SENSorS & iNStrumENtatioN 2018 SupplEmENt uExhibitor preview


Ultra high-precision current-sensing in space


Greater precision and stability from current-sense resistors are essential for sensing and instrumentation in Space sub-systems explains Roger Tall, product specialist, Charcroft Electronics


Roger Tall, product specialist, Charcroft Electronics


space applications with a four-terminal Kelvin-connection current resistor from Vishay Precision Group (VPG). Based on VPG’s proprietary Bulk Metal Z


T


Foil resistive technology, an advanced manufacturing process balances thermo- mechanical forces between the Bulk Metal Foil alloy and an engineered ceramic substrate. This results in the resistors combining the performance characteristics of low Temperature Coefficient of Resistance (TCR) and long-term stability with non-inductance and ESD insensitivity. The CSM3637F and VCS1625Z surface- mount chip resistors’ design also enables low capacitance values, fast thermal stabilisation and low noise performance. Designers for Space, Defence and


other high-end applications can also expect these resistors to deliver a significant reduction in the sensitivity to applied power changes including Power Coefficient of Resistance (PCR) and to thermal resistance. The combination of the


Bulk Metal Z Foil resistive element and the four- terminal Kelvin connection provides designers with a highly precise voltage which is directly proportional to measured current levels. The ultra high-precision current- sensing performance is enhanced with a


reduction in measurement error of the resistance. The


Kelvin connection allows the resistance of the lead wires and the solder joints to be sensed at, or close to, the active resistive element inside the resistor. The two current terminals conduct


electrical current through the resistor, while the other two sense terminals measure the


sensed voltage drop, VS. Dividing the VS by the known resistance, RS, provides the


S12


he introduction of an industry-first design provides precision current sensing and temperature stability in


sensed current, IS. It is the stability of ohmic resistance, RS, between the nodes which makes these resistors suitable for use in


efficient control and precise measurements. The measurement error in resistance is


minimised by having the the lead wires and solder joints measured inside the resistor, at or close to the resistive element. This means that the CSM3637F and VCS1625Z measurement accuracy is not influenced by changes to the current flow or fluctuations in ambient temperature or other environmental factors. Another potential


change in precision is caused by self-heating, ∆Tsh, which is caused by dissipating power, known as the Joule effect.


Whilst power dissipation will occur


proportionally to a rapid increase in current, it will take longer to register when the current decreases rapidly. Defined as Temperature Coefficient of Resistance (TCR) and Power Coefficient of Resistance (Power TCR), these are not measured on dynamic temperature changes but on resistors which have been stabilised through being inactive for some time.


AChiEvinG STAbiliTy VPG chooses to achieve the stability and other performance advantages delivered by four-terminal Kelvin-connection current resistors by cementing a Bulk Metal Foil alloy to a special ceramic substrate to create a thermo-mechanical balance of forces. Low repeatable and controllable TCR is


achieved through opposing physical phenomena within this structure. One of the reasons for this is that the Coefficient of Thermal Expansion (CTE) of the alloy and the substrate are different. When the temperature increases, compressive stress on the resistive alloy is created. The result is the low absolute TCR achieved by CSM3637F and VCS1625Z Bulk Metal Foil chip resistors. The stability of these Bulk Metal Foil


resistors is typically an absolute TCR of ± 0.2ppm/°C over a temperature range from –55°C to +125°C, +25°C. By ensuring a low absolute TCR, the resistors ensure precise resistance against a


background of changes to the ambient temperature and self-heating.


UlTRA-hiGh pRECiSion wiTh REliAbiliTy To increase reliability, the CSM3637F and VCS1625Z surface-mount chip resistors are screened and tested in accordance with NASA Goddard EEE-INST-002 guidelines and offer additional test options for EEE- INST-002 (Tables 2A and 3A, Film/Foil, Level 1) and MIL standards under VPG’s post-manufacturing A & B tests. With stringent test processes, designers can ensure a reliable set of data against the performance of the actual resistor performance. This is critical in high-end applications in Low Earth Orbit (LEO) as well as in defence systems. The benefits which are delivered by this


technology deliver a low TCR combined with long-term stability, in addition to non- inductance, insensitivity to ESD and low capacitance. Designers can also expect fast thermal stabilisation and low noise. All these benefits, in addition to testing to NASA guidelines, help designers to create instrumentation and sensor systems for reliable operation in Space as well as in other harsh and high-end applications. Meet Charcroft and VPG on Stand E1 at


Sensors & Instrumentation, NEC Birmingham on 25 and 26 September 2018.


Charcroft Electronics Stand E1 www.charcroft.com T: 01591 610408


September 2018 Instrumentation Monthly


Bottom view of the four-terminal Kelvin-connection current resistor


Top view of the CSM3637F ultra high-precision current sensing resistor

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