LM334MSLASHNOPB datasheet(7/25 Pages) TI1

7 / 25 page

LM134, LM234, LM334

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SNVS746C – MAY 2004 – REVISED MARCH 2005

Figure 14. Gain Adjustment

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

RSET or on the load resistor used to terminate the LM134. Slope error after trim will normally be less than ±1%.

To maintain this accuracy, however, a low temperature coefficient resistor must be used for RSET.

A 33 ppm/°C drift of RSET will give a 1% slope error because the resistor will normally see about the same

temperature variations as the LM134. Separating RSET from the LM134 requires 3 wires and has lead resistance

problems, so is not normally recommended. Metal film resistors with less than 20 ppm/°C 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 configuration can cancel the temperature-dependent

characteristic of the LM134. The circuit shown in Figure 15 balances the positive tempco of the LM134 (about

+0.23 mV/°C) with the negative tempco of a forward-biased silicon diode (about

−2.5 mV/°C).

Figure 15. Zero Tempco Current Source

The set current (ISET) is the sum of I1 and I2, each contributing approximately 50% of the set current, and IBIAS.

IBIAS is usually included in the I1 term by increasing the VR value used for calculations by 5.9%. (See

CALCULATING RSET.)

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