1 µF 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.
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.
A 47Ω resistor, in series with the 1 µF CL, is added to give
hysteresis effect which helps the input comparator provide
the excellent linearity (0.03% typical).
The output driver transistor acts to saturate pin 3 with an ON
resistance of about 50Ω. In case of overvoltage, the output
current is actively limited to less than 50 mA.
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, CF.
When the integrator’s output crosses the nominal threshold
level at pin 6 of the LM231/331, the timing cycle is initiated.
The voltage at pin 2 is regulated at 1.90 VDC 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.
The average current fed into the op amp’s summing point
(pin 2) is i x (1.1 RtCt) x f which is perfectly balanced with
−VIN/RIN. 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 VIN, as quickly as 2 output pulses’
spacing can be measured.
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, RIN 100 kΩ 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 RS at pin 2 is made up of a 12 kΩ fixed resis-
tor plus a 5 kΩ (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 Rt, RL and Ct.
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.
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 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).
A capacitor CIN is added from pin 7 to ground to act as a filter
for VIN. 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
The circuit of Figure 5 operates in the same way as Figure 4,
but with the necessary changes for high speed operation.
7
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