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LM331N Datasheet(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM331N
Description  Precision Voltage-to-Frequency Converters
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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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 001 mFto0 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 IfCIN is much less
than CL a step at VIN may cause fOUT to stop momentarily
A47X resistor in series with the 1 mFCL is added to give
hysteresis effect which helps the input comparator provide
the excellent linearity (003% 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 (11 RtCt) c f which is perfectly balanced with
b
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 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 of
Figure 4 operates in the same way as Figure 3
but with the necessary changes for high speed operation
fOUT e
b
VIN
209 V

RS
RIN

1
RtCt
TLH5680 – 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 VSe45V 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




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