TL/H/5680
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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
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
Typical Applications
TL/H/5680–1
*Use stable components with low temperature coefficients. See Typical Applications section.
fOUT e
VIN
2.09 V
#
RS
RL
#
1
RtCt
**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 RRD-B30M115/Printed in U. S. A.
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
LM131A/LM131 LM231A/LM231 LM331A/LM331
Supply Voltage 40V 40V 40V
Output Short Circuit to Ground Continuous Continuous Continuous
Output Short Circuit to VCC Continuous Continuous Continuous
Input Voltage b0.2V to aVS b0.2V to aVS b0.2V to aVS
TMIN TMAX TMIN TMAX TMIN TMAX
Operating Ambient Temperature Range b55§C to a125§C b25§C to a85§C 0§C to a70§C
Power Dissipation (PD at 25§C)
and Thermal Resistance (ijA)
(H Package) PD 670 mW
ijA 150§C/W
(N Package) PD 1.25W 1.25W
ijA 100§C/W 100§C/W
(M Package)PD 1.25W
iJA 85§C/W
Lead Temperature (Soldering, 10 sec.)
Dual-In-Line Package (Plastic) 260§C 260§C 260§C
Metal Can Package (TO-5) 260§C
ESD Susceptibility (Note 4)
Metal Can Package (TO-5) 2000V
Other Packages 500V 500V
Electrical Characteristics TAe25§C unless otherwise specified (Note 2)
Parameter Conditions Min Typ Max Units
VFC Non-Linearity (Note 3) 4.5V s VS s 20V g0.003 g0.01 % Full-
Scale
TMIN s TA s TMAX g0.006 g0.02 % Full-
Scale
VFC Non-Linearity VS e 15V, f e 10 Hz to 11 kHz g0.024 g0.14 %Full-
In Circuit ofFigure 1 Scale
Conversion Accuracy Scale Factor (Gain) VIN e b10V, RS e 14 kX
LM131, LM131A, LM231, LM231A 0.95 1.00 1.05 kHz/V
LM331, LM331A 0.90 1.00 1.10 kHz/V
Temperature Stability of Gain TMIN s TA s TMAX, 4.5V s VS s 20V
LM131/LM231/LM331 g30 g150 ppm/§C
LM131A/LM231A/LM331A g20 g50 ppm/§C
Change of Gain with VS 4.5VsVS s 10V 0.01 0.1 %/V
10V s VS s 40V 0.006 0.06 %/V
Rated Full-Scale Frequency VIN e b10V 10.0 kHz
Gain Stability vs Time TMIN s TA s TMAX g0.02 % Full-
(1000 Hrs) Scale
Overrange (Beyond Full-Scale) Frequency VIN e b11V 10 %
INPUT COMPARATOR
Offset Voltage g3 g10 mV
LM131/LM231/LM331 TMIN s TA s TMAX g4 g14 mV
LM131A/LM231A/LM331A TMIN s TA s TMAX g3 g10 mV
Bias Current b80 b300 nA
Offset Current g8 g100 nA
Common-Mode Range TMIN s TA s TMAX b0.2 VCCb2.0 V
2
Electrical Characteristics TAe25§C unless otherwise specified (Note 2) (Continued)
Parameter Conditions Min Typ Max Units
TIMER
Timer Threshold Voltage, Pin 5 0.63 0.667 0.70 c VS
Input Bias Current, Pin 5 VS e 15V
All Devices 0VsVPIN 5 s 9.9V g10 g100 nA
LM131/LM231/LM331 VPIN 5 e 10V 200 1000 nA
LM131A/LM231A/LM331A VPIN 5 e 10V 200 500 nA
VSAT PIN 5 (Reset) I e 5 mA 0.22 0.5 V
CURRENT SOURCE (Pin 1)
Output Current RSe14 kX, VPIN 1e0
LM131, LM131A, LM231, LM231A 126 135 144 mA
LM331, LM331A 116 136 156 mA
Change with Voltage 0VsVPIN 1s10V 0.2 1.0 mA
Current Source OFF Leakage
LM131, LM131A 0.01 1.0 nA
LM231, LM231A, LM331, LM331A 0.02 10.0 nA
All Devices TAeTMAX 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 VDC
LM331, LM331A 1.70 1.89 2.08 VDC
Stability vs Temperature g60 ppm/§C
Stability vs Time, 1000 Hours g0.1 %
LOGIC OUTPUT (Pin 3)
VSAT Ie5 mA 0.15 0.50 V
Ie3.2 mA (2 TTL Loads), TMINsTAsTMAX 0.10 0.40 V
OFF Leakage g0.05 1.0 mA
SUPPLY CURRENT
LM131, LM131A, LM231, VSe5V 2.0 3.0 4.0 mA
LM231A VSe40V 2.5 4.0 6.0 mA
LM331, LM331A VSe5V 1.5 3.0 6.0 mA
VSe40V 2.0 4.0 8.0 mA
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 3, with 4.0VsVSs40V, unless otherwise noted.
