Isolated Current Sensing in Motor Drives: Accuracy Meets Cost Efficiency

Introduction: Why Isolated Current Sensing Matters in Modern Motor Drives

The modern motor drive presents a fundamental challenge: how do you accurately measure phase current for commutation and protection without introducing ground loops that corrupt your measurement? This problem intensifies in three-phase inverters where high voltage switching creates electromagnetic noise that conventional current sensing cannot tolerate.

Traditional approaches—using opto-isolated current measurement or discrete amplifier chains—work, but they're expensive, power-hungry, and require significant PCB real estate. Engineers designing BLDC motors, industrial drives, traction inverters, and solar systems need a better solution: purpose-built isolated current sensing ICs that balance accuracy, cost, and integration efficiency.

The stakes are high. Inaccurate current measurement causes missed commutation timing in BLDC systems, sluggish field-oriented control (FOC) algorithms, and delayed over-current protection that fails when you need it most. Cost pressures demand solutions that don't inflate the bill-of-materials (BOM), particularly in high-volume consumer and industrial applications where every cent matters.

Novosense addresses this challenge with a portfolio of isolated amplifiers designed specifically for shunt-based current measurement: the high-reliability NSI1300D series (±50mV and ±250mV variants) for automotive and critical industrial systems, and the NSI1400 for cost-optimised applications. Both deliver the precision and bandwidth required for real-time motor control without the complexity of traditional multi-chip isolation architectures.

Novosense Isolated Current Sensing Portfolio: Overview

Before diving into specific part selection, understand the landscape. Novosense offers three core solutions for isolated current sensing:

The NSI1300D05 and NSI1300D25 are high-reliability isolated amplifiers designed for automotive-grade applications. Both carry RTMed (Reliability Test Methods for the Entire Device lifecycle) certification, AEC-Q100 qualification, and are pin-compatible with industry-standard TI AMC1300/1301 designs. The 05 variant accepts ±50mV input (suitable for low-current, high-precision applications), whilst the 25 variant handles ±250mV (appropriate for moderate to high-current systems). Both offer 310kHz bandwidth, making them suitable for PWM-based commutation analysis in fast-switching motor control systems.

The NSI1400 is the cost-optimised alternative. It delivers the same ±250mV input range and similar ±0.3% gain accuracy as the NSI1300D25, but without RTMed certification. It's suitable for industrial and consumer applications where automotive supply-chain traceability isn't critical. The NSI1400 also includes a ratiometric output option, a feature that simplifies integration with ratiometric ADCs and reduces the need for precision voltage reference circuitry.

For applications requiring extremely high bandwidth (5–21 MHz) and very low power consumption, the NSI1306M modulators offer an alternative architecture based on external clock modulation. These are appropriate for advanced control algorithms requiring sub-microsecond current sampling, but introduce additional design complexity compared to direct amplifier output.

This article focuses on the NSI1300D and NSI1400 amplifiers, the most practical solutions for the majority of motor drive applications.

Detailed Specifications & Selection Table

Selecting the right current sensing IC requires understanding key performance parameters. The table below compares the primary variants:

Parameter	NSI1300D05	NSI1300D25	NSI1400
Input Range	±50mV	±250mV	±250mV
Gain	41	8.2	8.2
Bandwidth	310kHz	310kHz	220kHz
Offset Error	±0.1mV	±0.2mV	±0.5mV
Offset Drift	-0.8–1.0µV/°C	-2–4µV/°C	±4µV/°C
Gain Error	±0.3%	±0.3%	±0.3%
Supply Voltage	3–5.5V	3–5.5V	3–5.5V
Bias Current	-22µA	-18µA	-0.1µA
Package	SOW8	SOW8, SOWW8	SOW8, SOP8
RTMed	Yes	Yes	No
TI Pin Compatible	AMC1300	AMC1300/1301	AMC1300/1301

The key trade-offs are straightforward. The NSI1300D05 accepts lower voltage inputs, enabling use of lower shunt resistance values—useful for applications where shunt power dissipation must be minimised. The NSI1300D25 and NSI1400 both handle higher input ranges, reducing the design burden for high-current systems. The NSI1300D series carries RTMed qualification; choose it if your supply chain or application requires automotive-grade reliability certification. The NSI1400 is the cost leader and suitable for applications where this certification isn't mandatory.

