Precision Current Sensing Solutions Guide
There are many control-system applications where the accurate measurement of current flow is required. In many of these applications the measurement circuits must provide accurate feedback of system current flow(s) across a very broad range of operating temperatures. Components in these applications require a minimal absolute temperature coefficient variation to ensure accurate feedback and efficient control system operation.
Solar and wind farms, aerospace, motor-control circuits, electric-vehicle charging stations, down-hole drilling and outdoor equipment are but a few of these applications.
This solution guide provides guidance for the selection of low-ohmic components that are constructed from materials that satisfy the design requirements of precision current measurement circuits operating in environments with a high degree of temperature variation. Guidance is also provided for selection of a high-quality difference amplifier.
- Precision Resistor - Considerations and criteria for specification and selection
- Difference Amp - Considerations and criteria for specification and selection
- High-Reliability, Precision Current Sensing Resistor Selection Guides and Brochures
- Examples of current sense circuits
- Accuracy and design advantages of Z-Foil precision film resistors over a wide range of temperature
- YouTube video resistor accuracy test demonstration
1. Precision Resistor - Considerations and criteria for specification and selection
Direct current sensing with low-ohmic resistors is based on ohms law (E=IR) and is accomplished by placing a resistor in series with the system circuitry that needs to be measured (often called a shunt resistor), a voltage is generated across the shunt resistor that is proportional to the load current. Measurement is accomplished via a high-quality difference amplifier. Precision resistors are available from Vishay Precision Group (VPG) and are design specifically for this application. Using a difference amp with a high-level Common Mode Rejection Ratio (CMRR) allows for a wide choice of options for the selection of the current measurement points. A very broad range of resistive values are available (0.003Ω to 500Ω) with precision tolerances of ±0.01% and small variations in resistance values over a broad range of temperatures (±2ppm/℃ from -55℃ to +125℃).
The factors to be considered when selecting a resistor are:
- Minimum and maximum current flow
- Desired voltage change across the resistor as a proportional function of current flow
- Measurement point within the system
- Required accuracy of the measurement, which is influenced by the resistors absolute tolerance and the allowable variation over operating temperature and operating life.
- Power dissipation budget
- Load life stability
- Reliability level (QPL, DSCC, EEE-INST-002, ESA, CECC, etc.)
Note: For the highest level of accuracy four-leaded “Kelvin Resistors” are available. The 4-terminal configuration eliminates the IR-drop error voltage that would be present in the voltage sense leads if a standard two-terminal resistor were used. They are commonly used in applications where the value of the sense resistor in 0.1Ω or less.
2. Difference Amp - Considerations and criteria for specification and selection
The input common-mode voltage is the most important specification when selecting a direct-current sensing solution. It is defined as the average voltage present at the input terminals of the amplifier. This specification is important because it limits the choice of differential amplifiers. For example, op amps and instrument amplifiers require an input common-mode voltage within their power supplies. Difference amplifiers, however, typically can accommodate input common-mode voltages in excess of their power supplies. This is useful in applications where the amplifier senses the shunt-resistor voltages in the presence of a large common-mode voltage and must interface with a low-voltage analog-to-digital converter (ADC). One of best difference amplifiers for measuring the voltage across the sense-resistor, when the desired measurement point is outside of the op amps supply rail voltage, is the Texas Instruments INA149 (Click to view data sheet). The INA149 is a precision unity-gain difference amplifier with a high Common-Mode Rejection Ratio (CMRR) for high input common mode voltage (CMV) applications. With 100dB (typ) CMRR, the INA149 can accurately measure small differential voltages in the presence of common-mode signals up to ±275V. The INA149 inputs are protected from momentary common-mode or differential over-voltage up to 500V making it the ideal solution for high-accuracy, high-voltage current sensing applications.
When monitoring load current the designer can choose to place the sense resistor in several places. When the measurement point (placement of the shunt-resistor) is near the system ground side, this is called low-side current measurement. The downside to this placement is that the system circuitry under measurement will be lifted for the voltage drop across the sensing resistor. Also, dynamic power consumption changes in the measurement path will cause this point to move up and down. The only benefit to low-side current measurement is that a simpler amplifier can be chosen with a lower CMRR.
High-side sensing is often preferable because it does not influence the system circuitry's path to ground. It's only downside is that it requires a difference amplifier with a high CMRR. Use of a difference amplifier with a high CMRR also permits the placement of the sense-resistor in a variety of areas.