Note 3: Nonlinearity is defined as the deviation of fOUT from VIN c (10 kHz/b10 VDC) 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, CT, use NPO ceramic, TeflonÉ, or polystyrene.
Note 4: Human body model, 100 pF discharged through a 1.5 kX resistor.
3
Functional Block Diagram
TL/H/5680–2
Pin numbers apply to 8-pin packages only. See connection diagram for LM231WM pin numbers.
FIGURE 1a
TeflonÉ registered trademark of DuPont
4
Typical Performance Characteristics
(All electrical characteristics apply for the circuit of Figure 3, unless otherwise noted.)
Nonlinearity Error, LM131
Family, as Precision V-to-F
Converter (Figure 3)
Nonlinearity Error, LM131
Family
Nonlinearity vs Power Supply
Voltage
Frequency vs Temperature,
LM131A
VREF vs Temperature,
LM131A
Output Frequency vs
VSUPPLY
100 kHz Nonlinearity Error,
LM131 Family (Figure 4)
Nonlinearity Error, LM131
(Figure 1)
Input Current (Pins 6, 7) vs
Temperature
Power Drain vs VSUPPLY
Output Saturation Voltage vs
IOUT (Pin 3)
Nonlinearity Error, Precision
F-to-V Converter (Figure 6)
TL/H/5680–3
5
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) con-
verter. A simplified block diagram of the LM131 is shown in
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,
Figure 2, which consists of the simplified block diagram of
the LM131 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, Vx, 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 te1.1 RtCt. During
this period, the current i will flow out of the switched current
source and provide a fixed amount of charge, Qei c t, into
the capacitor, CL. This will normally charge Vx 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 capaci-
tor CL will be gradually discharged by RL until Vx falls to the
level of V1. Then the comparator will trigger the timer and
start another cycle.
The current flowing into CL is exactly IAVE e i c (1.1cRtCt)
c f, and the current flowing out of CL is exactly Vx/RL j
VIN/RL. If VIN is doubled, the frequency will double to main-
tain this balance. Even a simple V-to-F converter can pro-
vide a frequency precisely proportional to its input voltage
over a wide range of frequencies.
TL/H/5680–4
FIGURE 2. Simplified Block Diagram of Stand-Alone
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 pro-
vides a stable 1.9 VDC output. This 1.9 VDC is well regulated
over a VS range of 3.9V to 40V. It also has a flat, low tem-
perature coefficient, and typically changes less than (/2%
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 ie1.90V/RS to flow. For
Rse14k, ie135 mA. The precision current reflector pro-
vides 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 RS flip-flop.
The timing function consists of an RS flip-flop, and a timer
comparator connected to the external RtCt network. When
the input comparator detects a voltage at pin 7 higher than
pin 6, it sets the RS flip-flop which turns ON the current
switch and the output driver transistor. When the voltage at
pin 5 rises to )/3 VCC, the timer comparator causes the RS
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 )/3 VCC, the flip-flop will not
be reset, and the current at pin 1 will continue to flow, in its
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
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 VDC for all values of
i between 10 mA to 500 mA. It can be used as a voltage
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 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 2 plus a few compo-
nents for improved performance.
A resistor, RINe100 kXg10%, has been added in the path
to pin 7, so that the bias current at pin 7 (b80 nA typical)
will cancel the effect of the bias current at pin 6 and help
provide minimum frequency offset.
The resistance RS at pin 2 is made up of a 12 kX fixed
resistor plus a 5 kX (cermet, preferably) gain adjust rheo-
stat. The function of this adjustment is to trim out the gain
tolerance of the LM131, and the tolerance of Rt, RL and Ct.
6
Typical Applications (Continued)
For best results, all the components should be stable low-
temperature-coefficient components, such as metal-film re-
sistors. The capacitor should have low dielectric absorption;
depending on the temperature characteristics desired, NPO
ceramic, polystyrene, Teflon or polypropylene are best
suited.
A capacitor CIN is added from pin 7 to ground to act as a
filter for VIN. A value of 0.01 mF to 0.1 mF will be adequate in
most cases; however, in cases where better filtering is re-
quired, a 1 mF capacitor can be used. When the RC time
constants are matched at pin 6 and pin 7, a voltage step at
VIN will cause a step change in fOUT. If CIN is much less
than CL, a step at VIN may cause fOUT to stop momentarily.
