Page 1
LM131A/LM131, LM231A/LM231, LM331A/LM331
Precision Voltage-to-Frequency Converters
LM131A/LM131, LM231A/LM231, LM331A/LM331 Precision Voltage-to-Frequency Converters
December 1994
General Description
The LM131/LM231/LM331 family of voltage-to-frequency
converters are ideally suited for use in simple low-cost circuits for analog-to-digital conversion, precision frequencyto-voltage conversion, long-term integration, linear frequency 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 advantages of the voltage-to-frequency conversion techniques, and is easy to apply in all standard voltage-to-frequency converter applications. Further, the LM131A/
LM231A/LM331A attains a new high level of accuracy versus temperature which could only be attained with expensive voltage-to-frequency modules. Additionally the LM131
is ideally suited for use in digital systems at low power supply voltages and can provide low-cost analog-to-digital conversion in 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 LM131/LM231/LM331 utilizes a new temperaturecompensated 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
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
V
.
CC
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
TL/H/5680– 1
1
t
*Use stable components with low temperature coefficients. See Typical Applications section.
**0.1mFor1mF, See ‘‘Principles of Operation.’’
f
OUT
e
V
IN
2.09 V
R
S
#
#
R
RtC
L
FIGURE 1. Simple Stand-Alone Voltage-to-Frequency Converter
g
with
0.03% Typical Linearity (fe10 Hz to 11 kHz)
C
1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
TL/H/5680
Page 2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 40V 40V 40V
Output Short Circuit to Ground Continuous Continuous Continuous
Output Short Circuit to V
Input Voltage
CC
Operating Ambient Temperature Range
i
i
i
D
D
jA
D
jA
D
JA
at 25§C)
jA
)
Power Dissipation (P
and Thermal Resistance (i
(H Package) P
(N Package) P
(M Package) P
Lead Temperature (Soldering, 10 sec.)
Dual-In-Line Package (Plastic) 260
Metal Can Package (TO-5) 260
ESD Susceptibility (Note 4)
Metal Can Package (TO-5) 2000V
Other Packages 500V 500V
LM131A/LM131 LM231A/LM231 LM331A/LM331
Continuous Continuous Continuous
b
0.2V toaV
T
MINTMAX
b
55§Ctoa125§C
S
b
0.2V toaV
T
MINTMAX
b
25§Ctoa85§C0
S
b
0.2V toaV
T
MINTMAX
Ctoa70§C
§
670 mW
150§C/W
1.25W 1.25W
100§C/W 100§C/W
1.25W
85§C/W
C 260§C 260§C
§
C
§
S
Electrical Characteristics T
e
25§C unless otherwise specified (Note 2)
A
Parameter Conditions Min Typ Max Units
VFC Non-Linearity (Note 3) 4.5VsV
T
MIN
VFC Non-Linearity V
In Circuit of
Figure 1
Conversion Accuracy Scale Factor (Gain) V
LM131, LM131A, LM231, LM231A 0.95 1.00 1.05 kHz/V
S
IN
s
20V
S
s
s
T
T
A
MAX
e
15V, fe10 Hz to 11 kHz
eb
10V, R
e
14 kX
S
g
g
g
0.003
0.006
0.024
g
0.01 % Full-
g
0.02 % Full-
g
0.14 %Full-
LM331, LM331A 0.90 1.00 1.10 kHz/V
s
Temperature Stability of Gain T
LM131/LM231/LM331
MIN
s
T
T
MAX
, 4.5VsV
A
LM131A/LM231A/LM331A
Change of Gain with V
S
4.5VsV
10VsV
Rated Full-Scale Frequency V
Gain Stability vs Time T
(1000 Hrs) Scale
Overrange (Beyond Full-Scale) Frequency V
s
10V 0.01 0.1 %/V
S
s
40V 0.006 0.06 %/V
S
eb
10V 10.0 kHz
IN
s
s
T
MIN
IN
eb
T
A
MAX
11V 10 %
s
20V
S
g
30
g
20
g
0.02 % Full-
g
150 ppm/§C
g
50 ppm/§C
INPUT COMPARATOR
Offset Voltage
LM131/LM231/LM331 T
LM131A/LM231A/LM331A T
Bias Current
Offset Current
Common-Mode Range T
MIN
MIN
MIN
g
s
s
T
T
A
s
s
MAX
s
T
T
A
MAX
s
T
T
A
MAX
b
0.2 V
3
g
4
g
3
b
80
g
8
g
10 mV
g
14 mV
g
10 mV
b
300 nA
g
100 nA
b
2.0 V
CC
Scale
Scale
Scale
2
Page 3
Electrical Characteristics T
e
25§C unless otherwise specified (Note 2) (Continued)