3. High-Reliability, Precision Current Sensing Resistor Selection Guides and Brochures
4. Accuracy and design advantages of Z-Foil precision film resistors over a wide range of temperatures
Vishay Foil Resistors achieved a technological breakthrough with the introduction of Bulk Metal® Z-Foil. Products built on this revolutionary technology deliver an absolute temperature coefficient of resistance (TCR) of ±0.2 ppm/°C (-55°C to +125°C, +25°C ref.) and ±0.05 ppm/°C (0°C to +60°C, +25°C ref.), one order of magnitude better than previous Foil technologies. The lower the absolute TCR, the better a resistor can maintain its precise value despite ambient temperature variations and self-heating when power is applied. By taking advantage of the overall stability and reliability of Vishay Foil resistors, designers can significantly reduce circuit errors and greatly improve overall circuit performance.
Key Features of Precision Film Resistors
- Temperature coefficient of resistance (TCR) for Z-Foil technology
- ±0.05 ppm/ºC typical (0ºC to +60ºC, +25ºC ref.)
- ±0.2 ppm/ºC typical (-55ºC to +125ºC, +25ºC ref.)
- Power coefficient of resistance for Z-Foil technology (Power PCR)
- “ΔR due to self heating”: ±5 ppm at rated power
- Load life stability: to ±0.005% (50 ppm) at +70ºC, 10,000 hours at rated power
- Resistance tolerance: to ±0.001% (10 ppm)
- Resistance range: 0.5 mΩ to 1.8 MΩ
- Electrostatic discharge (ESD) at least to 25 kV
- Non inductive, non capacitive design
- Rise time: 1 ns without ringing
- Thermal stabilization time <1 sec (nominal value achieved
- within 10 ppm of steady state value)
- Current noise: 0.010 μVRMS/volt of applied voltage (<-40 dB)
- Thermal EMF: 0.05 μV/ºC
- Voltage coefficient: <0.1 ppm/V
- Trimming operations increase resistance in precise steps but from remote locations so that the etched grid in the active area remains reliable and noise-free
- Lead (Pb) free, tin/lead and gold terminations are available
Range of Foil Resistor Products
- Surface-mount chips, molded resistors and networks
- Power resistors and current sensors
- Military established reliability (QPL, DSCC, EEE-INST-002, ESA, CECC)
- Leaded (through-hole)
- Trimming potentiometers
- Voltage dividers and networks
- Hybrid chips (wire-bondable chips)
- High temperature resistors (>220°C)
- Resistors for audio
Temperature Coefficient of Resistance (TCR)
Two predictable and opposing physical phenomena within the composite structure of the resistive alloy and its substrate are the key to the low absolute TCR capability of a Bulk Metal® Foil resistor:
- Resistivity of the resistive alloy changes directly with temperature in free air (resistance of the foil increases when temperature increases.)
- The Coefficient of Thermal Expansion (CTE) of the alloy and the substrate to which the foil alloy is cemented are different resulting in a compressive stress on the resistive alloy when temperature increases (resistance of the foil decreases due to compression caused by the temperature increases).
The TCR of the Foil resistor is achieved by matching two opposing effects - the inherent increase in resistance due to temperature increase vs. the compression-related decrease in resistance due to that same temperature increase. The two effects occur simultaneously resulting in an unusually low predictable, repeatable, and controllable TCR.
Improved TCR In Bulk Metal® Z-Foil Resistors to ±0.2 ppm/°C
Foil resistor technology has continued to progress over the years, with significant improvements in TCR. Figure 1 shows the typical TCR characteristics of the various foil alloys utilized by Vishay Foil to produce Bulk Metal® Foil resistors.
The original Alloy C Foil exhibits a negative parabolic response to temperature with a positive chord slope on the cold side and a negative chord slope on the hot side. Following was the Alloy K Foil which produced an opposite parabolic response with temperature with a negative chord slope on the cold side and a positive chord slope on the hot side. In addition, it provides a TCR approximately one half that of Alloy C Foil. The latest development are the the Alloys Z and Z1-Foil Technologies Breakthrough which has a similar parabolic response as the Alloy K Foil but produces TCR characteristics an order of magnitude better than Alloy C and five times better than Alloy K. Using this technology, extremely low TCR resistors have been developed that provide virtually zero response to temperature. These technological developments have resulted in a major improvement in TCR characteristics compared to what was available before, and what is available in any other resistor technology.
