2.0 Mounting and Thermal

Conductivity

The LM94021 can be applied easily in the same way as

other integrated-circuit temperature sensors. It can be glued

or cemented to a surface.

To ensure good thermal conductivity, the backside of the

LM94021 die is directly attached to the GND pin (Pin 2). The

temperatures of the lands and traces to the other leads of the

LM94021 will also affect the temperature reading.

Alternatively, the LM94021 can be mounted inside a sealed-

end metal tube, and can then be dipped into a bath or

screwed into a threaded hole in a tank. As with any IC, the

LM94021 and accompanying wiring and circuits must be

kept insulated and dry, to avoid leakage and corrosion. This

is especially true if the circuit may operate at cold tempera-

tures where condensation can occur. If moisture creates a

short circuit from the output to ground or V

from the LM94021 will not be correct. Printed-circuit coatings

are often used to ensure that moisture cannot corrode the

leads or circuit traces.

The thermal resistance junction to ambient (

θ

rameter used to calculate the rise of a device junction tem-

perature due to its power dissipation. The equation used to

calculate the rise in the LM94021’s die temperature is

where T

current, I

output voltage. For example, in an application where T

˚C, V

mV, and I

˚C, showing a self-heating error of only 0.021˚C. Since the

LM94021’s junction temperature is the actual temperature

being measured, care should be taken to minimize the load

current that the LM94021 is required to drive. Figure 1 shows

the thermal resistance of the LM94021.

3.0 Noise Considerations

The LM94021 has excellent noise rejection (the ratio of the

AC signal on V

tests, sine wave rejection of -54 dB or better was observed

over 200 Hz to 10 kHz; Also, -28 dB or better was observed

from 10 kHz to 1 MHz. A load capacitor on the output can

help filter noise; for example,a1nF load capacitor resulted

in -51 dB or better from 200 Hz to 1 MHz.

There is no specific requirement for the use of a bypass

capacitor close to the LM94021 because it does not draw

transient currents. For operation in very noisy environments,

some bypass capacitance may be required. The capacitance

does not need to be in close proximity to the LM94021. The

LM94021 has been bench tested successfully with a bypass

capacitor as far as 6 inches away. In fact, it can be powered

by a properly-bypassed logic gate.

4.0 Capacitive Loads

The LM94021 handles capacitive loading well. In an ex-

tremely noisy environment, or when driving a switched sam-

pling input on an ADC, it may be necessary to add some

filtering to minimize noise coupling. Without any precautions,

the LM94021 can drive a capacitive load less than or equal

to 1100 pF as shown in Figure 2. For capacitive loads

greater than 1100 pF, a series resistor may be required on

the output, as shown in Figure 3.

5.0 Output Voltage Shift

The LM94021 is very linear over temperature and supply

voltage range. Due to the intrinsic behavior of an NMOS/

PMOS rail-to-rail buffer, a slight shift in the output can occur

when the supply voltage is ramped over the operating range

of the device. The location of the shift is determined by the

relative levels of V

when V

This slight shift (a few millivolts) takes place over a wide

change (approximately 200 mV) in V

shift takes place over a wide temperature change of 5˚C to

20˚C, V

tions in the Electrical Characteristics table already include

this possible shift.

Device Number

NS Package

Number

Thermal

Resistance (

θ

LM94021BIMG

MAA05A

415˚C/W

FIGURE 1. LM94021 Thermal Resistance

20108615

FIGURE 2. LM94021 No Decoupling Required for

Capacitive Loads Less than 1100 pF.

20108633

C

LOAD

R

S

1.1 nF to

99 nF

3k

Ω

100 nF to

999 nF

1.5 k

Ω

1 µF

800

Ω

FIGURE 3. LM94021 with series resistor for capacitive

Loading greater than 1100 pF.

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