ADM1025A View Datasheet(PDF) - Analog Devices
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ADM1025A Datasheet PDF : 16 Pages
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ADM1025/ADM1025A
TEMPERATURE MEASUREMENT SYSTEM
Internal Temperature Measurement
The ADM1025/ADM1025A contains an on-chip band gap
temperature 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
temperatures 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°C to +127°C with a resolution of 1°C, although
temperatures below 0°C and above +100°C are outside the
operating temperature range of the device.
External Temperature Measurement
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 VBE, varies from device to device, and individual calibration
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 VBE when the device is operated at two differ-
ent currents. This is given by:
∆VBE = KT q × In(N )
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 4 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–.
I
N ؋ I IBIAS
VDD
Table III. Temperature Data Format
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
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 ∆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, then 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 twos 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 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, particularly
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.
D+
REMOTE
SENSING
TRANSISTOR D–
BIAS
DIODE
LOW-PASS
FILTER
fC = 65kHz
VOUT+
TO
ADC
VOUT–
Figure 4. Signal Conditioning for External Diode
Temperature Sensors
–10–
GND
D+
D–
GND
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
Figure 5. Arrangement of Signal Tracks
REV. C
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