ADT7467ARQZ-R7 datasheet(17/77 Pages) ONSEMI

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ADT7467

Rev. 3 | Page 17 of 77 | www.onsemi.com

Remote Temperature Measurement

The ADT7467 can measure the temperature of two remote

diode sensors or diode-connected transistors connected to

Pin 10 and Pin 11 or to Pin 12 and Pin 13.

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 each device and thus requires

individual calibration; therefore, the technique is unsuitable for

mass production. The technique used in the ADT7467 is to

measure the change in VBE when the device is operated at three

currents. This is given by

ΔVBE = kT/q × ln(N)

where:

k is Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvin.

N is the ratio of the two currents.

Figure 23 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. It could also

be a discrete transistor such as a 2N3904/2N3906.

N2 × I

IN1 × I IBIAS

D+

D–

LPF

VDD

VOUT+

VOUT–

fC = 65kHz

TO ADC

REMOTE

SENSING

TRANSISTOR

Figure 23. Signal Conditioning for Remote Diode Temperature Sensors

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 is connected to the

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

to the D− input and the base is connected to the D+ input.

Figure 25 and Figure 26 show how to connect the ADT7467 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

D− input.

To measure ΔVBE, the operating current through the sensor is

switched among three related currents. Shown in Figure 23,

N1 × I and N2 × I are different multiples of the current I. The

currents through the temperature diode are switched between

I and N1 × I, resulting in ΔVBE1; then they are switched between

I and N2 × I, resulting in ΔVBE2. The temperature can then be

calculated using the two ΔVBE measurements. This method can

also cancel the effect of series resistance on the temperature

measurement.

The resulting ΔVBE waveforms are passed through a 65 kHz

low-pass filter to remove noise and then sent to a chopper-

stabilized amplifier that amplifies and rectifies the waveform to

produce a dc voltage proportional to ΔVBE. The ADC digitizes

this voltage, and a temperature measurement is produced. To

reduce the effects of noise, digital filtering is performed by

averaging the results of 16 measurement cycles.

The results of remote temperature measurements are stored in

10-bit twos complement format, as listed in Table 7. The extra

resolution for the temperature measurements is held in the

Extended Resolution Register 2 (0x77). This produces tempera-

ture readings with a resolution of 0.25°C.

SERIES RESISTANCE CANCELLATION

Parasitic resistance to the ADT7467 D+ and D− inputs (seen in

series with the remote diode) is caused by a variety of factors,

including PCB track resistance and track length. This series

resistance appears as a temperature offset in the remote sensor’s

temperature measurement. This error typically causes a 0.5°C off-

set per 1 Ω of parasitic resistance in series with the remote diode.

The ADT7467 automatically cancels the effect of this series

resistance on the temperature reading, providing a more

accurate result without the need for user characterization of this

resistance. The ADT7467 is designed to automatically cancel,

typically up to 3 kΩ of resistance. By using an advanced

temperature measurement method, this is transparent to the

user. This feature allows resistances to be added to the sensor

path to produce a filter, allowing the part to be used in noisy

environments. See the Noise Filtering section for details.

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice involved placing a capacitor across the D+

and D− pins to help combat the effects of noise. However, large

capacitances affect the accuracy of the temperature measurement,

leading to a recommended maximum capacitor value of 1000 pF.

A capacitor of this value reduces the noise but does not eliminate

it, making use of the sensor difficult in a very noisy environment.

The ADT7467 has a major advantage over other devices for

eliminating the effects of noise on the external sensor. Using the

series resistance cancellation feature, a filter can be constructed

between the external temperature sensor and the device. The

effect of filter resistance seen in series with the remote sensor is

automatically canceled from the temperature result.

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