廬
APPLICATION BULLETIN
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VOLTAGE-TO FREQUENCY CONVERTERS
OFFER USEFUL OPTIONS IN A/D CONVERSION
Specialized Counting Techniques Achieve Improved Speed and Resolution
Voltage-to-frequency converters (VFCs) provide unique
characteristics when used as analog-to-digital (A/D) con-
verters. Their excellent accuracy, linearity, and integrating
input characteristics often provide performance attributes
unattainable with other converter types. By using efficient
frequency counting techniques, familiar speed/accuracy
tradeoffs can be averted.
Since an analog quantity represented as a frequency is
inherently a serial data stream, it is easily handled in large
multichannel systems. Frequency information can be trans-
mitted over long lines with excellent noise immunity using
low cost digital line transmitters and receivers. Voltage
isolation can be accomplished with low cost optical couplers
or transformers without loss in accuracy. Many channels of
frequency data can be efficiently steered to one counter
circuit using simple digital gating, avoiding expensive ana-
log multiplexing circuitry.
Like a dual-slope A/D converter, the VFC possesses a true
integrating input. While a successive approximation A/D
converter takes a 鈥渟napshot鈥?in time, making it susceptible
to noise peaks, the VFC鈥檚 input is constantly integrating,
smoothing the effects of noise or varying input signals.
When system requirements suggest the VFC as an appropri-
ate choice, a frequency measurement technique must also be
chosen which meets the conversion speed requirements.
While it is clearly not a 鈥渇ast鈥?converter, conversion speed
of a VFC system can be optimized by using efficient
counting techniques.
The frequency counting scheme shown in Figure 1 is the
most commonly used technique for converting the output of
a VFC to a numerical quantity. A gate time is created by
dividing a reference frequency down to a suitable period, T.
The output pulses of the VFC are simply accumulated
during the time the gate signal is high. If T is equal to one
second, for instance, the output count M is equal to the VFC
frequency. Other gate periods (often 0.1 seconds, 10 sec-
onds, etc.) are conveniently scaled by a decimal point shift
or a simple multiplying factor. The reset circuitry which
must be used to clear the counter before the next gate period
occurs is not shown in this simplified diagram.
Since the gate period is not synchronized to the VFC output
pulses, there is a potential counting inaccuracy of plus or
minus one count on M. This is easily seen by imagining a
sliding window of width T along the VFC output waveform.
91
V
IN
VFC
f
Counter M
M=3
M鹵1
f=
T
3鹵1
f
r
f=
10
Reference
Oscillator
f
r
梅
10
f
r
G
f
1
2
3
T
M=3
M=2
1
2
FIGURE 1. This Simplified Diagram of the Standard Counting Method Shows the Potential Inaccuracies That Can Occur.
Although the first gate counts three rising edges of the VFC output frequency, f, the second gate period counts only
two.
漏
1994 Burr-Brown Corporation
AB-066
Printed in U.S.A.May, 1994
Application Bulletin Number 91
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