Power Coefficient of Resistance (PCR)
The TCR of a resistor for a given temperature range is established by measuring the resistance at two different ambient temperatures: at room temperature and in a cooling chamber or oven. The ratio of relative resistance change and temperature difference gives the slope of ΔR/R = f (T) curve. This slope is usually expressed in parts per million per degree Centigrade (ppm/°C). In these conditions, a uniform temperature is achieved in the measured resistance. In practice, however, the temperature rise of the resistor is also partially due to self-heating as a result of the power it is dissipating (self-heating). As stipulated by the Joule effect, when current flows through a resistance, there will be an associated generation of heat. Therefore, the TCR alone does not provide the actual resistance change for precision resistor. Hence, another metric is introduced to incorporate this inherent characteristic – the Power Coefficient of Resistance (PCR). PCR is expressed in parts per million per Watt or in ppm at rated power. In the case of Z-based Bulk Metal® Foil, the PCR is 5 ppm typical at rated power or 4 ppm per Watt typical for power resistors.
For example: A foil power resistor with TCR of 0.2 ppm/ºC and PCR of 4 ppm/W, a temperature change of 50ºC (from +25ºC to +75ºC) at a power of 0.5 W will produce a ΔR/R of 50 x 0.2 + 0.5 x 4 = 12 ppm absolute change.
When power is applied to the resistor, self-heating occurs. Foil’s low TCR and PCR capabilities help to minimize this effect. But to achieve high-precision results, a rapid response to any changes in the environment or other stimuli is necessary. When the level of power is changed, one would desire the resistance value to adjust accordingly as quickly as possible. A rapid thermal stabilization is important in applications where the steady state value of resistance according to all internal and external factors must be achieved quickly to within a few ppm. While most resistor technologies may take minutes for such speed and precision of thermal stabilization to its steady state value, a Vishay Foil resistor is capable of almost immediate stabilization, down to within a few ppm in under a second. The exact response is dependent on the ambient temperature as well as the change in power applied; the heat flow when power is applied places mechanical stresses on the element and as a result causes temperature gradients. Regardless, Bulk Metal® Foil outperforms all other technologies by a large margin (see Figure 3).
A system or a device or one particular circuit element must perform for a specified period of time and at the end of that service period, it must still perform within specification. During its service life, it may have been subjected to some hostile working conditions and therefore may no longer be within purchased tolerance. One reason for specifying a tighter purchased tolerance than the end of life error budget tolerance is to allow room for service shifts. Another reason is that the error budget is more economically applied to resistors than to most other components. The Bulk Metal® Foil resistors are calibrated as accurately as 0.001% by selectively trimming various adjusting points that have been designed into the photo-etched pattern of the resistive element (see Figure 4). They provide predictable step increases in resistance to the desired tolerance level. Trimming the pattern at one of these adjusting points will force the current to seek another longer path, thus raising the resistance value of the element by a specific percentage. The trimming operations increase resistance in precise steps but from remote locations so that the etched grid in the active area remains reliable and noise-free (see Figure 5). In the fine adjusted areas, trimming affects the final resistance value by smaller and smaller amounts down to 0.001% and finally 0.0005% (5 ppm). This is the trimming resolution (see Figure 4).
Load Life Stability
Why are designers concerned about stability with applied load? Load life stability is the characteristic most relied upon to demonstrate a resistor’s long-term reliability. Military testing requirements to 10,000 hours with limits on amount of shift and the number of failures results in a failure rate demonstration. Precision Bulk Metal® Foil resistors have the tightest allowable limits. Whether military or not, the load life stability of Vishay Foil resistors is unparalleled and long-term serviceability is assured. The reason Vishay Foil resistors are so stable has to do with the materials of construction (Bulk Metal® Foil and high alumina substrate). For example, the S102C and Z201 resistors are rated at 0.3W at 125°C with an allowable ΔR of 150 ppm max after 2000 hours under load and 500 ppm max after 10,000 hours (see Figures 6 and 7 for the demonstrated behavior). Conversely, the ΔR is reduced by decreasing the applied power which lowers the element temperature rise in Vishay Foil resistors. Figure 6 shows the drift due to load life testing at rated power and Figure 7 shows the drift due to load life testing at varied power. Reducing the ambient temperature has a marked effect on load life results and Figure 8 shows the drift due to rated power at different ambient temperatures. The combination of lower power and ambient temperature is shown in Figure 9 for model S102C.
Figure 10 displays the results of our tests that has been in progress for 29 years. 50 sample S102C 10 kΩ resistors have been in a 70°C heating chamber while under 0.1W applied power for this entire duration. The average deviation in resistance is just 60 ppm. Figure 11 shows documented shelf life performances made by a customer for hermetically sealed VHP101 Foil resistors for over 8 years. The average deviation did not exceed 1 ppm.
