Datasheet LM334S (Analog Devices) - 6

LM134 Series
U U W U APPLICATIO S I FOR ATIO Lead Resistance
voltage across the 10k resistor will be 2.98V at 25°C, with a slope of 10mV/°C. The simplest way to convert this The sense voltage which determines the operating current signal to a Centigrade scale is to subtract a constant 2.73V of the LM134 is less than 100mV. At this level, thermo- in software. Alternately, a hardware conversion can be couple or lead resistance effects should be minimized by used, as shown in Figure 3, using an LT1009 as a level locating the current setting resistor physically close to the shifter to offset the output to a Centigrade scale. device. Sockets should be avoided if possible. It takes only 0.7Ω contact resistance to reduce output current by 1% at The resistor (RSET) used to set the operating current of the the 1mA level. LM134 in temperature sensing applications should have low temperature coefficient and good long term stability.
Start-Up Time
A 30ppm/°C drift in the resistor will change the slope of the The LM134 is designed to operate at currents as low as temperature sensor by 1%, assuming that the resistor is 1µA. This requires that internal biasing current be well at the same temperature as the sensor, which is usually the below that level because the device achieves its wide case since the resistor should be located physically close operating current range by using part of the operating to the LM134 to prevent errors due to wire resistance. A current as bias current for the internal circuitry. To ensure long term shift of 0.3% in the resistor will create a 1°C start-up, however, a fixed trickle current must be provided temperature error. The long term drift of the LM134 is internally. This is typically in the range of 20nA to 200nA typically much better than this, so stable resistors must be and is provided by the special ultralow I used for best long term performance. DDS FETs shown in the Schematic Diagrams as Q7 and Q8. The start-up time Calibration of the LM134 as a temperature sensor is of the LM134 is determined by the IDSS of these FETs and extremely easy. Referring to Figure 2, calibration is achieved the capacitor C1. This capacitor must charge to approxi- by trimming the termination resistor. This theoretically mately 500mV before Q3 turns on to start normal circuit trims both zero and slope simultaneously for Centigrade operation. This takes as long as (500mV)(50pF)/(20nA) = and Fahrenheit applications. The initial errors in the LM134 1.25ms for very low IDSS values. are directly proportional to absolute temperature, just like the actual output. This allows the sensor to be trimmed at
Using the LM134 as a Temperature Sensor
any temperature and have the slope error be corrected at Because it has a highly linear output characteristic, the the same time. Residual slope error is typically less than LM134 makes a good temperature sensor. It is particularly 1% after this single trim is completed. useful in remote sensing applications because it is a current output device and is therefore not affected by long VS ≥ 5V wire runs. It is easy to calibrate, has good long term V+ LM234-3 stability and can be interfaced directly with most data R acquisition systems, eliminating the expensive preampli- TO DATA R fiers required for thermocouples and platinum sensors. ACQUISITION SET SYSTEM V– 226Ω 10mV/°K A typical temperature sensor application is shown in 9.53k I = 1µA/°K Figure␣ 2. The LM134 operating current at 25°C is set at 1k CALIBRATE 134 F02 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
Figure 2 Kelvin Temperature Sensor
6