Typical Applications
(Continued)
DETAILS OF OPERATION FREQUENCY-TO-
VOLTAGE CONVERTERS
(FIGURES 5 AND 6
)
In these applications a pulse input at f
IN
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 I
AVERAGE
e
i
c
(1 1 R
t
C
t
)
c
f
In the simple circuit of
FIGURE 5
this current is filtered in
the network R
L
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 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
S
e
8V to 22V
but must be 10 kX for V
S
e
4 5V to 8V
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
100 kHz Full-Scale
g
0 03% Non-Linearity
TL H 5680– 7
R
L
c
(R
t
C
t
)
V
OUT
e
f
IN
c
2 09V
c
R
S
Use stable components with low temperature coefficients
V
OUT
e b
f
IN
c
2 09V
c
SELECT Rx
e
(V
S
b
2V)
0 2 mA
R
F
c
(R
t
C
t
)
R
S
TL H 5680 –8
FIGURE 5 Simple Frequency-to-Voltage Converter
10 kHz Full-Scale
g
0 06% Non-Linearity
Use stable components with low temperature coefficients
FIGURE 6 Precision Frequency-to-Voltage Converter
10 kHz Full-Scale with 2-Pole Filter
g
0 01%
Non-Linearity Maximum
8
NSC [ NATIONAL SEMICONDUCTOR ]
