LM34

www.ti.com

SNIS161D – MARCH 2000 – REVISED JANUARY 2016

7 Detailed Description

7.1 Overview

The LM34 series devices are precision integrated-circuit temperature sensors, whose output voltage is linearly

proportional to the Fahrenheit temperature. The LM34 device has an advantage over linear temperature sensors

calibrated in degrees Kelvin, because the user is not required to subtract a large constant voltage from its output

to obtain convenient Fahrenheit scaling. The LM34 device does not require any external calibration or trimming

to provide typical accuracies of ±1/2°F at room temperature and ±1-1

⁄2°F over a full −50°F to 300°F temperature

range. Lower cost is assured by trimming and calibration at the wafer level. The low output impedance, linear

output, and precise inherent calibration of the LM34 device makes interfacing to readout or control circuitry

especially easy. It can be used with single power supplies or with plus and minus supplies. Because the LM34

device draws only 75 µA from its supply, the device has very low self-heating, less than 0.2°F in still air.

The temperature sensing element is comprised of a simple base emitter junction that is forward biased by a

current source. The temperature sensing element is buffered by an amplifier and provided to the OUT pin. The

amplifier has a simple class-A output stage thus providing a low impedance output that can source 16

μA and

sink 1

μA.

typical 0.5-

Ω output impedance as shown in the Functional Block Diagram. Therefore, the LM34 device can only

source current and the sinking capability of the device is limited to 1 µA.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Capacitive Drive Capability

Like most micropower circuits, the LM34 device has a limited ability to drive heavy capacitive loads. The LM34

device, by itself, is able to drive 50 pF without special precautions. If heavier loads are anticipated, it is easy to

isolate or decouple the load with a resistor; see Figure 12. You can improve the tolerance of capacitance with a

series R-C damper from output to ground; see Figure 13. When the LM34 is applied with a 499-

Ω load resistor

(as shown Figure 18 and Figure 19), the device is relatively immune to wiring capacitance because the

capacitance forms a bypass from ground to input, not on the output. However, as with any linear circuit

connected to wires in a hostile environment, its performance can be affected adversely by intense

electromagnetic sources such as relays, radio transmitters, motors with arcing brushes, transients of the SCR,

and so on, as the wiring of the device can act as a receiving antenna and the internal junctions can act as

rectifiers. For best results in such cases, a bypass capacitor from VIN to ground and a series R-C damper, such

as 75

Ω in series with 0.2 μF or 1 μF from output to ground, are often useful. See Figure 23, Figure 24 and

Figure 26 for more details.

Copyright © 2000–2016, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM34