December 1994

LM131A LM131 LM231A LM231 LM331A LM331

Precision Voltage-to-Frequency Converters

General Description

The LM131 LM231 LM331 family of voltage-to-frequency

converters are ideally suited for use in simple low-cost cir-

cuits for analog-to-digital conversion precision frequency-

to-voltage conversion long-term integration linear frequen-

cy modulation or demodulation and many other functions

The output when used as a voltage-to-frequency converter

is a pulse train at a frequency precisely proportional to the

applied input voltage Thus it provides all the inherent ad-

vantages of the voltage-to-frequency conversion tech-

niques and is easy to apply in all standard voltage-to-fre-

quency converter applications Further the LM131A

LM231A LM331A attains a new high level of accuracy ver-

sus temperature which could only be attained with expen-

sive voltage-to-frequency modules Additionally the LM131

is ideally suited for use in digital systems at low power sup-

ply voltages and can provide low-cost analog-to-digital con-

version in microprocessor-controlled systems And the fre-

quency from a battery powered voltage-to-frequency con-

verter can be easily channeled through a simple photoisola-

tor to provide isolation against high common mode levels

The LM131 LM231 LM331 utilizes a new temperature-

compensated band-gap reference circuit to provide excel-

lent accuracy over the full operating temperature range at

power supplies as low as 4 0V The precision timer circuit

Typical Applications

has low bias currents without degrading the quick response

necessary for 100 kHz voltage-to-frequency conversion

And the output is capable of driving 3 TTL loads or a high

voltage output up to 40V yet is short-circuit-proof against

VCC

Features

Y Guaranteed linearity 0 01% max

Y Improved performance in existing voltage-to-frequency

conversion applications

Y Split or single supply operation

Y Operates on single 5V supply

Y Pulse output compatible with all logic forms

Y Excellent temperature stability g50 ppm C max

Y Low power dissipation 15 mW typical at 5V

Y Wide dynamic range 100 dB min at 10 kHz full scale

frequency

Y Wide range of full scale frequency 1 Hz to 100 kHz

Y Low cost

fOUT

e

VIN

2 09 V

RS

RL

1

RtCt

TL H 5680 – 1

Use stable components with low temperature coefficients See Typical Applications section

0 1mF or 1mF See ‘‘Principles of Operation ’’

FIGURE 1 Simple Stand-Alone Voltage-to-Frequency Converter

with g0 03% Typical Linearity (f e 10 Hz to 11 kHz)

C1995 National Semiconductor Corporation TL H 5680

RRD-B30M115 Printed in U S A