Applications Information (Continued)

A47

Ω resistor, in series with the 1 µF C

L, provides hyster-

esis, which helps the input comparator provide the excellent

linearity.

Details of Operation: 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

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 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.

00568001

*Use stable components with low temperature coefficients. See Typical

Applications section.

**0.1µF or 1µF, See “Principles of Operation.”

FIGURE 3. Simple Stand-Alone V-to-F Converter

with ±0.03% Typical Linearity (f = 10 Hz to 11 kHz)

00568005

*Use stable components with low temperature coefficients. See Typical Applications section.

**This resistor can be 5 k

Ω 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 4. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter