LM231A/LM231/LM331A/LM331
Precision Voltage-to-Frequency Converters

General Description

The LM231/LM331 family of voltage-to-frequency converters
are ideally suited for use in simple low-cost circuits for
analog-to-digital conversion, precision frequency-to-voltage
conversion, long-term integration, linear frequency modula-
tion 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 in-
put voltage. Thus, it provides all the inherent advantages of
the voltage-to-frequency conversion techniques, and is easy
to apply in all standard voltage-to-frequency converter appli-
cations. Further, the LM231A/LM331A attain a new high
level of accuracy versus temperature which could only be at-
tained with expensive voltage-to-frequency modules. Addi-
tionally the LM231/331 are ideally suited for use in digital
systems at low power supply voltages and can provide
low-cost

analog-to-digital

conversion

microprocessor-controlled systems. And, the frequency from
a battery powered voltage-to-frequency converter can be
easily channeled through a simple photoisolator to provide
isolation against high common mode levels.

The LM231/LM331 utilize a new temperature-compensated
band-gap reference circuit, to provide excellent accuracy

over the full operating temperature range, at power supplies
as low as 4.0V. The precision timer circuit has low bias cur-
rents without degrading the quick response necessary for
100 kHz voltage-to-frequency conversion. And the output
are capable of driving 3 TTL loads, or a high voltage output
up to 40V, yet is short-circuit-proof against V

Features

Guaranteed linearity 0.01% max

Improved performance in existing voltage-to-frequency
conversion applications

Split or single supply operation

Operates on single 5V supply

Pulse output compatible with all logic forms

Excellent temperature stability,

50 ppm/°C max

Low power dissipation, 15 mW typical at 5V

Wide dynamic range, 100 dB min at 10 kHz full scale
frequency

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

Low cost

Typical Applications

Teflon

is a registered trademark of DuPont

DS005680-1

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

0.1µF or 1µF, See "Principles of Operation."

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

with

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

June 1999

LM231A/LM231/LM331A/LM331

Precision

oltage-to-Frequency

Converters

DS005680

www.national.com

Absolute Maximum Ratings

(Note 1)

If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

LM231A/LM231

LM331A/LM331

Supply Voltage

40V

Output Short Circuit to Ground

Continuous

Output Short Circuit to V

Continuous

Input Voltage

-0.2V to +V

MIN

MAX

MIN

MAX

Operating Ambient Temperature Range

-25°C to +85°C

0°C to +70°C

Power Dissipation (P

at 25°C)

and Thermal Resistance (

)

(N Package) P

1.25W

100°C/W

Lead Temperature (Soldering, 10 sec.)

Dual-In-Line Package (Plastic)

260°C

ESD Susceptibility (Note 4)

N Package

500V

Electrical Characteristics

=25°C unless otherwise specified (Note 2)

Parameter

Conditions

Min

Typ

Max

Units

VFC Non-Linearity (Note 3)

4.5V

20V

0.003

0.01

% Full-

Scale

MIN

MAX

0.006

0.02

% Full-

Scale

VFC Non-Linearity

= 15V, f = 10 Hz to 11 kHz

0.024

0.14

%Full-

In Circuit of

Figure 1

Scale

Conversion Accuracy Scale Factor (Gain)

= -10V, R

= 14 k

LM231, LM231A

0.95

1.00

1.05

kHz/V

LM331, LM331A

0.90

1.00

1.10

kHz/V

Temperature Stability of Gain

MIN

MAX

, 4.5V

20V

LM231/LM331

150

ppm/°C

LM231A/LM331A

ppm/°C

Change of Gain with V

4.5V

10V

0.01

0.1

%/V

10V

40V

0.006

0.06

%/V

Rated Full-Scale Frequency

= -10V

10.0

kHz

Gain Stability vs Time

MIN

MAX

0.02

% Full-

(1000 Hrs)

Scale

Overrange (Beyond Full-Scale) Frequency

= -11V

INPUT COMPARATOR

Offset Voltage

LM231/LM331

MIN

MAX

LM231A/LM331A

MIN

MAX

Bias Current

-80

-300

Offset Current

100

Common-Mode Range

MIN

MAX

-0.2

-2.0

TIMER

Timer Threshold Voltage, Pin 5

0.63

0.667

0.70

x V

Input Bias Current, Pin 5

= 15V

All Devices

PIN 5

9.9V

100

LM231/LM331

PIN 5

= 10V

200

1000

LM231A/LM331A

PIN 5

= 10V

200

500

www.national.com

Electrical Characteristics

(Continued)

