VFC110

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 VREF output is useful for offsetting the transfer function

and exciting sensors. Figure 3 shows VREF 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 VREF

can decrease frequency stability due to internal heating.

2

NC

V

IN

112

11

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 CINT, and input resistor RIN. The

integrator’s output voltage is proportional to the

charge stored in CINT. An input voltage develops an

input current of VIN/RIN, which is forced to flow

through CINT. This current charges CINT, 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, IREF, (approximately

1mA) to the integrator input during the one-shot

period, TOS. 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 TO is the one-shot period and fOUT 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