Applications: Motor Drives, Solar Inverters, UPS, and Industrial Automation

BLDC Motor Commutation and Field Oriented Control

The most common application for isolated current sensing is real-time phase current measurement in BLDC motors. Here, the motor controller must measure current flowing through each phase to determine motor position (via back-EMF analysis) and adjust PWM timing for commutation.

Traditional approaches use hall-effect sensors or high-frequency PWM ripple injection—techniques that work but introduce complexity and latency. Novosense isolated amplifiers offer a cleaner alternative: measure current directly across the motor phase shunt resistor, with the isolated amplifier sitting between the power stage and the motor controller's analogue-to-digital converter (ADC).

The 310kHz bandwidth of the NSI1300D series is critical here. Motor drives typically operate PWM switching frequencies between 16 kHz and 100 kHz. The 310kHz bandwidth ensures that the isolated amplifier captures the fast current transients during each PWM cycle, enabling the controller to calculate instantaneous phase current rather than average current. This precision is essential for field-oriented control (FOC) algorithms, which require cycle-by-cycle current feedback for optimal torque production.

In a practical three-phase BLDC system, you would typically use three NSI1300D or NSI1400 instances—one per phase—to measure phase currents simultaneously. The simultaneous measurement capability eliminates the latency and complexity of multiplexing a single current channel across three phases.

Over-Current Protection in Three-Phase Inverters

Industrial motor drives must implement rapid over-current protection (OCP) to prevent catastrophic failure when phase current exceeds design limits. Novosense isolated amplifiers enable hardware-level OCP without additional complexity.

The flat frequency response of the NSI1300D (310kHz bandwidth) ensures that the output faithfully represents current transients. If a phase short-circuit or load jam occurs, the amplifier output rises immediately, allowing the microcontroller to detect the fault and shut down power stages before they fail thermally or electrically. The response time is typically in the microsecond range—fast enough for real-time protection in industrial automation systems.

The low offset error (±0.1mV for NSI1300D05, ±0.2mV for NSI1300D25) is crucial for OCP threshold detection. If offset error is large, the protection threshold becomes unreliable. Novosense's tight offset specifications ensure that you can set OCP thresholds accurately without excessive safety margins that would reduce system utilisation.

Solar Inverter Grid Current Monitoring

Utility-connected solar inverters require precise DC link and three-phase AC output current measurement for grid synchronisation and maximum power point tracking (MPPT). Here, isolated current sensing serves a different purpose than motor commutation—the goal is accurate steady-state measurement rather than cycle-by-cycle transient capture.

The NSI1300D and NSI1400 are well-suited for this application. The 310kHz (NSI1300D) and 220kHz (NSI1400) bandwidth allows capture of grid frequency harmonic content (50/60 Hz fundamental plus harmonics up to ~10 kHz), providing detailed current waveform visibility for power quality analysis and grid synchronisation algorithms.

The low offset drift (-0.8–1.0µV/°C for NSI1300D05, -2–4µV/°C for NSI1300D25) is particularly valuable in solar applications. Grid-connected inverters often operate continuously during daylight hours, with ambient temperatures varying 20–30°C throughout the day. Without adequate drift compensation, offset errors accumulate, degrading current measurement accuracy over time. Novosense's temperature-stable designs minimise this problem.

Uninterruptible Power Supply (UPS) Battery Current Monitoring

Battery-backed UPS systems require continuous, accurate charge and discharge current monitoring for battery management and seamless AC utility/battery switchover. Unlike motor drives (which tolerate some measurement latency) or solar inverters (which operate in well-defined outdoor environments), UPS systems operate 24/7 in unpredictable temperature and load conditions.

The NSI1300D series excels here. RTMed qualification ensures long-term reliability in continuous-duty applications. The tight offset drift specifications allow firmware-based temperature compensation, enabling sub-0.1% measurement accuracy even as ambient temperature varies. The 310kHz bandwidth exceeds the requirements of DC current monitoring (which needs only ~1 kHz), providing margin for future system upgrades.

