AD7294BCPZRL datasheet(25/48 Pages) AD

25 / 48 page

AD7294

Data Sheet

Rev. I | Page 24 of 47

increased about the preset (VREF × 1.2)/12.5. If this occurs, the

ISENSE OVERRANGE pin is set to a high logic level enabling

appropriate action to be taken to prevent any damage to the

external circuitry.

The setpoint threshold level is fixed internally in the AD7294,

and the current sense amplifier saturates above this level. The

comparator also triggers if a voltage of less than AVDD is applied

to the RSENSE or VPP pin.

TEMPERATURE SENSOR

The AD7294 contains one local and two remote temperature

sensors. The temperature sensors continuously monitor the

three temperature inputs and new readings are automatically

available every 5 ms.

The on-chip, band gap temperature sensor measures the temper-

ature of the system. Diodes are used in conjunction with the two

remote temperature sensors to monitor the temperature of other

critical board components.

LIMIT

REGISTERS

TEMP

SENSOR

ALERT

D2+

D2–

D1+

D1–

AD7294

CAP

REMOTE

SENSING

TRANSISTORS

16 × I

I-BIAS

MUX

VDD

BIAS DIODE

TO ADC

4 × I

fC = 65kHz

LPF

Figure 45. Internal and Remote Temperature Sensors

The temperature sensor module on the AD7294 is based on the

three current principle (see Figure 45), 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.

Each input integrates, in turn, over a period of several hundred

microseconds. This takes place continuously in the background,

leaving the user free to perform conversions on the other channels.

When integration is complete, a signal passes to the control logic

to initiate a conversion automatically. If the ADC is in command

mode, the temperature conversion is performed as soon as the

next conversion is completed. In autocycle mode, the conversion

is inserted into an appropriate place in the current sequence; see

the Register Setting section for further details. If the ADC is

idle, the conversion takes place immediately.

Three registers store the result of the last conversion on each

temperature channel; these can be read at any time. In addition,

in command mode, one or both of the two external channel

registers can be read out as part of the output sequence.

Remote Sensing Diode

The AD7294 is designed to work with discrete transistors,

2N3904 and 2N3906. If an alternative transistor is used, the

AD7294 operates as specified provided the following conditions

are adhered to.

Ideality Factor

The ideality factor, nf, of the transistor is a measure of the

deviation of the thermal diode from ideal behavior. The

AD7294 is trimmed for an nf value of 1.008. Use the following

equation to calculate the error introduced at a Temperature T

(°C) when using a transistor whose nf does not equal 1.008:

ΔT = (nf − 1.008) × (273.15 K + T)

To factor this in, the user can write the ΔT value to the offset

from, the temperature measurement.

Base Emitter Voltage

The AD7294 operates as specified provided that the base-

emitter voltage is greater than 0.25 V at 8 μA at the highest

operating temperature, and less than 0.95 V at 128 μA for the

lowest operating temperature.

Base Resistance

The base resistance should be less than 100 Ω.

hFE Variation

Use a transistor with small variation in hFE (approximately 50 to

150). Small variation in hFE indicates tight control of the VBE

characteristics.

For RF applications, the use of high Q capacitors functioning as a

filter protects the integrity of the measurement. These capacitors,

such as Johanson Technology 10 pF high Q capacitors: Reference

Code 500R07S100JV4T, should be connected between the base and

the emitter, as close to the external device as possible. However,

large capacitances affect the accuracy of the temperature measure-

ment; thus, the recommended maximum capacitor value is 100 pF.

In most cases, a capacitor is not required; the selection of any

capacitor is dependent on the noise frequency level.

2N3904

NPN

AD7294

D+

D–

10pF

Figure 46. Measuring Temperature Using an NPN Transistor

10pF

2N3906

PNP

AD7294

D+

D–

Figure 47. Measuring Temperature Using a PNP Transistor

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