29-12-2012, 06:41 PM
Voltage-to-Frequency and Frequency-to-Voltage CONVERTER
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DESCRIPTION
The VFC32 voltage-to-frequency converter provides
an output frequency accurately proportional to its
input voltage. The digital open-collector frequency
output is compatible with all common logic families.
Its integrating input characteristics give the VFC32
excellent noise immunity and low nonlinearity.
Full-scale output frequency is determined by an external
capacitor and resistor and can be scaled over a
wide range. The VFC32 can also be configured as a
frequency-to-voltage converter.
The VFC32 is available in 14-pin plastic DIP, SO-14
surface-mount, and metal TO-100 packages. Commercial,
industrial, and military temperature range models
are available.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and
installation procedures can cause damage.
ESD damage can range from subtle performance degradation to
complete device failure. Precision integrated circuits may be
more susceptible to damage because very small parametric
changes could cause the device not to meet its published specifications.
APPLICATION INFORMATION
Figure 1 shows the basic connection diagram for frequencyto-
voltage conversion. R1 sets the input voltage range. For a
10V full-scale input, a 40kW input resistor is recommended.
Other input voltage ranges can be achieved by changing the
value of R1.
R1 should be a metal film type for good stability. Manufacturing
tolerances can produce approximately ±10% variation
in output frequency. Full-scale output frequency can be
trimmed by adjusting the value of R1—see Figure 3.
The full-scale output frequency is determined by C1. Values
shown in Figure 1 are for a full-scale output frequency of
10kHz. Values for other full-scale frequencies can be read
from Figure 2. Any variation in C1—tolerance, temperature
drift, aging—directly affect the output frequency. Ceramic
NPO or silver-mica types are a good choice.
For full-scale frequencies above 200kHz, use larger capacitor
values as indicated in Figure 2, with R1 = 20kW.
FREQUENCY OUTPUT PIN
The frequency output terminal is an open-collector logic
output. A pull-up resistor is usually connected to a 5V logic
supply to create standard logic-level pulses. It can, however,
be connected to any power supply up to +VCC. Output pulses
have a constant duration and positive-going during the oneshot
period. Current flowing in the open-collector output
transistor returns through the Common terminal. This terminal
should be connected to logic ground.
FREQUENCY-TO-VOLTAGE CONVERSION
Figure 4 shows the VFC32 connected as a frequency-tovoltage
converter. The capacitive-coupled input network C3,
R6 and R7 allow standard 5V logic levels to trigger the
comparator input. The comparator triggers the one-shot on
the falling edge of the frequency input pulses. Threshold
voltage of the comparator is approximately –0.7V. For
frequency input waveforms less than 5V logic levels, the
R6/R7 voltage divider can be adjusted to a lower voltage to
assure that the comparator is triggered.
The value of C1 is chosen from Figure 2 according to the
full-scale input frequency. C2 smooths the output voltage
waveform. Larger values of C2 reduce the ripple in the
output voltage. Smaller values of C2 allow the output voltage
to settle faster in response to a change in input frequency.
Resistor R1 can be trimmed to achieve the desired output
voltage at the full-scale input frequency.
PRINCIPLES OF OPERATION
The VFC32 operates on a principle of charge balance. The
signal input current is equal to VIN/R1. This current is
integrated by input op amp and C2, producing a downward
ramping integrator output voltage. When the integrator output
ramps to the threshold of the comparator, the one-shot is
triggered. The 1mA reference current is switched to the
integrator input during the one-shot period, causing the
integrator output ramp upward. After the one-shot period,
the integrator again ramps downward.