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ADM1025A Datasheet(PDF) 13 Page - ON Semiconductor

Part No. ADM1025A
Description  Low Cost PC Hardware Monitor ASIC
Download  21 Pages
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

ADM1025A Datasheet(HTML) 13 Page - ON Semiconductor

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Preliminary Technical Data
Rev. P5 | Page 13 of 21| www.onsemi.com
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 6. 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.
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 different
currents. This is given by:
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 15 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
temperature 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−.
Figure 15. Signal Conditioning for External Diode Temperature Sensors
Table 6. Temperature Data Format
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 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, then to a
chopperstabilized 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.
An external temperature measurement takes nominally 34.8 ms.

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