ADM1025/ADM1025A
TEMPERATURE MEASUREMENT SYSTEM
Internal Temperature Measurement
Table III. Temperature Data Format
The ADM1025/ADM1025A contains an on-chip bandgap tem-
perature sensor, whose output is digitized by the on-chip ADC.
The temperature data is stored in the Local Temperature Value
Register (address 27h). As both positive and negative tempera-
tures can be measured, the temperature data is stored in two’s
complement format, as shown in Table III. Theoretically, the
temperature sensor and ADC can measure temperatures from
–128°C to +127°C with a resolution of 1°C, although tempera-
tures below 0°C and above 100°C are outside the operating
temperature range of the device.
External Temperature Measurement
Temperature
–128°C
–125°C
–100°C
–75°C
–50°C
–25°C
0°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+127°C
Digital Output
1000 0000
1000 0011
1001 1100
1011 0101
1100 1110
1110 0111
0000 0000
0000 1010
0001 1001
0011 0010
0100 1011
0110 0100
0111 1101
0111 1111
The ADM1025/ADM1025A can measure temperature using an
external diode sensor or diode-connected transistor, connected to
Pins 9 and 10.
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 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 ADM1025/ADM1025A is to measure
the change in V
BE
when the device is operated at two differ-
ent currents.
This is given by:
∆V
BE
=
KT/q
×
ln(N)
where:
K
is Boltzmann’s constant
q
is charge on the carrier
T
is absolute temperature in Kelvins
N
is ratio of the two currents
Figure 11 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 tem-
perature monitoring on some microprocessors, but it could
equally well be a discrete transistor.
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.
Bit 6 of Status Register 2 (42h) is set if a remote diode fault is
detected. The ADM1025/ADM1025A detects shorts from D+
to GND or supply, as well as shorts/opens between D+/D–.
V
DD
I
N
I
I
BIAS
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.
If the sensor is used in a very noisy environment, a capacitor of
value up to 1 nF may be placed between the D+ and D– inputs
to filter the noise.
To measure
∆V
BE
, 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 pro-
portional to
∆V
BE
. This voltage is measured by the ADC to give
a temperature output in 8-bit two’s complement format. To
further reduce the effects of noise, digital filtering is performed
by averaging the results of sixteen measurement cycles. An
external temperature measurement takes nominally 34.8 ms.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments and care
must be taken to protect the analog inputs from noise, particu-
larly when measuring the very small voltages from a remote
diode sensor. The following precautions should be taken:
1. Place the ADM1025/ADM1025A 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 four to eight 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.
3. Use wide tracks to minimize inductance and reduce noise
pickup. 10 mil track minimum width and spacing is
recommended.
GND
10MIL
10MIL
D+
10MIL
10MIL
D–
10MIL
10MIL
GND
10MIL
D+
REMOTE
SENSING
TRANSISTOR
V
OUT+
TO
ADC
D–
BIAS
DIODE
V
OUT–
LOW-PASS
FILTER
f
C
= 65kHz
Figure 11. Signal Conditioning for External Diode
Temperature Sensors
Figure 12. Arrangement of Signal Tracks
–10–
REV. A
AD [ ANALOG DEVICES ]
