Frequency-to-Voltage Converter uses Sample-and-Hold
to Improve Response and Ripple
Frequency-to-Voltage
Converter uses Sample-
and-Hold to Improve
Response and Ripple
Most frequency-to-voltage (F-to-V) converters suffer from
the classical tradeoff of ripple versus speed of response
For example the basic F-to-V converter shown below has
13 mVp-p of ripple and a rather slow 0 6 second settling
time when C
FILTER
is 1
mF
If you want less ripple than that
the response time will be even slower If you want quicker
response it is easy to decrease C
FILTER
but the ripple will
increase by the same factor
The improved circuit in
Figure 2
makes an end-run around
these compromises A low-cost sample-and-hold circuit
National Semiconductor
Linear Brief 45
April 1979
such as LF398 can sample the F-to-V鈥檚 output at the peak
of its ripple and hold it until the next cycle The LF398 has
fairly low output ripple (rms) but it does have some short
duration noise spikes and glitches which can be removed
easily with a simple output filter The ripple at the output of
the active filter V6 is smaller than 1 mV peak but the set-
tling time for a step change of input frequency is only 60 ms
or ten times quicker than the 鈥樷€榖asic鈥欌€?FVC with C
FILTER
e
1
mF
V
OUT
e
f
IN
c
J
output
ripple
p-p
e
R
J
(1 9V) (1 1R C )
1
(1 9V) (1 1R C )
C
R
J
R
L
c
c
S
t t
c
c
t t
FILTER
S
TL H 8494 鈥?1
FIGURE 1 Basic Frequency-to-Voltage Converter
LB-45
TL H 8494 鈥?2
FIGURE 2 Improved F-to-V Converter Using Sample-and-Hold
C
1995 National Semiconductor Corporation
TL H 8494
RRD-B30M115 Printed in U S A
1
2