Figure 11 shows documented shelf life performances made by a customer for hermetically sealed VHP101 Foil resistors for over 8 years. The average deviation did not exceed 1 ppm.
For evaluation of load life stability, the two parameters which must be mentioned together – power rating and ambient temperature – can be joined into one single parameter for a given style of resistor. If the steady state temperature rise can be established, it can be added to the ambient temperature, and the sum will represent the combined (load induced + ambient) temperature. For instance, the Vishay Foil resistor S102C has a temperature rise of 9°C per 0.1W of applied power. This leads to the following example calculations:
If T = 75°C, P = 0.2W, and t = 2000 hrs.;
Then self-heating = 9°C x 2 = 18°C.
18°C rise + 75°C ambient = 93°C total ΔR.
R max = 80 ppm from the curve of Figure 12.
Figure 12 shows, for a given duration of load life test, how the drift increases with the level of the applied combined temperature. As explained above, the combined temperature comprises the effect of power induced temperature rise and the ambient temperature. The curve shows maximum drift.
High Speed and Response Time
The equivalent circuit of a resistor, as shown in Figure 13, combines a resistor in series with an inductance and in parallel with a capacitance (PLC). Resistors can perform like an R/C circuit, filter or inductor depending on their geometry. In spiraled and wire-wound resistors, these reactances are created by the loops and spaces formed by the spirals or turns of wire. Figure 14 shows how the capacitance and inductance increase as the resistance value increases due to continually increasing the number of spirals or turns. Certain assembly techniques attempt to mitigate the inductance in wire-wound resistors but all have only limited effect. On the other hand, in planar resistors such as the Bulk Metal® Foil resistors, the geometry of the lines of the resistor patterns is intentionally designed to counteract these reactances. Figure 15 shows a typical serpentine pattern of a planar resistor. Opposing current directions in adjacent lines reduces mutual inductance while geometry-related inter-line capacitances in series reduces overall capacitance. Both inductance and capacitance produce reactance proportional to the operating frequency and it changes the effective resistance and the phase between the current and voltage in the circuit.
Both inductive and capacitive reactances distort input signals, particularly in pulse applications. Figure 16 shows the current response to a voltage pulse comparing a fast Bulk Metal® Foil resistor to a slower wire-wound resistor. Here a pulse width of one nanosecond would have been completely missed by the wire-wound resistor while the Vishay Foil resistor achieves full replication in the time allotted.
In frequency applications, these reactive distortions also cause changes in apparent resistance (impedance) with changes in frequency. Figure 17 shows a family of curves relating the AC resistance to the DC resistance in Bulk Metal® Foil resistors. Very good response is seen in the 100Ω range out to 100 MHz and all values have a good response out to 1 MHz. The performance curves for other resistor technologies can be expected to show considerably more distortion (particularly wire-wounds).
Resistors, depending on construction, can be a source of noise. This unintended signal addition is measurable and independent of the presence of a fundamental signal.
Figures 18-20 illustrate the effects of resistor noise on a fundamental signal. Resistors made of conductive particles in a non-conductive binder are the most likely to generate noise. In carbon composition and thick film resistors, conduction takes place at points of contact between the conductive particles within the binder matrix. Where these point-to-point contacts are made constitutes a high resistance conduction site which is the source for noise. These sites are sensitive to any distortion resulting from expansion mismatch, moisture swelling, mechanical strain, and voltage input levels. The response to these outside influences is an unwanted signal as the current finds its way through the matrix. Figure 21 illustrates this current path.
Resistors made of metal alloys, such as Bulk Metal® Foil resistors, are the least likely to be noise sources. Here, the conduction is across the inter-granular boundaries of the alloy. The intergranular current path
from one or more metal crystals to another involves multiple and long current paths through the boundaries reducing the chance for noise generation. Figure 22 illustrates this current path. In addition, the photolithography and fabrication techniques employed in the manufacture of Bulk Metal® Foil resistors results in more uniform current paths than is found in some other resistor constructions. Spiraled resistors, for example, have more geometric variations that contribute to insertion of noise signals. Bulk Metal® Foil resistors have the lowest noise of any resistor technology, with the noise level being essentially immeasurable. Signal purity can be a function of the selection of resistor technology for pre-amp and amplifier applications. Vishay Foil resistors offer the best performance for low noise audio applications.
