EVAL-ADM1026EB datasheet(20/56 Pages) AD

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ADM1026

Rev. A | Page 20 of 56

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.

10k

Ω

0.1

µF

ADM1026

+12V

0.1

µF

Ω

VREF

NDT3055

VREF

Figure 33. VREF Interface Circuit for VREF Loads > 2 mA

TEMPERATURE MEASUREMENT SYSTEM

Local Temperature Measurement

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

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.

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.

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

()

log

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.

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.

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.

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.

EVAL-ADM1026EB Datasheet(PDF) 20 Page - Analog Devices

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