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

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

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

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

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

for V

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

a step change in f

V

A47

Ω resistor, in series with the 1 µF C

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

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

−V

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

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

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

The circuit of

Figure 5 operates in the same way as Figure 4,

but with the necessary changes for high speed operation.

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