LM231, LM331
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SNOSBI2B –JUNE 1999–REVISED MARCH 2013
Details of Operation: Precision V-To-F Converter (Figure 16)
In this circuit, integration is performed by using a conventional operational amplifier and feedback capacitor, CF.
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 × (1.1 RtCt) × f which is perfectly balanced
with −VIN/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 converter; nor does the LM231/331 bias
current or offset current. Instead, the offset voltage and offset current of the operational amplifier are the only
limits on how small the signal can be accurately converted. Since op-amps with voltage offset well below 1 mV
and offset currents well below 2 nA are available at low cost, this circuit is recommended for best accuracy for
small signals. This circuit also responds immediately to any change of input signal (which a stand-alone circuit
does not) so that the output frequency will be an accurate representation of VIN, as quickly as 2 output pulses'
spacing can be measured.
In the precision mode, excellent linearity is obtained because the current source (pin 1) is always at ground
potential and that voltage does not vary with VIN 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 VIN).
The circuit of Figure 17 operates in the same way as Figure 16, but with the necessary changes for high speed
operation.
*Use stable components with low temperature coefficients. See APPLICATIONS INFORMATION.
**This resistor can be 5 kΩ or 10 kΩ for VS=8V to 22V, but must be 10 kΩ for VS=4.5V to 8V.
***Use low offset voltage and low offset current op-amps for A1: recommended type LF411A
Figure 16. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter
DETAILS OF OPERATION: F-to-V CONVERTERS
(Figure 18 and Figure 19)
In these applications, a pulse input at fIN is differentiated by a C-R network and the negative-going edge at pin 6
causes the input comparator to trigger the timer circuit. Just as with a V-to-F converter, the average current
flowing out of pin 1 is IAVERAGE = i × (1.1 RtCt) × f.
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