Typical Applications (Continued)
DETAILS OF OPERATION, FREQUENCY-TO-
VOLTAGE CONVERTERS(FIGURES 5 AND 6)
0.1 second time constant, and settling of 0.7 second to
0.1% accuracy.
In these applications, a pulse input at f is differentiated by
IN
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.
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
e
c
c
(1.1 R C )
t
is I
i
f.
AVERAGE
t
In the simple circuit of FIGURE 5, this current is filtered in
e
than 10 mV peak, but the response will be slow, with a
the network R
100 kX and 1 mF. The ripple will be less
L
*Use stable components with low temperature coefficients.
See Typical Applications section.
e
**This resistor can be 5 kX or 10 kX for V
8V to 22V,
S
e
but must be 10 kX for V
4.5V to 8V.
S
***Use low offset voltage and low offset current op amps for A1:
recommended types LF411A or LF356.
TL/H/5680–6
FIGURE 4. Precision Voltage-to-Frequency Converter,
g
100 kHz Full-Scale, 0.03% Non-Linearity
TL/H/5680–7
R
R
R
L
F
e
c
c
c
e b
c
c
c
(R C )
V
OUT
f
2.09V
(R C )
t t
V
f
2.09V
IN
OUT
IN
t t
TL/H/5680–8
R
S
S
b
*Use stable components with low temperature coefficients.
(V
2V)
S
e
SELECT Rx
0.2 mA
*Use stable components with low temperature coefficients.
FIGURE 5. Simple Frequency-to-Voltage Converter,
g
10 kHz Full-Scale, 0.06% Non-Linearity
FIGURE 6. Precision Frequency-to-Voltage Converter,
g
10 kHz Full-Scale with 2-Pole Filter, 0.01%
Non-Linearity Maximum
8
NSC [ National Semiconductor ]
