LM231, LM331

SNOSBI2C – JUNE 1999 – REVISED SEPTEMBER 2015

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Typical Applications (continued)

9.2.2 Precision V-To-F Converter

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 LMx31, the timing cycle is

initiated.

The average current fed into the summing point of the op-amp (pin 2) is i × (1.1 RtCt) × f which is perfectly

balanced with

−VIN/RIN. In this circuit, the voltage offset of the LMx31 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 the spacing of the

2 output pulses 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.

**This resistor can be 5 k

Ω or 10 kΩ for VS = 8 V to 22 V, but must be 10 kΩ for VS = 4.5 V to 8 V.

***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

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