CSM2512S

Vishay Foil Resistors

www.vishayfoilresistors.com

Document Number: 63145

2

Revision: 3-May-11

ABOUT CSM

(Low Ohm Value 10 m

Ω to 100 mΩ)

The CSM2512S series of low value current sense resistors

provides power and precision in a four terminal, surface

mount configuration. Its all welded construction is made up

terminations. For low value resistors in precision applications

it is necessary to use four-terminal Kelvin connections to

obtain a precise voltage drop across the resistive element.

In these applications, the contact resistance and the terminal

resistance may have the same order of magnitude or be

even greater than that of the element resistance itself. Thus,

significant error is introduced because the high temperature

coefficient of resistance of the leads and the contact

resistance

are

unavoidably

incorporated

into

the

measurements when the current sense resistor has only two

leads.

Because the ability to measure low values to tolerances of

0.1% or tighter is a concern to both the manufacturer and the

user, many situations require coordination of measurement

standards between both parties. Coordination is often

accomplished by exchange of serialized units with recorded

readings to align measurement practices and specific

reference standards. The problem is compounded when

high-precision current sensors under moderate to high

power experience self-heating (Joule effect) which causes

the in-service resistance value to be different from that

obtained using low current measurement equipment.

Therefore, the measurement conditions must be defined and

accepted at the time of spec preparation-that is, resistance

value as determined by specified current and measured

IR-drop following a specified period of stabilization.

Measurement equipment is available from a number of

sources with varying stated accuracies. Traditional passive

current sensors and shunts generate heat under power,

which changes their resistance, and thus their voltage

output. The CSM’s low absolute TCR reduces errors due to

temperature gradients, thus reducing a major source of

uncertainty in current measurement. The CSM can withstand

unconventional environmental conditions, including the

extremely

high

temperatures

and

radiation-rich

environments of down-hole oil exploration and well logging,

or the deep-sea underwater repeaters in cross-ocean

communications. The stability of the CSM can be further

enhanced by post-manufacturing operations (PMO), such as

temperature cycling, short-time overload, and accelerated

resistors. The device features a low thermal electromotive

force (EMF) that is critical in many precision applications.

Thermal EMF in DC applications induces a voltage offset in

the resistor that is equivalent to adding a small battery into

the circuit.

element with welded copper terminations, plated for

soldering. The terminations make a true continuous contact

with the resistive layer along the entire side of the resistive

element, thereby minimizing temperature variations. Also,

the resistor element is designed to uniformly dissipate power

without creating hot spots, and the welded terminations

material is compatible with the element material. These

design factors result in a very low thermal-EMF(<3 µV/°C)

resistor, because in addition to the low thermal EMF

compatibility of the metals, the uniformity and thermal

efficiency of the design minimizes the temperature

differential across the resistor, thereby assuring low thermal

EMF generation at the leads. This further reduces the

“battery effect” exhibited by most current-sensing or

voltage-reference resistors. Thus, the parasitic voltage

generated at the junction of two dissimilar metals, which is

especially important in low-value current-sensing resistors, is

minimized, while the pure current-to-voltage conversion is

protected from such interference in DC applications.

The stability problems associated with analog circuits are

very pervasive, but knowledgeable selection of a few

high-quality resistors, networks, or trimming potentiometers

in critical locations can greatly improve circuit performance,

long-term application-related performance, as well as the

designer’s peace-of-mind. Additionally, the overall system

cost is often reduced when a knowledgeable designer

concentrates costs in a few exceptionally stable components

with minimal deviation and whose load and environmental

stability can often eliminate the necessity of additional

compensating circuitry or temperature-controlling systems.

The higher reliability and better overall system performances

also achieve excellent product results in the field, enhancing

market acceptance and product reputation. Designers often

unnecessarily pay for tighter tolerances than required simply

to accommodate the resistance stability shifts they know to

be imminent in an application due to the large

application-related changes in the components they

selected. Selection of a high-stability component like the

CSM in these applications eliminates the need for shift

allowance due to “planned instability” and allows the use of

looser initial tolerances than would otherwise be necessary.