=25°C unless otherwise specified (Note 2)

Parameter

Conditions

Min

Typ

Max

Units

TIMER

SAT PIN 5

(Reset)

I = 5 mA

0.22

0.5

CURRENT SOURCE (Pin 1)

Output Current

=14 k

, V

PIN 1

LM231, LM231A

126

135

144

µA

LM331, LM331A

116

136

156

µA

Change with Voltage

PIN 1

10V

0.2

1.0

µA

Current Source OFF Leakage

LM231, LM231A, LM331, LM331A

0.02

10.0

All Devices

MAX

2.0

50.0

Operating Range of Current (Typical)

(10 to 500)

µA

REFERENCE VOLTAGE (Pin 2)

LM231, LM231A

1.76

1.89

2.02

LM331, LM331A

1.70

1.89

2.08

Stability vs Temperature

ppm/°C

Stability vs Time, 1000 Hours

0.1

LOGIC OUTPUT (Pin 3)

SAT

I=5 mA

0.15

0.50

I=3.2 mA (2 TTL Loads), T

MIN

MAX

0.10

0.40

OFF Leakage

0.05

1.0

µA

SUPPLY CURRENT

LM231, LM231A

=5V

2.0

3.0

4.0

LM331, LM331A

=40V

2.5

4.0

6.0

=5V

1.5

3.0

6.0

=40V

2.0

4.0

8.0

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating
the device beyond its specified operating conditions.

Note 2: All specifications apply in the circuit of

Figure 4, with 4.0V

40V, unless otherwise noted.

Note 3: Nonlinearity is defined as the deviation of f

OUT

from V

x (10 kHz/-10 V

) when the circuit has been trimmed for zero error at 10 Hz and at 10 kHz, over

the frequency range 1 Hz to 11 kHz. For the timing capacitor, C

, use NPO ceramic, Teflon

, or polystyrene.

Note 4: Human body model, 100 pF discharged through a 1.5 k

resistor.

www.national.com

Typical Performance Characteristics

(Continued)

Typical Applications

PRINCIPLES OF OPERATION OF A SIMPLIFIED
VOLTAGE-TO-FREQUENCY CONVERTER

The LM231/331 are monolithic circuits designed for accu-
racy

and

versatile

operation

when

applied

voltage-to-frequency

(V-to-F)

converters

frequency-to-voltage (F-to-V) converters. A simplified block
diagram of the LM231/331 is shown in

Figure 3 and consists

of a switched current source, input comparator, and 1-shot
timer.

The operation of these blocks is best understood by going
through the operating cycle of the basic V-to-F converter,
Figure 3, which consists of the simplified block diagram of
the LM231/331 and the various resistors and capacitors con-
nected to it.

The voltage comparator compares a positive input voltage,
V1, at pin 7 to the voltage, V

, at pin 6. If V1 is greater, the

comparator will trigger the 1-shot timer. The output of the
timer will turn ON both the frequency output transistor and
the switched current source for a period t=1.1 R

. During

this period, the current i will flow out of the switched current
source and provide a fixed amount of charge, Q=i x t, into
the capacitor, C

. This will normally charge V

up to a higher

level than V1. At the end of the timing period, the current i will
turn OFF, and the timer will reset itself.

Now there is no current flowing from pin 1, and the capacitor
C

will be gradually discharged by R

until V

falls to the level

of V1. Then the comparator will trigger the timer and start an-
other cycle.

The current flowing into C

is exactly I

AVE

= i x (1.1xR

) x

f, and the current flowing out of C

is exactly V

If V

is doubled, the frequency will double to maintain this

balance. Even a simple V-to-F converter can provide a fre-
quency precisely proportional to its input voltage over a wide
range of frequencies.

DETAIL OF OPERATION, FUNCTIONAL BLOCK
DIAGRAM (

Figure 2)

The block diagram shows a band gap reference which pro-
vides a stable 1.9 V

output. This 1.9 V

is well regulated

over a V

range of 3.9V to 40V. It also has a flat, low tem-

perature coefficient, and typically changes less than

over a 100°C temperature change.