When a junction is formed by two dissimilar metals and is heated, a voltage is generated due to the different levels of molecular activity within these metals. This electromotive force, induced by temperature, is called Thermal EMF and is usually measured in micro-volts. A useful purpose of this Thermal EMF is the measurement of temperature using a thermocouple and micro-volt meter. In resistors, this Thermal EMF is considered a parasitic effect interfering with pure resistance (especially at low values when DC is applied). It is often caused by the dissimilarity of the materials used in the resistor construction, especially at the junction of the resistor element and the lead materials. The Thermal EMF performance of a resistor can be degraded by external temperature differences between the two junctions, dissymmetry of power distribution within the element, and the dissimilarity of the molecular activity of the metals involved. One of the key features feature of the Vishay Foil resistor is its low Thermal EMF design. The flattened paddle leads (in through hole design) make intimate contact with the chip thereby maximizing heat transfer and minimizing temperature variations. The resistor element is designed to uniformly dissipate power without creating hot spots and the lead material is compatible with the element material. These design factors result in a very low Thermal EMF resistor. Figures 23 and 24 display the various design characteristics that give these resistors an extremely low thermal EMF.
Electrostatic Discharge (ESD)
Electrostatic discharge (ESD) can be defined as a rapid transfer of charge between bodies at different electrical potentials – either by direct contact, arcing, or induction – in an attempt to become electrically neutral. The human threshold for feeling an ESD is 3000V, so any discharge that can be felt is above this voltage level. Because the duration of this high voltage spike is less than a microsecond long, the net energy is small compared to the size of the human body over which it is spread. From the human body’s point of view, ESD does no harm. But when the discharge is across a small electronic device, the relative energy density is so great that many components can be damaged by ESD at levels as low as 3000V or even 500V.
ESD damage is generally divided into three categories:
- Parametric Failure – the ESD event alters the resistance of the component causing it to shift from its required tolerance. This failure does not directly pertain to functionality; thus a parametric failure may be present even if the device is still functional.
- Catastrophic Damage – the ESD event causes the device to immediately stop functioning. This may occur after one or a number of ESD pulses, and may have many causes, such as human body discharge or the mere presence of an electrostatic field.
- Latent Damage – the ESD event causes moderate damage to the device, which is not noticeable, as the device appears to be functioning correctly. However, the load life of the device is dramatically reduced, as further degradation caused by operating stresses may cause the device to fail during service. This defect is of greatest concern as it is very difficult to detect by visual inspection or re-measurement.
In resistors, ESD sensitivity is a function of their size. The smaller the resistor, the less space there is to spread the energy pulsed through it from the ESD. This energy concentration in a small area of a resistor’s active element causes it to heat up, which could lead to irreversible damage. With the growing trend of miniaturization, electronic devices, including resistors, are becoming smaller and smaller, causing them to be more prone to ESD damage. Thus, the superiority of Bulk Metal® Foil precision resistors over Thin Film resistors, when subjected to ESD, is attributed mainly to their greater thickness (foil is 100 times thicker than Thin Film), and therefore the heat capacity of the resistive foil layer is much higher compared to the thin film layer. Thin film is created through particle deposition processes (evaporation or sputtering), while foil is a bulk alloy with a crystalline structure created through hot and cold rolling of the melt.
Tests performed have indicated that foil chip resistors can withstand ESD events at least to 25 kV (data available), while thin film and thick film chip resistors have been seen to undergo catastrophic failures at electric potentials as low as 3000 V (parametric failures at even much less). If the application is likely to confront the resistor with ESD pulses of significant magnitude, the best resistor choice is Bulk Metal® Foil.
Non-Measurable Voltage Coefficient
As mentioned earlier in our section on resistor noise, resistors can change value due to applied voltage. The term used to describe the rate of change of resistance with changing voltage is known as voltage coefficient. Resistors of different constructions have noticeably different voltage coefficients. In the extreme case, the effect in a carbon composition resistor is so noticeable that the resistance value varies greatly as a function of the applied voltage. Bulk Metal® Foil resistor elements are insensitive to voltage variation and the designer can count on Vishay Foil resistors having the same resistance under varying circuit voltage level conditions. The inherent bulk property of the metal alloy provides a non-measurable voltage coefficient.
All In One Resistor
The ten reasons to specify Foil resistors are inherent in the design and are not a function of manufacturing variables or a selection process. This combination of parameters is not available in any other resistor technology. Vishay Foil Resistors provides a unique, inherent combination of performance characteristics resulting in unmatched performance and high reliability, satisfying the needs of today’s expanding requirements.
Consider Vishay Foil Resistors for all of your low TCR needs. Special orders may be placed for low TCR, low value resistors, and tight TCR tracking of individual resistors and network combinations. Contact the Application Engineering Department to discuss your requirements for these and any other TCR applications.