Non-Isolated Current Sensing Solutions

Many electronic devices are required to measure current, either to support one of their features or as a means of regulation for safety and control.  In the former case, energy measurement tools and power management systems are common examples, while in the latter case, inverter feedback loops and over current shutoff circuits come to mind.  In all of these examples, the current being measured can exist in a wide variety of conditions and span orders of magnitude.  For example, a high power inverter may be monitoring the current flow in two phases of an AC switching circuit.  These currents could exceed hundreds of amperes and flow from potentials of hundreds of volts.  At the other end of the spectrum, a fuel gauge in a wearable might be required to track microamperes flowing out of a lithium battery.  Designing circuits to accommodate such a broad swath of applications requires a deep tool chest of components and the ability to choose appropriate topologies while considering cost, performance, and available physical space.

Current Sensing Topologies

Measuring current flowing through a conductor can be accomplished in two ways: sensing the magnetic field that results from the movement of charge, and using Ohm’s law to derive an unknown current from a known resistance and a measured voltage drop.  Using magnetic fields comes with the benefit of electrical isolation, a feature that could be critically important for user safety or in high voltage environments.  The tradeoff for this isolation is that quantifying magnetic fields requires a specialized Hall effect sensor or an inductor along with non-trivial signal conditioning circuitry and calibration requirements.  All of these result in increased cost and reduced accuracy.

On the other hand, using Ohm’s law for inferring current flow doesn’t provide electrical isolation, but greatly simplifies the requirements of the measurement circuitry and uses a comparatively low cost sensing device, a resistor.  The other tradeoff with non-isolated sensing is power dissipation and dynamic range.  In order to minimize heat generation and non-linearity in the sensing resistor, extremely low resistance values are preferred.  As a result, the voltage drop resulting from the flow of current is similarly very small.  In order to use this low magnitude signal, high gain amplifiers are needed along with low noise design and layout techniques.

Non-isolated sensing can be accomplished using two techniques, shown in the figure below.  In the first schematic, discrete op-amps are configured as a typical differential amplifier.  The current to be measured creates a small voltage drop across the shunt resistor, and the output of the op amp circuit is proportional to this difference and in turn, the current.  In the second schematic, a specialized IC for current sensing is employed.  Internally, it is similar to the discrete version, containing stages of op-amps in differential form.  Unlike the discrete version, only an external shunt resistor is required with no additional supporting components.  In addition, the current sense amplifier is optimized specifically for low noise, wide common mode range, and minimal offset.

When comparing the CSA solution to discrete op amp designs, it is important to consider the tolerances of all the supporting passives.  As shown below, the CSA outperforms the op amp in every case, regardless of how tight the passive tolerances are specified.  At the extreme end, the CSA is more accurate than the 5% tolerant op amp design by nearly 20%. 

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Figure 2. OpAmp circuit method and CSA gain accuracy comparison (ΔT = 100 ° C)

Shunt Resistors

The low resistance sensing element used in non-isolated current measurement is commonly referred to as a shunt.  Unlike traditional resistors made from carbon, ceramic and metal films with values typically ranging from several ohms to several million ohms, shunts are generally constructed from metal alloys with resistance values on the order of milli-ohms.  This is due to the thermal limitations after which the shunt accuracy begins to degrade.  As shown in the figure below, ROHM offers a wide variety of shunts for current sensing applications with power ratings up to 15 Watts and resistance values from 0.1 milliohms to over 100 milliohms.

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Figure 3. ROHM shunt resistors power rating (W) vs resistance (mOhm)

One of the most important factors influencing current measurement accuracy is the temperature dependence of the shunt.  ROHM’s LTR100L series offers a power rating of 4W with a temperature coefficient less than 300 ppm/℃ across the -65 to 155℃ range.  In addition to reduced variation, ROHM’s shunt designs are also optimized for maximum thermal dissipation.  As shown in the figure below, the absolute temperature rise in an iso-current condition is nearly 25% lower for the ROHM devices. 

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Figure 4. ROHM temperature rise compared to other competing shunts

Current Sense Amplifiers

When constrained by physical space or when accuracy is paramount, dedicated current sense amplifiers (CSA) are the best choice.  No supporting passives or protection components are required, as CSA’s connect directly to the shunt and provide a proportional to current output.  These amplifiers are designed to maintain constant gain (within 1%) over the entire temperature range, and are available in numerous configurations to meet any application.  Compared to discrete designs, dedicated CSA’s exhibit wider common mode range and lower offset.  As shown in the figure below, ROHM offers CSA’s in single and dual channel configurations, with multiple gain options and common mode input ranges from 26V all the way to 80V. 

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Figure 5. ROHM’s portfolio of current sense amplifiers

When compared to competing op amp designs, the absolute accuracy of ROHM’s CSA solution shines.  As shown below, the ROHM CSA is nearly 5% more accurate across the entire current range even when the supporting passives variation is ignored.  Note that for all current measurements, the error by definition approaches an asymptote at zero load. 

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Figure 6. Accuracy improvement of CSA compared to competitive OpAmp

Low Offset Op Amps

In applications where cost is paramount, discrete signal conditioning circuits can be built using op amps.  These circuits also offer added flexibility in design where a CSA isn’t available to meet the exact gain requirements or specialized functionality.  Input voltage offset is one of the most critical factors affecting accuracy of differential amplifiers.  To eliminate this error in current measuring applications, ROHM has developed specialized low offset op amps.  As shown in the figure below, these devices exhibit more than an order of magnitude improvement compared to standard competitors.

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Figure 7. Specialized low offset op amps for improved current sense accuracy

To illustrate the importance of input voltage offset as it relates to current sensing accuracy, a series of simulations was carried out for different offsets across the entire current sensing range.  As shown in the results below, the total current sensing error is directly linked to the offset voltage.  Even at the extreme end of only 1mV of input offset, the current sensing error climbs to a few percent at the low measurement end. 

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Figure 8. Current sensing error as a function of op amp input voltage offset

For accurate current sensing, specialized low offset op amps are a necessity, and ROHM’s product lineup can satisfy the needs of any such application. These op amps are also available in one to four channel packages to help minimize space overhead.  Even though the total component count and sensing error will likely exceed the CSA solution, compelling performance can still be achieved using discrete non-isolated current measurement.

Design Support

From initial concept to final production, ROHM offers a complete suite of tools and products to aid in the design and implementation of current sensing solutions.  In addition to the aforementioned devices, ROHM has developed many of the other components often found in current measurement systems, including input protection, analog to digital converters, power supplies, and supervisory ICs.  Application notes are available for various current sensing topologies along with simulation tools for developing the initial schematics and choosing the appropriate components.  Once the PCB design stage is underway, ROHM also offers complete component libraries for all of their parts to speed up the placement and routing process.  Finally, in the validation stage, ROHM’s thermal simulation platform is freely available for catching potential heat issues before moving into production.  This is especially important for maintaining shunt performance in systems that demand high accuracy. 

ROHM Semiconductor