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LM334MX Datasheet(PDF) 6 Page  National Semiconductor (TI) 


6 page Application Hints (Continued) NOISE Current noise generated by the LM134 is approximately 4 times the shot noise of a transistor. If the LM134 is used as an active load for a transistor amplifier, input referred noise will be increased by about 12dB. In many cases, this is ac ceptable and a single stage amplifier can be built with a volt age gain exceeding 2000. LEAD RESISTANCE The sense voltage which determines operating current of the LM134 is less than 100mV. At this level, thermocouple or lead resistance effects should be minimized by locating the current setting resistor physically close to the device. Sock ets should be avoided if possible. It takes only 0.7 â„¦ contact resistance to reduce output current by 1% at the 1 mA level. SENSING TEMPERATURE The LM134 makes an ideal remote temperature sensor be cause its current mode operation does not lose accuracy over long wire runs. Output current is directly proportional to absolute temperature in degrees Kelvin, according to the fol lowing formula: Calibration of the LM134 is greatly simplified because of the fact that most of the initial inaccuracy is due to a gain term (slope error) and not an offset. This means that a calibration consisting of a gain adjustment only will trim both slope and zero at the same time. In addition, gain adjustment is a one point trim because the output of the LM134 extrapolates to zero at 0ËšK, independent of R SET or any initial inaccuracy. This property of the LM134 is illustrated in the accompanying graph. Line abc is the sensor current before trimming. Line a'b'c' is the desired output. A gain trim done at T2 will move the output from b to b' and will simultaneously correct the slope so that the output at T1 and T3 will be correct. This gain trim can be done on R SET or on the load resistor used to terminate the LM134. Slope error after trim will normally be less than Â±1%. To maintain this accuracy, however, a low temperature coefficient resistor must be used for R SET. A 33 ppm/ËšC drift of R SET will give a 1% slope error because the resistor will normally see about the same temperature variations as the LM134. Separating R SET from the LM134 requires 3 wires and has lead resistance problems, so is not normally recommended. Metal film resistors with less than 20 ppm/ËšC drift are readily available. Wire wound resistors may also be used where best stability is required. APPLICATION AS A ZERO TEMPERATURE COEFFICENT CURRENT SOURCE Adding a diode and a resistor to the standard LM134 con figuration can cancel the temperaturedependent character istic of the LM134. The circuit shown in Figure 3 balances the positive tempco of the LM134 (about +0.23 mV/ËšC) with the negative tempco of a forwardbiased silicon diode (about âˆ’2.5 mV/ËšC). The set current (I SET) is the sum of I1 and I2, each contribut ing approximately 50% of the set current, and I BIAS.IBIAS is usually included in the I 1 term by increasing the VR value used for calculations by 5.9%. (See CALCULATING R SET.) The first step is to minimize the tempco of the circuit, using the following equations. An example is given using a value of +227ÂµV/ËšC as the tempco of the LM134 (which includes the I BIAS component), and âˆ’2.5 mV/ËšC as the tempco of the di ode (for best results, this value should be directly measured or obtained from the manufacturer of the diode). With the R 1 to R2 ratio determined, values for R1 and R2 should be determined to give the desired set current. The formula for calculating the set current at T = 25ËšC is shown below, followed by an example that assumes the forward voltage drop across the diode (V D) is 0.6V, the voltage across R 1 is 67.7mV (64 mV + 5.9% to account for IBIAS), and R 2/R1 = 10 (from the previous calculations). DS0056974 FIGURE 2. Gain Adjustment DS00569728 FIGURE 3. Zero Tempco Current Source www.national.com 6 
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