VAR

Vishay Foil Resistors

www.vishayfoilresistors.com

Document Number: 63140

2

Revision: 22-Nov-10

While the regular foil resistors are already widely

acknowledged as the leading resistors for audio applications,

the special “naked Z-foil resistor” design without mold or

encapsulation, adds an additional dimension for reducing

signal distortion and increasing clarity in signal processing.

Our application engineering department is available to

advise and to make recommendations. For non-standard

technical requirements and special applications, please

contact us.

Bulk Metal Foil resistors owe their low current noise and high

linearity to the type of material they’re made of: a several

microns thick cold-rolled metal foil. Every real-world resistor

possesses certain nonlinearity of its electrical resistance

(nonlinearity of volt-ampere characteristic). The degree of

nonlinearity depends on two factors:

• Micro factors: internal microstructure of resistive material.

• Macro factors (laser trimming cuts cleanness, micro

cracking in resistive element resulting from laser trimming,

quality of contacts between resistive element and

terminals, etc.).

As for the microstructure, the most linear materials are pure

metals and metal alloys in bulk, such as the foil in Bulk

deposited in the form of very Thin (nanometer range) Films,

they are less linear. Even less linear are composite materials

like resistive cermets in Thick-Film resistors or carbon

compositions in carbon composition resistors.

The macro factors that cause nonlinearity in other types of

resistors aren’t relevant to foil resistors. Indeed, laser

trimming of foil resistors consists of cutting shorting jumpers

and does not damage current carrying portions of resistive

element. Terminals in foil resistors are an integral part of the

foil resistive element. This insures high-quality contact

between resistive element and terminals

Several types of noise are found in resistors:

Thermal noise is caused by thermal movement of electrons

in resistive material and gets worse as resistance and

temperature increase. Thermal noise can be reduced by

reducing resistance, temperature, or signal bandwidth.

Shot or fluctuation noise is caused by the discrete nature

of charge carriers and fluctuation of their number in the unit

of volume. Shot noise can be reduced by reducing bandwidth

or increasing current. The spectral density of voltage in both

thermal and shot noises is uniformly distributed in entire

range of frequencies (“white” noise). The level of these types

of noise does not depend on resistor type (resistive element

material).

Current (excess, flicker) noise has 1/f type spectral density

of voltage (“pink” noise). Its level essentially depends on

resistor material. Current noise can be reduced by a)

avoiding use of the low frequency band, b) reducing current,

c) increasing the volume of resistive material, i.e. by using

resistors with higher rated power than is needed for proper

power dissipation, or d) using less noisy resistive materials.

Carbon composition resistors are the noisiest such device

type followed by Thick Film and Thin Film resistors. The least

noisy are bulk metals and metal alloys (foil, wire). At that,

negligible capacitance/inductance of foil resistors (when

compared to wirewound resistors) significantly reduces

probability of self-excitation or "ringing" of amplification

choice for low-noise applications.

FIGURE 1 - TYPICAL RESISTANCE /

TEMPERATURE CURVE

(Z-FOIL)

+ 500

+ 200

+ 100

0

- 100

- 200

- 300

- 500

- 55

- 25

+ 25

+ 60

+ 75

+ 100

+ 125

Ambient Temperature (°C)

ΔR

R

(ppm)

0

0.05 ppm/°C

- 0.1 ppm/°C

0.1 ppm/°C

0.14 ppm/°C

0.2 ppm/°C

- 0.16 ppm/°C

- 400

+ 300

+ 400

TCR chord slopes for different temperature ranges

TABLE 1 - RESISTANCE VERSUS TCR

RESISTANCE VALUE

(

Ω)

TYPICAL TCR AND MAXIMUM SPREAD (ppm/°C)

TIGHTEST TOLERANCE

(%)

100 to < 100K

± 0.2 ± 1.8

0.01

50 to < 100

± 0.2 ± 2.8

0.01

10 to < 50

± 0.2 ± 3.8

0.02

Note:

(1) Pb containing terminations are not RoHS compliant, exemptions may apply

(2) For non-standard requests, please contact application engineering at foil@vishaypg.com