A
Parameter Conditions Min Typ Max Units
TIMER
Timer Threshold Voltage, Pin 5 0.63 0.667 0.70
Input Bias Current, Pin 5 V
All Devices 0V
LM131/LM231/LM331 V
LM131A/LM231A/LM331A V
V
(Reset) Ie5 mA 0.22 0.5 V
SAT PIN 5
e
15V
S
PIN 5
PIN 5
s
s
V
9.9V
PIN 5
e
10V 200 1000 nA
e
10V 200 500 nA
g
10
g
c
100 nA
CURRENT SOURCE (Pin 1)
Output Current R
LM131, LM131A, LM231, LM231A 126 135 144 mA
e
S
14 kX,V
PIN 1
e
0
LM331, LM331A 116 136 156 mA
Change with Voltage 0VsV
s
10V 0.2 1.0 mA
PIN 1
Current Source OFF Leakage
LM131, LM131A 0.01 1.0 nA
LM231, LM231A, LM331, LM331A 0.02 10.0 nA
All Devices T
e
T
A
MAX
2.0 50.0 nA
Operating Range of Current (Typical) (10 to 500) mA
REFERENCE VOLTAGE (Pin 2)
LM131, LM131A, LM231, LM231A 1.76 1.89 2.02 V
LM331, LM331A 1.70 1.89 2.08 V
Stability vs Temperature
Stability vs Time, 1000 Hours
g
60 ppm/§C
g
0.1 %
LOGIC OUTPUT (Pin 3)
V
SAT
OFF Leakage
Ie5 mA 0.15 0.50 V
e
I
3.2 mA (2 TTL Loads), T
MIN
s
s
T
T
A
MAX
0.10 0.40 V
g
0.05 1.0 mA
SUPPLY CURRENT
LM131, LM131A, LM231, V
LM231A V
LM331, LM331A V
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
Note 3: Nonlinearity is defined as the deviation of f
over the frequency range 1 Hz to 11 kHz. For the timing capacitor, C
Note 4: Human body model, 100 pF discharged through a 1.5 kX resistor.
e
5V 2.0 3.0 4.0 mA
S
e
40V 2.5 4.0 6.0 mA
S
e
5V 1.5 3.0 6.0 mA
S
e
V
40V 2.0 4.0 8.0 mA
S
Figure 3
OUT
, with 4.0VsV
from V
s
40V, unless otherwise noted.
S
c
(10 kHz/b10 VDC) when the circuit has been trimmed for zero error at 10 Hz and at 10 kHz,
IN
, use NPO ceramic, TeflonÉ, or polystyrene.
T
DC
DC
V
S
3
Page 4
Functional Block Diagram
Pin numbers apply to 8-pin packages only. See connection diagram for LM231WM pin numbers.
TeflonÉregistered trademark of DuPont
FIGURE 1a
TL/H/5680– 2
4
Page 5
Typical Performance Characteristics
(All electrical characteristics apply for the circuit of
Nonlinearity Error, LM131
Family, as Precision V-to-F
Converter (
Figure 3
)
Figure 3
Nonlinearity Error, LM131
Family
, unless otherwise noted.)
Nonlinearity vs Power Supply
Voltage
Frequency vs Temperature,
LM131A
100 kHz Nonlinearity Error,
LM131 Family (
Power Drain vs V
Figure 4
SUPPLY
)
V
vs Temperature,
REF
LM131A
Nonlinearity Error, LM131
(
Figure 1
)
Output Saturation Voltage vs
I
(Pin 3)
OUT
Output Frequency vs
V
SUPPLY
Input Current (Pins 6, 7) vs
Temperature
Nonlinearity Error, Precision
F-to-V Converter (
Figure 6
)
TL/H/5680– 3
5
Page 6
Typical Applications (Continued)
PRINCIPLES OF OPERATION OF A SIMPLIFIED
VOLTAGE-TO-FREQUENCY CONVERTER
The LM131 is a monolithic circuit designed for accuracy and
versatile operation when applied as a voltage-to-frequency
(V-to-F) converter or as a frequency-to-voltage (F-to-V) converter. A simplified block diagram of the LM131 is shown in
Figure 2
Figure 2
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,
, which consists of the simplified block diagram of
the LM131 and the various resistors and capacitors connected to it.
The voltage comparator compares a positive input voltage,
V1, at pin 7 to the voltage, V
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
this period, the current i will flow out of the switched current
source and provide a fixed amount of charge, Q
the capacitor, C
level than V1. At the end of the timing period, the current i
. This will normally charge Vxup to a higher
L
, at pin 6. If V1 is greater, the
x
e
1.1 RtCt. During
e
ict, into
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 RLuntil Vxfalls to the
L
level of V1. Then the comparator will trigger the timer and
start another cycle.
