VFC110

SBVS021A − OCTOBER 1988 − REVISED APRIL 2007

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6

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, RPU, 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 25k

Ω internal input resistor. Pin 14 (the noninverting

amplifier input) should be referred directly to the negative

side of VIN. 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 VIN, easily

accommodating any ground-drop errors. The input

impedance

loading

VIN

is

equal

to

the

input

resistor—approximately 25k

Ω.

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, COS, should be

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 CINT

equal to COS 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 RIN.

The integrator capacitor serves as a charge bucket, where

charge is accumulated from the input, VIN, 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

0.01

µF. 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.

2

14

V

IN

112

11

One− Shot

V

REF

−V

S

Analog Ground

413

NC

3

6

8

Logic Ground

f

OUT

0MHz to 4MHz

10

+V

S

7

NC

5

NC

50pF(1)

NC

R

PU

680

Ω

−15V

+15V

V

L

+5V

25k

Ω(1)

0V to

+10V

NOTE: (1) Nominal values (

±20%).

Figure 1. 4MHz Full-Scale Operation