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Vishay Foil Resistors
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
Metal ? Foil resistors. When the same materials are
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.
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
circuit.This is why Bulk Metal ? foil resistors are such a good
choice for low-noise applications.
FIGURE 1 - TYPICAL RESISTANCE /
TEMPERATURE CURVE
(Z-FOIL)
Δ R
- 100
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
+ 500
+ 400
+ 300
+ 200
+ 100
R 0
(ppm)
TCR chord slopes for different temperature ranges
0.05 ppm/°C
Several types of noise are found in resistors:
- 200
- 300
- 0.1 ppm/°C
0.1 ppm/°C
0.14 ppm/°C
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
- 400
- 500
- 55
- 0.16 ppm/°C 0.2 ppm/°C
- 25 0 + 25 + 60 + 75 + 100
Ambient Temperature (°C)
+ 125
reducing resistance, temperature, or signal bandwidth.
TABLE 1 - RESISTANCE VERSUS TCR (- 55 °C to + 125 °C, + 25 °C Ref.) (2)
RESISTANCE VALUE
( ? )
100 to < 100K
50 to < 100
10 to < 50
TYPICAL TCR AND MAXIMUM SPREAD (ppm/°C)
± 0.2 ± 1.8
± 0.2 ± 2.8
± 0.2 ± 3.8
TIGHTEST TOLERANCE
(%)
0.01
0.01
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
www.vishayfoilresistors.com
2
For any questions, contact: foil@vishaypg.com
Document Number: 63140
Revision: 03-Jan-12
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