ADM1025A datasheet(13/21 Pages) ONSEMI | Low Cost PC Hardware Monitor ASIC

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Preliminary Technical Data

ADM1025/ADM1025A

Rev. P5 | Page 13 of 21| www.onsemi.com

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 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.

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 different

currents. This is given by:

)

(

VBE

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 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

Temperature

Digital Output

−128°C

1000 0000

−125°C

1000 0011

−100°C

1001 1100

−75°C

1011 0101

−50°C

1100 1110

−25°C

1110 0111

0°C

0000 0000

+10°C

0000 1010

+25°C

0001 1001

+50°C

0011 0010

+75°C

0100 1011

+100°C

0110 0100

+125°C

0111 1101

+127°C

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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