
be
when the device is operated at two
different currents. This is given by
where:
K is Boltzmanns constant.
q is charge on the carrier.
T is absolute temperature in kelvins.
N is the ratio of the two currents.
Figure 13 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows the
external sensor as a substrate transistor, provided for temperature
monitoring on some microprocessors. It could equally well be a
discrete transistor such as a 2N3904/06.
D+
D
REMOTE
SENSING
TRANSISTOR
I
N I
I
BIAS
V
DD
V
OUT+
TO ADC
V
OUT
BIAS
DIODE
LOW-PASS
FILTER
f
C
= 65kHz
THERMDA
THERMDC
CPU
Figure 13. Signal Conditioning for Remote Diode Temperature Sensors
DV
KT q
N
be
=
?/DIV>
( )
ln
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