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LM234Z-6

Constant Current Source and Temperature Sensor

Linear Technology 

LM134 Series

APPLICATIO S I FOR ATIO

Lead Resistance

The sense voltage which determines the operating current

of the LM134 is less than 100mV. At this level, thermo-

couple or lead resistance effects should be minimized by

locating the current setting resistor physically close to the

device. Sockets should be avoided if possible. It takes only

0.7Ω contact resistance to reduce output current by 1% at

the 1mA level.

Start-Up Time

The LM134 is designed to operate at currents as low as

1µA. This requires that internal biasing current be well

below that level because the device achieves its wide

operating current range by using part of the operating

current as bias current for the internal circuitry. To ensure

start-up, however, a fixed trickle current must be provided

internally. This is typically in the range of 20nA to 200nA

and is provided by the special ultralow IDDS FETs shown in

the Schematic Diagrams as Q7 and Q8. The start-up time

of the LM134 is determined by the IDSS of these FETs and

the capacitor C1. This capacitor must charge to approxi-

mately 500mV before Q3 turns on to start normal circuit

operation. This takes as long as (500mV)(50pF)/(20nA) =

1.25ms for very low IDSS values.

Using the LM134 as a Temperature Sensor

Because it has a highly linear output characteristic, the

LM134 makes a good temperature sensor. It is particularly

useful in remote sensing applications because it is a

current output device and is therefore not affected by long

wire runs. It is easy to calibrate, has good long term

stability and can be interfaced directly with most data

acquisition systems, eliminating the expensive preampli-

fiers required for thermocouples and platinum sensors.

A typical temperature sensor application is shown in

Figure␣ 2. The LM134 operating current at 25°C is set at

298µA by the 226Ω resistor, giving an output of 1µA/°K.

The current flows through the twisted pair sensor leads to

the 10k termination resistor, which converts the current

output to a voltage of 10mV/°K referred to ground. The

voltage across the 10k resistor will be 2.98V at 25°C, with

a slope of 10mV/°C. The simplest way to convert this

signal to a Centigrade scale is to subtract a constant 2.73V

in software. Alternately, a hardware conversion can be

used, as shown in Figure 3, using an LT1009 as a level

shifter to offset the output to a Centigrade scale.

The resistor (RSET) used to set the operating current of the

LM134 in temperature sensing applications should have

low temperature coefficient and good long term stability.

A 30ppm/°C drift in the resistor will change the slope of the

temperature sensor by 1%, assuming that the resistor is

at the same temperature as the sensor, which is usually the

case since the resistor should be located physically close

to the LM134 to prevent errors due to wire resistance. A

long term shift of 0.3% in the resistor will create a 1°C

temperature error. The long term drift of the LM134 is

typically much better than this, so stable resistors must be

used for best long term performance.

Calibration of the LM134 as a temperature sensor is

extremely easy. Referring to Figure 2, calibration is achieved

by trimming the termination resistor. This theoretically

trims both zero and slope simultaneously for Centigrade

and Fahrenheit applications. The initial errors in the LM134

are directly proportional to absolute temperature, just like

the actual output. This allows the sensor to be trimmed at

any temperature and have the slope error be corrected at

the same time. Residual slope error is typically less than

1% after this single trim is completed.

VS ≥ 5V

TO DATA

ACQUISITION

SYSTEM

10mV/°K 9.53k

1k

CALIBRATE

I = 1µA/°K

V+

LM234-3

R

RSET

V–

226Ω

134 F02

Figure 2 Kelvin Temperature Sensor

6

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