4423

SBFS020A − JANUARY 1978 − REVISED JUNE 2004

www.ti.com

6

After selecting the capacitor for a particular frequency,

the value of the required resistor can be obtained by

using the resistor selection curve shown in Figure 7 or

by Equation (3).

R

+

3.785f(C

) 0.001)

42.05

* 2f(C ) 0.001)

Where:

R is in k

Ω.

f is in Hz.

C is in

µF.

1k

10k

100k

0.001

10k

1k

100

10

1

0.01

0.1

1

10

100

Frequency (Hz)

Figure 7.

RECOMMENDED CAPACITOR TYPES

There are various types of capacitors available for use.

There are polarized (DC) capacitors and nonpolarized

(AC) capacitors available. Of these two types, polarized

capacitors cannot be used with the 4423 to set the

frequencies.

Commonly-available nonpolarized capacitors include

NPO

ceramic,

silver

mica,

Teflon,

polystyrene,

polycarbonate, mylar, and ceramic disc. A comparison

of capacitor types is shown in Table 2.

Table 2. Comparison of Nonpolarized Capacitors

CAPACITOR

TYPE

CAPACITANCE

RANGE

(

µF)

TEMPERATURE

COEFFICIENTS

(ppm/

5C)

DISSIPATION

FACTOR

(%)

NPO Ceramic

5pF to 0.1

µF

30

0.05

Silver Mica

5pF to 0.047

µF

60

0.05

Teflon

0.001 to 100

200

0.01

Polystyrene

0.001 to 500

100

0.03

Polycarbonate

0.001 to 1000

90

0.08

Metallized

Teflon

0.001 to 100

60

0.1

Metallized

Polycarbonate

0.001 to 1000

10

0.4

Mylar

0.001 to 1000

700

0.7

Metallized

Mylar

0.001 to 2000

700

1

Ceramic Disc

5pF to 0.5

µF

10,000

3

Choosing a capacitor to use with the 4423 depends

mainly on the application, error budget, and cost

budget. Note that the specifications of the 4423 do not

include

the

error

contribution

of

the

external

components. Errors sourced by external components

normally have to be added to the 4423 specifications.

As a general selection criteria, TI recommends the use

of Table 2. If the capacitor is found unsuitable due to its

size, cost, or availability, then move down the list for the

next best selection. In any case, do not choose or use

any capacitors with dissipation factors greater than 1%.

Such capacitors would stop 4423 oscillation.

DISSIPATION FACTOR (DF)

A capacitor can be modeled by an ideal capacitor in

parallel with an internal resistor whose value depends

on its dissipation factor (DF). Mathematically, internal

resistor R is given by Equation (4).

R

+

1

2

pfC(DF)

Where:

R is in

Ω.

f is in Hz.

C is in farads.

(3)

(4)