VFC110
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SBVS021A − OCTOBER 1988 − REVISED APRIL 2007
OPERATION
Figure 1 shows the connections required for operation at
a full-scale output frequency of 4MHz. Only power supply
bypass capacitors and an output pull-up resistor, R
PU
, are
required for this mode of operation. A 0V to 10V input
voltage produces a 0Hz to 4MHz output frequency. The
internal input resistor, one-shot and integrator capacitors
set the full-scale output frequency. The input is applied to
the summing junction of the integrator amplifier through
the 25kΩ internal input resistor. Pin 14 (the noninverting
amplifier input) should be referred directly to the negative
side of V
IN
. The common-mode range of the integrating
amplifier is limited to approximately −1V to +1V referred to
analog ground. This allows the noninverting input to
Kelvin-sense the common connection of V
IN
, easily
accommodating any ground-drop errors. The input
impedance loading V
IN
is equal to the input
resistor—approximately 25kΩ.
connected to logic ground. The one-shot connection
(pin 6) is not short-circuit protected. Short-circuits to
ground may damage the device.
The integrator capacitor’s value does not directly affect the
output frequency, but determines the magnitude of the
voltage swing on the integrator’s output. Using a C
INT
equal to C
OS
provides an integrator output swing from 0V
to approximately 1.5V.
COMPONENT SELECTION
Selection of the external resistor and capacitor type is
important. Temperature drift of an external input resistor
and one-shot capacitor will affect temperature stability of
the output frequency. NPO ceramic capacitors will
normally produce the best results. Silver-mica types will
result in slightly higher drift, but may be adequate in many
applications. A low temperature coefficient film resistor
should be used for R
IN
.
The integrator capacitor serves as a
charge bucket,
where
charge is accumulated from the input, V
IN
, and that charge
is drained during the one-shot period. While the size of the
bucket (capacitor value) is not critical, it must not leak.
Capacitor leakage or dielectric absorption can affect the
linearity and offset of the transfer function. High-quality
ceramic capacitors can be used for values less than
0.01µF. Use caution with higher value ceramic capacitors.
High-k ceramic capacitors may have voltage
nonlinearities which can degrade overall linearity.
Polystyrene, polycarbonate, or mylar film capacitors are
superior for high values.
OPERATION AT LOWER FREQUENCIES
The VFC110 can be operated at lower frequencies simply
by limiting the input voltage to less than the nominal 10V
full-scale input. To maintain a 10V FS input and highest
accuracy, however, external components are required
(see Table 1). Small adjustments may be required in the
nominal values indicated. Integrator and one-shot
capacitors are added in parallel to internal capacitors.
Figure 2 illustrates the connections required for 100kHz
full-scale output. The one-shot capacitor, C
OS
, should be
+V
S
+15V
NC
1
12
11
10
8
2
14
0V to
V
+10V
IN
25kΩ
(1)
50pF
(1)
One− Shot
V
L
+5V
R
PU
680Ω
f
OUT
0MHz to 4MHz
7 Logic Ground
5 NC
V
REF
4
−15V
−V
S
NOTE: (1) Nominal values (±20%).
13
3
NC
6
NC
Analog Ground
Figure 1. 4MHz Full-Scale Operation
6
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