The current flowing into C
c
f, and the current flowing out of CLis exactly Vx/R
.IfVINis doubled, the frequency will double to main-
V
IN/RL
tain this balance. Even a simple V-to-F converter can pro-
is exactly I
L
AVE
e
ic(1.1cRtCt)
L
vide a frequency precisely proportional to its input voltage
over a wide range of frequencies.
FIGURE 2. Simplified Block Diagram of Stand-Alone
TL/H/5680– 4
Voltage-to-Frequency Converter Showing LM131 and
External Components
DETAIL OF OPERATION, FUNCTIONAL BLOCK
DIAGRAM (
FIGURE 1a
)
The block diagram shows a band gap reference which provides a stable 1.9 V
over a V
perature coefficient, and typically changes less than (/2%
over a 100
range of 3.9V to 40V. It also has a flat, low tem-
S
C temperature change.
§
The current pump circuit forces the voltage at pin 2 to be at
1.9V, and causes a current i
e
R
14k, ie135 mA. The precision current reflector pro-
s
vides a current equal to i to the current switch. The current
output. This 1.9 VDCis well regulated
DC
e
1.90V/RSto flow. For
switch switches the current to pin 1 or to ground depending
on the state of the R
flip-flop.
S
The timing function consists of an RSflip-flop, and a timer
comparator connected to the external R
the input comparator detects a voltage at pin 7 higher than
pin 6, it sets the R
switch and the output driver transistor. When the voltage at
pin 5 rises to )/3 V
flip-flop to reset. The reset transistor is then turned ON and
flip-flop which turns ON the current
S
, the timer comparator causes the R
CC
network. When
tCt
S
the current switch is turned OFF.
However, if the input comparator still detects pin 7 higher
than pin 6 when pin 5 crosses )/3 V
be reset, and the current at pin 1 will continue to flow, in its
, the flip-flop will not
CC
attempt to make the voltage at pin 6 higher than pin 7. This
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
j
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 50X. 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 mAto500mA. It can be used as a voltage
for all values of
DC
reference for other components, but care must be taken to
ensure that current is not taken from it which could reduce
the accuracy of the converter.
PRINCIPLES OF OPERATION OF BASIC VOLTAGETO-FREQUENCY CONVERTER (
The simple stand-alone V-to-F converter shown in
includes all the basic circuitry of
FIGURE 1
Figure 2
)
Figure 1
plus a few compo-
nents for improved performance.
A resistor, R
to pin 7, so that the bias current at pin 7 (
e
100 kXg10%, has been added in the path
IN
b
80 nA typical)
will cancel the effect of the bias current at pin 6 and help
provide minimum frequency offset.
The resistance R
resistor plus a 5 kX (cermet, preferably) gain adjust rheo-
at pin 2 is made up of a 12 kX fixed
S
stat. The function of this adjustment is to trim out the gain
tolerance of the LM131, and the tolerance of R
t,RL
and Ct.
6
Page 7
Typical Applications (Continued)
For best results, all the components should be stable lowtemperature-coefficient components, such as metal-film resistors. The capacitor should have low dielectric absorption;
depending on the temperature characteristics desired, NPO
ceramic, polystyrene, Teflon or polypropylene are best
suited.
A capacitor C
filter for V
most cases; however, in cases where better filtering is required, a 1 mF 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
IN
than C
L
A47Xresistor, in series with the 1 m FCL, 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 (
In this circuit, integration is performed by using a conventional operational amplifier and feedback capacitor, C
When the integrator’s output crosses the nominal threshold
level at pin 6 of the LM131, the timing cycle is initiated.
is added from pin 7 to ground to act as a
IN
. A value of 0.01 mFto0.1mF will be adequate in
IN
.IfCINis much less
, a step at VINmay cause f
FIGURE 3
OUT
to stop momentarily.
OUT
)
The average current fed into the op amp’s summing point
(pin 2) is i
b
c
(1.1 RtCt)cf which is perfectly balanced with
VIN/RIN. In this circuit, the voltage offset of the LM131
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 LM131 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 V
output pulses’ spacing 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 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
.
F
as a function of V
The circuit of
).
IN
Figure 4
operates in the same way as
but with the necessary changes for high speed operation.
, as quickly as 2
IN
or f
OUT
. (In the
IN
Figure 3
,
b
V
R
1
IN
2.09 V
e
4.5V to 8V.
S
S
#
#
R
RtC
IN
t
TL/H/5680– 5
e
f
OUT
*Use stable components with low temperature coefficients. See Typical Applications section.
**This resistor can be 5 kX or 10 kX for V
***Use low offset voltage and low offset current op amps for A1: recommended types LM108, LM308A, LF411A
e
8V to 22V, but must be 10 kX for V
S
FIGURE 3. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter
7
Page 8
Typical Applications (Continued)
DETAILS OF OPERATION, FREQUENCY-TOVOLTAGE CONVERTERS
(FIGURES 5 AND 6
)
In these applications, a pulse input at fINis 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
In the simple circuit of
the network R
than 10 mV peak, but the response will be slow, with a
e
ic(1.1 RtCt)cf.
