ADM1026
If the VREF output is not being used, it should be left uncon-
nected. Do not connect VREF to GND using a capacitor. The
internal output buffer on the voltage reference is capacitively
loaded, which can cause the voltage reference to oscillate. This
affects temperature readings reported back by the ADM1026.
The recommended interface circuit for the VREF output is shown
in Figure 33.
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 ADM1026 is to measure the change
in Vbe when the device is operated at two different currents,
given by
+12V
ADM1026
24
K ×T
ΔVbe
=
× logn(N )
10kΩ
V
REF
q
NDT3055
where K is Boltzmann’s constant, q is the charge on the carrier,
T is the absolute temperature in Kelvins, and N is the ratio of
the two currents.
V
REF
0.1µF
50Ω
10µF
0.1µF
Figure 34 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, but it could
equally well be a discrete transistor such as a 2N3904.
Figure 33. VREF Interface Circuit for VREF Loads > 2 mA
TEMPERATURE MEASUREMENT SYSTEM
Local Temperature Measurement
If a discrete transistor is used, the collector is not 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.
The ADM1026 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 1Fh). As both positive and negative temper-
atures can be measured, the temperature data is stored in twos
complement format, as shown in Table 7. Theoretically, the
temperature sensor and ADC can measure temperatures from
−128°C to +127°C with a resolution of 1°C. Temperatures below
TMIN and above TMAX are outside the operating temperature
range of the device, however, so local temperature measure-
ments outside this range are not possible. Temperature
measurement from −128°C to +127°C is possible using a
remote sensor.
To prevent ground noise from 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.
To measure ΔVbe, 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, and to a
chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce a
DC voltage proportional to ΔVbe. 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. A remote temperature measurement
takes nominally 2.14 ms.
Remote Temperature Measurement
The ADM1026 can measure the temperature of two remote
diode sensors, or diode-connected transistors, connected to
Pins 25 and 26, or 27 and 28.
Pins 25 and 26 are a dedicated temperature input channel.
Pins 27 and 28 can be configured to measure a diode sensor by
clearing Bit 3 of Configuration Register 1 (Address 00h) to 0.
If this bit is 1, then Pins 27 and 28 are AIN8 and AIN9
.
Rev. A | Page 20 of 56
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