Datasheet

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SBVS021A − OCTOBER 1988 − REVISED APRIL 2007
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8
Using the Enable input, several VFCs’ outputs can be
connected to a single output line. All disabled VFCs will
have a high output impedance; one active VFC can then
transmit on the output line. Since the disabled VFCs are
not oscillating, they cannot interfere or lock with the
operating VFC. Locking can occur when one VFC
operates at nearly the same frequency as—or a multiple
of—a nearby VFC. Coupling between the two may cause
them to lock to the same or exact multiple frequency. It then
takes a small incremental input voltage change to unlock
them. Locking cannot occur when unneeded VFCs are
disabled.
REFERENCE VOLTAGE
The V
REF
output is useful for offsetting the transfer function
and exciting sensors. Figure 3 shows V
REF
used to offset
the transfer function of the VFC110 to achieve a bipolar
input voltage range. Sub-surface zener reference circuitry
is used for low noise and excellent temperature drift.
Output current is specified to 10mA and current-limited to
approximately 20mA. Excessive or variable loads on V
REF
can decrease frequency stability due to internal heating.
2
NC
V
IN
11211
One−Shot
V
REF
4 13
5V
3 6
8
f
OUT
7
5
R
PU
C
OS
R
1
14
NC
+5V+15V
10
−
15V
R
2
I
REF
(∼1mA)
Figure 3. Offsetting the Frequency Output
PRINCIPLE OF OPERATION
The VFC110 uses a charge-balance technique to
achieve high accuracy. The heart of this technique is
an analog integrator formed by the integrator op amp,
feedback capacitor C
INT
, and input resistor R
IN
. The
integrator’s output voltage is proportional to the
charge stored in C
INT
. An input voltage develops an
input current of V
IN
/R
IN
, which is forced to flow
through C
INT
. This current charges C
INT
, causing the
integrator output voltage to ramp negatively.
When the output of the integrator ramps to 0V, the
comparator trips, triggering the one-shot. This
connects the reference current, I
REF
, (approximately
1mA) to the integrator input during the one-shot
period, T
OS
. This switched current causes the
integrator output to ramp positively until the one-shot
period ends. Then the cycle starts again.
The oscillation is regulated by the balance of current
(or charge) between the input current and the
time-averaged reset current. The equation of current
balance is:
where T
O
is the one-shot period and f
OUT
is the
oscillation frequency.
I
IN
+ I
IREF
Duty Cycle
V
IN
R
IN
+ I
REF
f
OUT
T
O
Integrator
Output
(Pin 12)
0V
f
OUT
1/f
OUT
T
OS
Effect of
Smaller C
INT