Laser Tracker Atmospheric Error Fundamentals and Sensor Hardware Selection
Atmospheric refraction errors in laser tracker metrology demand precise temperature, pressure, and humidity compensation to maintain sub-millimeter precision.
Four Wire Kelvin Sensing
Meaning
Precision resistance measurement bypasses the error introduced by the resistance of the test leads and contact terminals. Utilizing four wire kelvin sensing provides high-accuracy resistance values by separating the current source path from the voltage measurement circuit. This technique eliminates the test lead voltage drop from the calculation because the voltage measurement terminals draw almost zero current.
The operational range of this method spans from milliohm shunt resistors to low-level semiconductor channels.
Circuit Topology
Two separate pairs of conductors connect the test instrument to the resistor under test. The force pair delivers a known current through the component, while the sense pair connects directly to a high-impedance voltmeter. This arrangement ensures that the voltage drop across the current-carrying leads does not affect the measurement.
Small-scale designs require careful layout to keep the measurement traces isolated up to the component pads.
Measurement Protocol
Implementing this configuration requires that the four separate contacts are made as close to the target resistance body as possible. Thermal electromotive forces at the junction points can introduce dc offsets that distort the final measurement. Alternating the polarity of the test current helps to cancel these thermal offsets through mathematical averaging.
Automated test rigs use custom spring-loaded pogo pins to make reliable, low-wear contacts on production boards.
System Integration
Integrating these measurement tracks on high-density circuit boards requires careful isolation from high-speed digital buses. Designers must route the force and sense lines symmetrically to avoid differential noise pickup. Differential filtering at the analog-to-digital converter input stabilizes the reading.