FIGURE 5
e
100 kX and 1 mF. The ripple will be less
L
, this current is filtered in
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 5
However, for input frequencies below 200 Hz, this circuit will
have worse ripple than
Figure 5
. 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.
*Use stable components with low temperature coefficients.
See Typical Applications section.
**This resistor can be 5 kX or 10 kX for V
but must be 10 kX for V
***Use low offset voltage and low offset current op amps for A1:
recommended types LF411A or LF356.
e
4.5V to 8V.
S
e
S
8V to 22V,
.
FIGURE 4. Precision Voltage-to-Frequency Converter,
100 kHz Full-Scale,
R
L
e
c
V
f
OUT
2.09V
IN
*Use stable components with low temperature coefficients.
c
c
(RtCt)
R
S
TL/H/5680– 7
FIGURE 5. Simple Frequency-to-Voltage Converter,
10 kHz Full-Scale,
g
0.06% Non-Linearity
TL/H/5680– 6
g
0.03% Non-Linearity
eb
V
OUT
SELECT Rx
c
f
IN
e
*Use stable components with low temperature coefficients.
FIGURE 6. Precision Frequency-to-Voltage Converter,
10 kHz Full-Scale with 2-Pole Filter,
8
R
F
c
c
2.09V
(V
S
0.2 mA
b
2V)
(RtCt)
R
S
Non-Linearity Maximum
TL/H/5680– 8
g
0.01%
Page 9
Typical Applications (Continued)
Light Intensity to Frequency Converter
*L14F-1, L14G-1 or L14H-1, photo transistor (General Electric Co.) or similar
Temperature to Frequency Converter
Long-Term Digital Integrator Using VFC
TL/H/5680– 9
TL/H/5680– 10
Basic Analog-to-Digital Converter Using
Voltage-to-Frequency Converter
TL/H/5680– 11
TL/H/5680– 12
9
Page 10
Typical Applications (Continued)
Analog-to-Digital Converter with Microprocessor
Remote Voltage-to-Frequency Converter with 2-Wire Transmitter and Receiver
TL/H/5680– 13
Voltage-to-Frequency Converter with Square-Wave Output Usingd2 Flip-Flop
TL/H/5680– 14
Voltage-to-Frequency Converter with Isolators
10
TL/H/5680– 15
TL/H/5680– 16
Page 11
Typical Applications (Continued)
Voltage-to-Frequency Converter with Isolators
Voltage-to-Frequency Converter with Isolators
TL/H/5680– 17
Voltage-to-Frequency Converter with Isolators
11
TL/H/5680– 18
TL/H/5680– 19
Page 12
Connection Diagrams
Metal Can Package
Note: Metal case is connected to pin 4 (GND.)
Order Number LM131H/883 or LM131AH/883
See NS Package Number H08C
Dual-In-Line Package
TL/H/5680– 20
TL/H/5680– 21
Order Number LM231AN, LM231N, LM331AN,
or LM331N
See NS Package Number N08E
Small-Outline Package
TL/H/5680– 24
Top View
Order Number LM231WM
See NS Package Number M14B
12
Page 13
Schematic Diagram
TL/H/5680– 22
13
Page 14
14
Page 15
Physical Dimensions inches (millimeters)
Order Number LM131H/883 or LM131AH/883
Metal Can Package (H)
NS Package H08C
14-Pin Small Outline Package (M)
Order Number LM231WM
NS Package M14B
15
Page 16
Physical Dimensions inches (millimeters) (Continued)
Order Number LM231AN, LM231N, LM331AN, or LM331N
Dual-In-Line Package (N)
NS package N08E
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 OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or 2. A critical component is any component of a life
systems which, (a) are intended for surgical implant support device or system whose failure to perform can
into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life
failure to perform, when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can effectiveness.
be reasonably expected to result in a significant injury
to the user.
National Semiconductor National Semiconductor National Semiconductor National Semiconductor
Corporation Europe Hong Kong Ltd. Japan Ltd.
1111 West Bardin Road Fax: (
Arlington, TX 76017 Email: cnjwge@tevm2.nsc.com Ocean Centre, 5 Canton Rd. Fax: 81-043-299-2408
Tel: 1(800) 272-9959 Deutsch Tel: (
LM131A/LM131, LM231A/LM231, LM331A/LM331 Precision Voltage-to-Frequency Converters
Fax: 1(800) 737-7018 English Tel: (
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.
Fran3ais Tel: (
Italiano Tel: (
a
49) 0-180-530 85 86 13th Floor, Straight Block, Tel: 81-043-299-2309
a
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a
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