ADT75ARM datasheet(21/24 Pages) AD

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ADT75

Rev. 0 | Page 21 of 24

APPLICATION INFORMATION

THERMAL RESPONSE TIME

The time required for a temperature sensor to settle to a

specified accuracy is a function of the thermal mass of the

sensor and the thermal conductivity between the sensor and the

object being sensed. Thermal mass is often considered

equivalent to capacitance. Thermal conductivity is commonly

specified using the symbol Q, and can be thought of as thermal

resistance. It is commonly specified in units of degrees per watt

of power transferred across the thermal joint. Thus, the time

required for the ADT75 to settle to the desired accuracy is

dependent on the package selected, the thermal contact

established in that particular application, and the equivalent

power of the heat source. In most applications, it is best to

determine empirically the settling time.

SELF-HEATING EFFECTS

The temperature measurement accuracy of the ADT75 might

be degraded in some applications due to self-heating. Errors can

be introduced from the quiescent dissipation and power

dissipated when converting. The magnitude of these

temperature errors is dependent on the thermal conductivity of

the ADT75 package, the mounting technique, and the effects of

airflow. At 25°C, static dissipation in the ADT75 is typically

798.6 μW operating at 3.3 V. In the 8-lead MSOP package

mounted in free air, this accounts for a temperature increase

due to self-heating of

ΔT = PDISS × θJA = 798.6 μW × 205.9°C/W = 0.16°C

It is recommended that current dissipated through the device be

kept to a minimum, because it has a proportional effect on the

temperature error.

Using the power-down mode can reduce the current dissipated

through the ADT75 subsequently reducing the self-heating

affect. When the ADT75 is in power-down mode and operating

at 25°C, static dissipation in the ADT75 is typically 78.6 μW

with VDD = 3.3 V and the power-up/conversion rate is 1 SPS

(sample per second). In the 8-lead MSOP package mounted in

free air, this accounts for a temperature increase due to self-

heating of

ΔT = PDISS × θJA = 78.6 μW × 205.9°C/W = 0.016°C

SUPPLY DECOUPLING

The ADT75 should be decoupled with a 0.1 μF ceramic

capacitor between VDD and GND. This is particularly important

when the ADT75 is mounted remotely from the power supply.

Precision analog products, such as the ADT75, require a well-

filtered power source. Because the ADT75 operates from a

single supply, it might seem convenient to tap into the digital

logic power supply. However, the logic supply is often a switch-

mode design, which generates noise in the 20 kHz to 1 MHz

range. In addition, fast logic gates can generate glitches

hundreds of mV in amplitude due to wiring resistance and

inductance.

If possible, the ADT75 should be powered directly from the

system power supply. This arrangement, shown in Figure 22,

isolates the analog section from the logic switching transients.

Even if a separate power supply trace is not available, generous

supply bypassing reduces supply-line induced errors. Local

supply bypassing consisting of a 0.1 μF ceramic capacitor is

critical for the temperature accuracy specifications to be

achieved. This decoupling capacitor must be placed as close as

possible to the ADT75 VDD pin.

0.1

μF

ADT75

TTL/CMOS

LOGIC

CIRCUITS

POWER

SUPPLY

Figure 22. Use Separate Traces to Reduce Power Supply Noise

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