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LM231 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM231 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 15 page Typical Applications (Continued) condition will usually apply under start-up conditions or in the case of an overload voltage at signal input. It should be noted that during this sort of overload, the output frequency will be 0; as soon as the signal is restored to the working range, the output frequency will be resumed. The output driver transistor acts to saturate pin 3 with an ON resistance of about 50 Ω. In case of overvoltage, the output current is actively limited to less than 50 mA. The voltage at pin 2 is regulated at 1.90 V DC for all values of i between 10 µA to 500 µA. It can be used as a voltage ref- erence for other components, but care must be taken to en- sure that current is not taken from it which could reduce the accuracy of the converter. PRINCIPLES OF OPERATION OF BASIC VOLTAGE- TO-FREQUENCY CONVERTER ( Figure 1) The simple stand-alone V-to-F converter shown in Figure 1 includes all the basic circuitry of Figure 3 plus a few compo- nents for improved performance. A resistor, R IN=100 kΩ±10%, has been added in the path to pin 7, so that the bias current at pin 7 (−80 nA typical) will cancel the effect of the bias current at pin 6 and help provide minimum frequency offset. The resistance R S at pin 2 is made up of a 12 kΩ fixed resis- tor plusa5k Ω (cermet, preferably) gain adjust rheostat. The function of this adjustment is to trim out the gain tolerance of the LM231/331, and the tolerance of R t,RL and Ct. For best results, all the components should be stable low-temperature-coefficient components, such as metal-film resistors. The capacitor should have low dielectric absorp- tion; depending on the temperature characteristics desired, NPO ceramic, polystyrene, Teflon or polypropylene are best suited. A capacitor C IN is added from pin 7 to ground to act as a filter for V IN. A value of 0.01 µF to 0.1 µF will be adequate in most cases; however, in cases where better filtering is required, a 1 µF capacitor can be used. When the RC time constants are matched at pin 6 and pin 7, a voltage step at V IN will cause a step change in f OUT.IfCIN is much less than CL, a step at V IN may cause fOUT to stop momentarily. A47 Ω resistor, in series with the 1 µF C L, is added to give hysteresis effect which helps the input comparator provide the excellent linearity (0.03% typical). DETAIL OF OPERATION OF PRECISION V-TO-F CONVERTER ( Figure 4) In this circuit, integration is performed by using a conven- tional operational amplifier and feedback capacitor, C F. When the integrator’s output crosses the nominal threshold level at pin 6 of the LM231/331, the timing cycle is initiated. The average current fed into the op amp’s summing point (pin 2) is i x (1.1 R tCt) x f which is perfectly balanced with −V IN/RIN. In this circuit, the voltage offset of the LM231/331 input comparator does not affect the offset or accuracy of the V-to-F converter as it does in the stand-alone V-to-F con- verter; nor does the LM231/331 bias current or offset cur- rent. Instead, the offset voltage and offset current of the op- erational amplifier are the only limits on how small the signal can be accurately converted. Since op amps with voltage off- set well below 1 mV and offset currents well below 2 nA are available at low cost, this circuit is recommended for best ac- curacy for small signals. This circuit also responds immedi- ately to any change of input signal (which a stand-alone cir- cuit does not) so that the output frequency will be an accurate representation of V IN, as quickly as 2 output pulses’ spacing can be measured. In the precision mode, excellent linearity is obtained be- cause the current source (pin 1) is always at ground potential and that voltage does not vary with V IN or fOUT. (In the stand-alone V-to-F converter, a major cause of non-linearity is the output impedance at pin 1 which causes i to change as a function of V IN). The circuit of Figure 5 operates in the same way as Figure 4, but with the necessary changes for high speed operation. www.national.com 7 |
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