AD7294 datasheet(18/45 Pages) AD

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AD7294

Preliminary Technical Data

Rev. PrB | Page 18 of 45

TEMPERATURE SENSOR

The AD7294 consists of one local and two remote temperature

sensors. The analog input multiplexer can alternately select

either the on-chip band gap temperature sensor, to measure the

temperature of the system, or one of the two remote diode

temperature sensors. The 12-bit ADC digitizes these signals,

and the results are stored in the TSENSEINT, TSENSE1, and TSENSE2

registers. These results are compared with their respective

DATALOW, DATAHIGH, and hysteresis registers. Out-of-limit

comparisons generate flags, and further information on these

are stored in the alert registers. A result that exceeds the high

temperature limit, the low temperature limit, or an external

diode fault causes the ALERT output to assert high.

LIMIT

REGISTERS

MUX

TEMP

SENSOR

ALERT

D1 (+)

D1 (-)

D0 (-)

AD7294

D0 (+)

CAP

REMOTE

SENSING

TRANSISTORS

N*I

I-BIAS

MUX

VDD

BIAS

DIODE

TO ADC

LOW PASS FILTER

fc = 65 KHz

Figure 20. Internal and Remote Temperature Sensors

The temperature sensor module on the AD7294 is based on the

3-current principle, see Figure 20, where three currents are

passed through a diode and the forward voltage drop is

measured at each diode, allowing the temperature to be

calculated free of errors caused by series resistance.

Temperature Measurement Method

The AD7294 can measure the temperature of two remote diode

sensors or diode-connected transistors connected from D0(+)

to D0(−) and from D1(+) to D1(−).

The forward voltage of a diode or diode-connected transistor

operated at 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; therefore, the technique is unsuitable for

mass production.

The technique used in the AD7294 is to measure the change in

VBE when the device is operated at three different currents, see

Figure 20.

This is given by

ΔVBE = KT/q × 1n(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.

If a discrete transistor is used for T1 and T0, such as a

2N3904/2N3906, 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. Figure 21 and Figure 22

show how to connect the AD7294 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.

2N3904

NPN

AD7294

D+

D–

Figure 21. Measuring Temperature Using an NPN Transistor

2N3906

PNP

AD7294

D+

D–

Figure 22. Measuring Temperature Using a PNP Transistor

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 10-bit, twos

complement format.

Series Resistance Cancellation

Parasitic resistance to the D+ and D− inputs to the AD7294,

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 offset per ohm of parasitic resistance in series with the

remote diode.

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