A 47X resistor, in series with the 1 mF CL, 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 3)
In this circuit, integration is performed by using a conven-
tional operational amplifier and feedback capacitor, CF.
When the integrator’s output crosses the nominal threshold
level at pin 6 of the LM131, the timing cycle is initiated.
The average current fed into the op amp’s summing point
(pin 2) is i c (1.1 RtCt) c f which is perfectly balanced with
bVIN/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 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 be-
low 2 nA are available at low cost, this circuit is recommend-
ed for best accuracy for small signals. This circuit also re-
sponds 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 VIN, 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 poten-
tial and that voltage does not vary with VIN 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 VIN).
The circuit ofFigure 4 operates in the same way asFigure 3,
but with the necessary changes for high speed operation.
fOUT e
bVIN
2.09 V
#
RS
RIN
#
1
RtCt
TL/H/5680–5
*Use stable components with low temperature coefficients. See Typical Applications section.
**This resistor can be 5 kX or 10 kX for VSe8V to 22V, but must be 10 kX for VSe4.5V to 8V.
***Use low offset voltage and low offset current op amps for A1: recommended types LM108, LM308A, LF411A
FIGURE 3. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter
7
Typical Applications (Continued)
DETAILS OF OPERATION, FREQUENCY-TO-
VOLTAGE CONVERTERS (FIGURES 5 AND 6)
In these applications, a pulse input at fIN 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 IAVERAGE e i c (1.1 RtCt) c f.
In the simple circuit of FIGURE 5, this current is filtered in
the network RL e 100 kX and 1 mF. 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 inFigure 5.
However, for input frequencies below 200 Hz, this circuit will
have worse ripple thanFigure 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.
TL/H/5680–6
*Use stable components with low temperature coefficients.
See Typical Applications section.
**This resistor can be 5 kX or 10 kX for VSe8V to 22V,
but must be 10 kX for VSe4.5V to 8V.
***Use low offset voltage and low offset current op amps for A1:
recommended types LF411A or LF356.
FIGURE 4. Precision Voltage-to-Frequency Converter,
100 kHz Full-Scale, g0.03% Non-Linearity
TL/H/5680–7
VOUT e fIN c 2.09V c
RL
RS
c (RtCt)
*Use stable components with low temperature coefficients.
FIGURE 5. Simple Frequency-to-Voltage Converter,
10 kHz Full-Scale, g0.06% Non-Linearity
VOUT e bfIN c 2.09V c
RF
RS
c (RtCt) TL/H/5680–8
SELECT Rx e
(VS b 2V)
0.2 mA
*Use stable components with low temperature coefficients.
FIGURE 6. Precision Frequency-to-Voltage Converter,
10 kHz Full-Scale with 2-Pole Filter, g0.01%
Non-Linearity Maximum
8
Typical Applications (Continued)
Light Intensity to Frequency Converter
TL/H/5680–9
*L14F-1, L14G-1 or L14H-1, photo transistor (General Electric Co.) or similar
Temperature to Frequency Converter
TL/H/5680–10
Long-Term Digital Integrator Using VFC
TL/H/5680–11
Basic Analog-to-Digital Converter Using
Voltage-to-Frequency Converter
TL/H/5680–12
9
Typical Applications (Continued)
Analog-to-Digital Converter with Microprocessor
TL/H/5680–13
Remote Voltage-to-Frequency Converter with 2-Wire Transmitter and Receiver
TL/H/5680–14
Voltage-to-Frequency Converter with Square-Wave Output Usingd2 Flip-Flop
TL/H/5680–15
Voltage-to-Frequency Converter with Isolators
TL/H/5680–16
10
Typical Applications (Continued)
Voltage-to-Frequency Converter with Isolators
TL/H/5680–17
Voltage-to-Frequency Converter with Isolators
TL/H/5680–18
Voltage-to-Frequency Converter with Isolators
TL/H/5680–19
11
Connection Diagrams
Metal Can Package
TL/H/5680–20
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/5680–21
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
Schematic Diagram
TL/H/5680–22
13
14
Physical Dimensions inches (millimeters)
Metal Can Package (H)
Order Number LM131H/883 or LM131AH/883
NS Package H08C
14-Pin Small Outline Package (M)
Order Number LM231WM
NS Package M14B
15
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Physical Dimensions inches (millimeters) (Continued)
Dual-In-Line Package (N)
Order Number LM231AN, LM231N, LM331AN, or LM331N
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