Z-foil surface-mount current sensing chip resistor
25 February 2011
It features a 1W power rating, low TCR to ±0.05 ppm/°C, and power TCR of 5ppm at rated power.
Vishay Precision Group has announced that its Vishay Foil Resistors division has released a new ultra-high-precision Z-Foil surface-mount current sensing chip resistor with low TCR of ±0.05 ppm/°C from 0°C to +60°C and ±0.2 ppm/°C from -55°C to +125°C, +25°C ref., power TCR (PCR) of 5 ppm at rated power, and tolerances to ±0.2% (0.05% and 0.1% are available).
For high-power applications, the VCS1625ZP offers a power rating of 1W at +70°C and 5A maximum current rating, and a four-terminal Kelvin configuration for increased accuracy.
VCS1625ZP features a wide resistance range from 0.3Ω to 10Ω. Vishay foil resistors are not restricted to standard values and specific ‘as required’ values (1.234Ω versus 1Ω) can be supplied at no extra cost or delivery time.
The resistor features a rise time of 1.0ns, with effectively no ringing, short time overload of <0.005% (50ppm), current noise of 0.010μVRMS/V of applied voltage (<-40 dB), and a voltage co-efficient of <0.1 ppm/V.
Offering the utmost in ESD immunity, the device withstands electrostatic discharges at least to 25kV for increased reliability, and offers a non-inductive (<0.08 μH), non-capacitive design.
The VCS1625ZP can be offered with gold terminations and with additional temperature treatments to extend the operating temperature from +150°C to above +220°C.
The Z-Foil technology of the VCS1625ZP provides a significant reduction of the resistive component's sensitivity to ambient temperature variation (TCR) and applied power changes (PCR - "ΔR due to self-heating"), allowing designers to guarantee a high degree of stability and accuracy in fixed-resistor applications. The resistor's load-life stability to ±0.02% (200 ppm) at +70°C for 2,000 hours at rated power is an order of magnitude better than typical current sensing resistors (0.5% for 1,000 hours). The device's improved stability makes it suitable for tightened-stability reference voltage and precision current sensing applications in forced-balance electronic scales, measurement instrumentation, bridge networks, and medical and test equipment. In addition, the VCS1625ZP complies with EEE-INST-002 (MIL-PRF 55342) for military and space applications.
The resistor's design results in a very low thermal EMF of 0.05μV/°C typical, which is critical in precision applications. The VCS1625ZPS's all-welded construction is composed of a Bulk Metal Foil resistive element with plated copper terminations. The flattened terminations make intimate contact with the resistive layer along the entire side of the resistive element, thereby minimising temperature variations. In addition to the low thermal EMF compatibility of the device's metals, the uniformity and thermal efficiency of the design minimise the temperature differential across the resistor, thereby assuring low thermal EMF generation at the terminations. This further reduces the thermal EMF voltage, or ‘battery effect’, exhibited by most current sensing or voltage reference resistors.
The stability problems associated with analogue circuits are very pervasive, but the selection of a few high-quality current sensing resistors in critical locations can greatly improve circuit performance. Selection of a high-stability component like VCS1625ZP in these applications eliminates the need for shift allowance due to ‘planned instability’, and allows the use of looser initial tolerances than would be necessary with current sensing resistors based on other technologies.
Many analogue applications require performance under unusually high-stress conditions over extended periods of time. This calls for more than just selecting a standard device and applying it to a circuit. The standard device may turn out to be all that is needed, but an analysis of the projected service conditions should be made, and it may well dictate a routine of stabilisation known as post manufacturing operations, or PMO.
The PMO operations that will be discussed are only applicable to Bulk Metal Foil resistors. Short time overload, accelerated load life, and temperature cycling are the three PMO exercises that do the most to remove service life anomalies. VPG's Bulk Metal Foil resistors are inherently stable as manufactured, but these PMO exercises may improve performance by small but significant amounts. Users are encouraged to contact Vishay Foil Resistors' applications engineering team for assistance in choosing the PMO operations that are right for their applications.
When current sensing has important implications – such as force-balance electronic scales measuring valuable gems or pharmaceuticals, or to control the guidance of missiles with electro-static gyros – the current represents system conditions where errors have serious consequences.
These systems don't just measure digitally significant on/off stages. Their continuously changing analogue outputs in current mean that the sense resistors are constantly changing their power dissipation, which in turn changes their operating temperatures independently of ambient temperature. Therefore, the inherent TCR of the sense resistors registers constantly changing resistance and present current sensing errors superimposed upon the current that would represent the real current output of the main analogue sensors. One way to eliminate these errors, although expensive, is to develop a correction factor for each individual piece of sensing equipment. A more practical way is to use very-low-TCR current sense resistors, such as the VCS1625ZP, that isolate the current output from varying power dissipation effects.
These devices are characterised by extremely low excess noise when compared to other types of resistors. Additionally, the current in adjacent current-carrying paths runs in opposing directions, cancelling the parasitic inductance of these paths.
Also, path-to-path capacitances are connected in series, which has the effect of minimising the parasitic capacitance of the resistor. These low-inductance/capacitance resistors are characterised by non-measurable peak-to-peak signal distortions.
Samples and production quantities of the VCS1625ZP are available now, with lead times of five working days for samples and six weeks for standard orders.
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