The current pump circuit forces the voltage at pin 2 to be at
1.9V, and causes a current i=1.90V/R

to flow. For R

=14k,

i=135 µA. The precision current reflector provides a current
equal to i to the current switch. The current switch switches
the current to pin 1 or to ground depending on the state of
the R

flip-flop.

The timing function consists of an R

flip-flop, and a timer

comparator connected to the external R

network. When

the input comparator detects a voltage at pin 7 higher than
pin 6, it sets the R

flip-flop which turns ON the current

switch and the output driver transistor. When the voltage at
pin 5 rises to

, the timer comparator causes the R

flip-flop to reset. The reset transistor is then turned ON and
the current switch is turned OFF.

However, if the input comparator still detects pin 7 higher
than pin 6 when pin 5 crosses

, the flip-flop will not be

reset, and the current at pin 1 will continue to flow, in its at-
tempt to make the voltage at pin 6 higher than pin 7. This

Power Drain vs V

SUPPLY

DS005680-34

Output Saturation Voltage vs
I

OUT

(Pin 3)

DS005680-35

Nonlinearity Error, Precision
F-to-V Converter (

Figure 7)

DS005680-36

DS005680-4

FIGURE 3. Simplified Block Diagram of Stand-Alone

Voltage-to-Frequency Converter and

External Components

www.national.com

Typical Applications

(Continued)

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

. 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

for all values of

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

=100 k

10%, has been added in the path to

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

at pin 2 is made up of a 12 k

fixed resis-

tor plus a 5 k

(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

, R

and C

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

is added from pin 7 to ground to act as a filter

for V

. A value of 0.01 µF to 0.1 µF will be adequate in most

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

will cause

a step change in f

OUT

. If C

is much less than C

, a step at

may cause f

OUT

to stop momentarily.

A 47

resistor, in series with the 1 µF C

, is added to give

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

) x f which is perfectly balanced with

-V

. 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 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

, 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

or f

OUT

. (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

The circuit of

Figure 5 operates in the same way as Figure 4,

but with the necessary changes for high speed operation.

www.national.com

Typical Applications

(Continued)

DETAILS OF OPERATION, FREQUENCY-TO-
VOLTAGE CONVERTERS (

Figure 6 and Figure 7)

In these applications, a pulse input at f

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
I

AVERAGE

= i x (1.1 R

) x f.

In the simple circuit of

Figure 6, this current is filtered in the

network R

= 100 k

and 1 µF. The ripple will be less than 10

mV peak, but the response will be slow, with a 0.1 second
time constant, and settling of 0.7 second to 0.1% accuracy.

In the precision circuit, an operational amplifier provides a
buffered output and also acts as a 2-pole filter. The ripple will
be less than 5 mV peak for all frequencies above 1 kHz, and
the response time will be much quicker than in

Figure 6.

However, for input frequencies below 200 Hz, this circuit will
have worse ripple than

Figure 6. The engineering of the filter

time-constants to get adequate response and small enough
ripple simply requires a study of the compromises to be
made. Inherently, V-to-F converter response can be fast, but
F-to-V response can not.

DS005680-6

Use stable components with low temperature coefficients.

See Typical Applications section.

This resistor can be 5 k

or 10 k

for V

=8V to 22V, but must be 10 k

for V

=4.5V to 8V.

***

Use low offset voltage and low offset current op amps for A1: recommended types LF411A or LF356.

FIGURE 5. Precision Voltage-to-Frequency Converter,

100 kHz Full-Scale,

0.03% Non-Linearity

www.national.com

Physical Dimensions

inches (millimeters) unless otherwise noted

LIFE SUPPORT POLICY

NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com

National Semiconductor
Europe

Fax: +49 (0) 1 80-530 85 86

Email: europe.support@nsc.com

Deutsch Tel: +49 (0) 1 80-530 85 85
English

Tel: +49 (0) 1 80-532 78 32

Français Tel: +49 (0) 1 80-532 93 58
Italiano

Tel: +49 (0) 1 80-534 16 80

National Semiconductor
Asia Pacific Customer
Response Group
Tel: 65-2544466
Fax: 65-2504466
Email: sea.support@nsc.com

National Semiconductor
Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507

www.national.com

Dual-In-Line Package (N)

Order Number LM231AN, LM231N, LM331AN, or LM331N

NS Package N08E

LM231A/LM231/LM331A/LM331

Precision

oltage-to-Frequency

Converters

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM231AN

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM231AN