ADM1063ASU datasheet(23/36 Pages) AD

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ADM1063

Rev. 0 | Page 23 of 36

TEMPERATURE MEASUREMENT SYSTEM

The ADM1063 contains an on-chip, band gap temperature

sensor, whose output is digitized by the on-chip, 12-bit ADC.

Theoretically, the temperature sensor and ADC can measure

temperatures from −128°C to +127°C with a resolution of

0.125°C. Because this exceeds the operating temperature range

of the device, local temperature measurements outside this

range are not possible. Temperature measurements from

−128°C to +127°C are possible using a remote sensor. The

output code is in offset binary format, with −128°C given by

Code 0x400, 0°C given by Code 0x800, and +127°C given by

Code 0xC00.

As with the other analog inputs to the ADC, a limit register is

provided for each of the temperature input channels. Therefore,

a temperature limit can be set such that if it is exceeded, a warning

is generated and available as an input to the sequencing engine.

This enables users to control their sequence or monitor functions

based on an overtemperature or undertemperature event.

REMOTE TEMPERATURE MEASUREMENT

The ADM1063 can measure the temperature of two remote

diode sensors or diode-connected transistors connected to the

DxN and DxP pins.

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, making the technique unsuitable for

mass production. The technique used in the ADM1063 is to

measure the change in Vbe when the device is operated at two

different currents.

This is given by

∆Vbe = kT/q × ln(N)

where:

is Boltzmann’s constant.

is charge on the carrier.

is absolute temperature in Kelvin.

is ratio of the two currents.

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 be a

discrete transistor such as a 2N3904 or 2N3906.

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 DxN input and the emitter is connected to the

DxP input. If an NPN transistor is used, the emitter is connected

to the DxN input and the base is connected to the DxP input.

Figure 35 and Figure 36 show how to connect the ADM1063

to an NPN or PNP transistor for temperature measurement. To

prevent ground noise from 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

DxN input.

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 and through

a chopper-stabilized amplifier that amplifies and rectifies the

waveform to produce a dc voltage proportional to ∆Vbe. This

voltage is measured by the ADC to produce a temperature

output in 12-bit offset binary. 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 600 ms. The results of remote temperature

measurements are stored in 12-bit, offset binary format, as

shown in Table 10. This provides temperature readings

with a resolution of 0.125°C.

Table 10. Temperature Data Format

Temperature

Digital Output (Hex)

Digital Output (Bin)

−128 °C

400

010000000000

−125 °C

418

010000011000

−100 °C

4E0

010011100000

−75 °C

5A8

010110101000

−50 °C

600

011000000000

−25 °C

670

011001110000

−10 °C

7B0

011110110000

0 °C

800

100000000000

+10.25 °C

852

100001010010

+25.5 °C

8CC

100011001100

+50.75 °C

996

100110010110

+75 °C

A58

101001011000

+100 °C

B48

101101001000

+125 °C

BE8

101111101000

+128 °C

C00

110000000000

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