ADM1028ARQ View Datasheet(PDF) - Analog Devices
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ADM1028ARQ Datasheet PDF : 16 Pages
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ADM1028
TEMPERATURE MEASUREMENT SYSTEM
Internal Temperature Measurement
The ADM1028 contains an on-chip bandgap temperature sen-
sor. The on-chip ADC performs conversions on the output of
this sensor and outputs the temperature data in 8-bit two’s
complement format. The format of the temperature data is
shown in Table I.
Table I. Temperature Data Format
Temperature
–128°C
–125°C
–100°C
–75°C
–50°C
–25°C
–1°C
0°C
+1°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
1111 1111
0000 0000
0000 0001
0000 1010
0001 1001
0011 0010
0100 1011
0110 0100
0111 1101
0111 1111
External Temperature Measurement
The ADM1028 can measure the temperature of an external
diode sensor or diode-connected transistor, connected to Pins 9
and 10.
Pins 9 and 10 are a dedicated temperature input channel. The
default functions of Pins 11 and 12 are as THERM outputs to
indicate over-temperature conditions.
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, making the technique unsuitable
for mass production.
The technique used in the ADM1028 is to measure the change
in VBE when the device is operated at two different currents.
This is given by:
∆VBE = 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 3 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 tempera-
ture 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.
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 1000 pF may be placed between the D+ and D–
inputs to filter the noise.
To measure ∆VBE, 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, thence to a
chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce a dc
voltage proportional to ∆VBE. 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 16 measurement cycles.
An external temperature measurement nominally takes 9.6 ms.
VDD
I
N ؋ I IBIAS
D+
REMOTE
SENSING
TRANSISTOR D–
BIAS
DIODE
LOW-PASS
FILTER
fC = 65kHz
Figure 3. Signal Conditioning
VOUT+
TO
ADC
VOUT–
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 ADM1028 as close as possible to the remote sens-
ing diode. Provided that the worst noise sources such as clock
generators, data/address buses and CRTs are avoided, this
distance can be 4 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.
3. Use wide tracks to minimize inductance and reduce noise
pickup. Ten mil track minimum width and spacing is rec-
ommended.
4. Try to minimize the number of copper/solder joints, which
can cause thermocouple effects. Where copper/solder joints
are used, make sure that they are in both the D+ and D–
path and at the same temperature.
Thermocouple effects should not be a major problem as 1°C
corresponds to about 200 µV, and thermocouple voltages are
about 3 µV/oC of temperature difference. Unless there are
two thermocouples with a big temperature differential between
them, thermocouple voltages should be much less than 200 µV.
5. Place 0.1 µF bypass and 2200 pF input filter capacitors close
to the ADM1028.
6. If the distance to the remote sensor is more than 8 inches, the
use of twisted-pair cable is recommended. This will work up
to about 6 to 12 feet.
–8–
REV. A
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