ADM1026JSTZ-R7 datasheet(19/55 Pages) ONSEMI

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ADM1026

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If the VREF output is not being used, it should be left

unconnected. 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 32.

Figure 32. VREF Interface Circuit for VREF Loads > 2 mA

10k

0.1F

ADM1026

+12V

0.1F

10F

50

VREF

NDT3055

VREF

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

temperatures can be measured, the temperature data is stored

in twos complement format, as shown in Table 8.

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 measurements 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:

(eq. 10)

DVbe +

log n (N)

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 33 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

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

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.

To measure

DVbe, 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

DVbe. 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.

Figure 33. Signal Conditioning for Remote Diode Temperature Sensors

C1*

D+

D–

REMOTE

SENSING

TRANSISTOR

N x I

IBIAS

VDD

VOUT+

TO ADC

VOUT–

BIAS

DIODE

LOW−PASS FILTER

fC = 65kHz

CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

C1 = 2.2nF TYPICAL, 3nF MAX.

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