Inexpensive VFC features good linearity and dynamic range
A voltage to frequency converter circuit builds on the work of two analog greats.
Jordan Dimitrov, Toronto, ON, Canada; Edited by Paul Rako and Fran Granville -- EDN, December 1, 2011
VFCs (voltage-to-frequency converters) were favorite circuits of the late analog gurus Bob Pease and Jim Williams (references 1 to 5). In tribute to them, this Design Idea reveals a circuit that provides good performance at a low price. You can obtain all of the parts for a few dollars from a local electronics shop.
The circuit has high input impedance,
works with a single power supply,
and connects directly to microcontrollers.
The linearity error is less than
0.1% for frequencies as high as 700 kHz,
and the dynamic range is 60 dB. The
circuit exploits the integrator, comparator,
and one-shot architecture (Figure
1). The output frequency is proportional
to the input voltage: f=(1/VCCtOS)
VIN, where VCC is the power supply,
5V, and tOS is the duration of the pulse
that the one-shot generates, according to the equation tOS=0.7×ROS×COS.
You must filter and regulate the power
supply, VCC. If the magnitude of the
power supply changes, the slope of the
calibration curve also changes. The
components you use for the integrator,
CINT and RINT, do not participate in the
equation so they need not be either
accurate or stable. However, capacitors
CINT and COS must have low dielectric
absorption.
You build a start-up circuit with
switch S1 and the timing network comprising
R1, C1, and R2. This step ensures
that the circuit will oscillate with any
value of input voltage. After you turn
on the power supply, the switch stays
closed for approximately 1 sec, keeping
CINT completely discharged. When
the switch opens, CINT starts charging
by a fixed current, which the magnitude
of the input voltage defines. The result is a rising ramp at the integrator’s
output. When the ramp reaches 2.5V,
IC2 generates a pulse because 2.5V
is the threshold level of the Schmitt
trigger at the 1B input of IC2. Because
the pulse magnitude is larger than the
input voltage, the current through CINT
reverses, and CINT partially discharges
(Figure 2).
When the pulse is over, the integrator
starts another rising ramp, and the
cycle repeats. Because of the built-in
Schmitt trigger, the circuit requires no
separate comparator IC. Most applications can go without any adjustment. You adjust the full-scale frequency
using only the trimming potentiometer, which is part of ROS in Figure 1.You can select different frequency
spans (Table 1), each requiring its own
values for CINT and ROS. The spans have
different linearity. The table shows
the linearity error as a percentage of
the full-scale frequency for 11 equally
spaced values of the input value in a
range from 2 mV to 2V.
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References |
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Talkback
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The output of the one-shot can have several mVs of offset. It would be better to use two sections of the 4066 to switch Rint between two reference points.
Einar Abell - 2011-19-12 01:32:21 PST -
IT may be reliable using the '221 oneshot, but I have experienced schmit trigger problems using the MC14538, which is a similar function. The problem was a variable trigger point, which brought the switching point into the noise window in my application. The cure was to use the same product from a different maker. This is a good reason to design using multisourced parts. The second source saved a whole production run of circuit boards.
William Ketel - 2011-5-12 14:30:43 PST
