ADM1032
FUNCTIONAL DESCRIPTION
Figure 2 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. To prevent ground noise interfering with the measure-
ment, 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 typi-
cally 2200 pF, but should be no more than 3000 pF. See the
section on Layout Considerations for more information on C1.
The ADM1032 is a local and remote temperature sensor and
over-temperature alarm. When the ADM1032 is operating
normally, the on-board A-to-D converter operates in a free-
running mode. The analog input multiplexer alternately selects
either the on-chip temperature sensor to measure its local tem-
perature, or the remote temperature sensor. These signals are
digitized by the ADC and the results stored in the Local and
Remote Temperature Value Registers.
The measurement results are compared with local and remote,
high, low and THERM temperature limits, stored in nine on-
chip registers. Out-of-limit comparisons generate flags that are
stored in the Status Register, and one or more out-of limit results
will cause the ALERT output to pull low. Exceeding THERM
temperature limits cause the THERM output to assert low.
To measure ∆VBE, the sensor is switched between operating cur-
rents 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 pro-
portional to ∆VBE. This voltage is measured by the ADC to give
a temperature output in two’s complement format. To further
reduce the effects of noise, digital filtering is performed by aver-
aging the results of 16 measurement cycles.
The limit registers can be programmed, and the device con-
trolled and configured, via the serial System Management Bus
(SMBus). The contents of any register can also be read back via
the SMBus.
Control and configuration functions consist of:
•
Switching the device between normal operation and
standby mode.
Signal conditioning and measurement of the internal tempera-
ture sensor is performed in a similar manner.
•
•
Masking or enabling the ALERT output.
Selecting the conversion rate.
TEMPERATURE DATA FORMAT
MEASUREMENT METHOD
One LSB of the ADC corresponds to 0.125°C, so the ADC can
measure from 0°C to 127.875°C. The temperature data format
is shown in Tables I and II.
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, or the base-emitter
voltage of a transistor, operated at constant current. Unfortu-
nately, this technique requires calibration to null out the effect
of the absolute value of VBE, which varies from device to device.
The results of the local and remote temperature measurements
are stored in the Local and Remote Temperature Value Registers,
and are compared with limits programmed into the Local and
Remote High and Low Limit Registers.
The technique used in the ADM1032 is to measure the change
in VBE when the device is operated at two different currents.
Table I. Temperature Data Format (Local Temperature and
Remote Temperature High Byte)
This is given by:
KT
Temperature
Digital Output
∆VBE = n
× In N
(
)
(
)
f
q
0°C
1°C
0 000 0000
0 000 0001
0 000 1010
0 001 1001
0 011 0010
0 100 1011
0 110 0100
0 111 1101
0 111 1111
where:
K is Boltzmann’s constant (1.38 × 10–23).
10°C
25°C
50°C
75°C
100°C
125°C
127°C
q is charge on the electron (1.6 × 10–19 Coulombs).
T is absolute temperature in Kelvins.
N is ratio of the two currents.
nf is the ideality factor of the thermal diode.
The ADM1032 is trimmed for an ideality factor of 1.008.
V
DD
I
N ꢄ I
I
BIAS
D+
C1*
D–
V
OUT+
TO ADC
REMOTE
SENSING
TRANSISTOR
V
BIAS
DIODE
OUT–
LOW-PASS FILTER
fC = 65kHz
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
C1 = 2.2nF TYPICAL, 3nF MAX.
Figure 2. Input Signal Conditioning
–5–
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