Design Considerations: PCB Layout, Filtering, and Thermal Management

Implementing isolated current sensing correctly requires attention to three areas: input stage filtering, output impedance matching, and thermal management.

Input stage filtering: The shunt current path carries high-frequency switching noise from PWM transitions. If this noise isn't filtered before reaching the isolated amplifier input, it appears in the output, corrupting your current measurements. Recommended practice: place a low-pass RC filter immediately adjacent to the shunt resistor, with R = 10–100Ω and C = 100 nF. This suppresses switching noise above ~100 kHz whilst passing the fundamental current signal. Calculate the corner frequency as f = 1/(2πRC) and ensure it's well above your PWM switching frequency but below the amplifier bandwidth.

Output impedance matching: NSI1300D and NSI1400 output impedance is typically 10–100Ω. Most microcontroller ADCs have input impedance greater than 1 kΩ, so direct connection is acceptable. If your ADC input impedance is lower (common in high-speed analogue signal processors) or if trace length exceeds 10 cm, add a 10–100Ω series resistor and 100 nF shunt capacitor at the ADC input to reduce high-frequency coupling and ringing.

Thermal management: NSI1300D and NSI1400 are small-signal ICs with typical thermal dissipation less than 100 mW. Passive cooling (standard PCB copper layer) is sufficient. However, offset drift has a temperature coefficient (-0.8–1.0µV/°C for NSI1300D05, -2–4µV/°C for NSI1300D25). In applications where temperature varies significantly (outdoor solar inverters, industrial drives with wide ambient range), implement firmware-based temperature compensation: measure on-chip temperature via an additional temperature sensor and apply a linear offset correction based on temperature change from calibration point.

Cost Analysis: NSI1300D vs Traditional Isolated Current Sensing Solutions

Traditional multi-chip isolation architectures typically comprise four components: a high-frequency transformer (for isolation), an opto-isolator or magnetic isolator (for signal), a precision op-amp (for amplification), and discrete resistor networks (for gain setting and biasing). This adds up to approximately 10–15 components per current measurement channel.

Novosense's integrated approach (one IC per channel) reduces component count to 1, plus support passives (shunt resistor, input filter, output filter). For a three-phase system:

• Traditional approach: 3 channels × (transformer + isolator + op-amp + resistors) = 12–20 components

• Novosense approach: 3 × NSI1300D + shunt resistors + filters = 6–9 components

The BOM savings extend beyond component count. The integrated approach occupies significantly less PCB area (NSI1300D in SOW8 is roughly 1 cm² including bypass capacitors; a discrete isolation chain can easily occupy 3–5 cm²). Reduced PCB area lowers manufacturing cost and improves reliability (fewer solder joints, less layout complexity).

The NSI1400 variant further improves cost economics for price-sensitive applications. Compared to the NSI1300D series, the NSI1400 offers similar electrical performance at a 15–25% lower cost point, making it the natural choice for consumer and industrial applications where supply-chain certification isn't required.

Competitive Positioning: NSI1300D/NSI1400 vs TI AMC1300/1301

The TI AMC1300 and AMC1301 are the industry-standard isolated current-sensing amplifiers. Novosense NSI1300D and NSI1400 are pin-compatible replacements, enabling drop-in migration with zero firmware changes.

Key advantages of Novosense over TI:

Novosense NSI1300D and NSI1400 offer superior supply-chain resilience. Post-Nexperia acquisition by Qualcomm, the semiconductor industry has consolidated rapidly. Novosense provides a viable alternative to TI designs, protecting against single-source dependency and supply disruptions. Many OEMs are actively qualifying Novosense alternatives to TI designs.

The NSI1400 cost variant is particularly attractive. TI does not offer a true cost-optimised AMC1300 alternative; users must choose between automotive-grade (expensive) and no isolation at all. Novosense's NSI1400 fills this gap, offering industrial-grade reliability at consumer prices.

Both NSI1300D and NSI1400 offer competitive electrical specifications: identical ±0.3% gain accuracy, similar offset drift, and comparable bandwidth to TI equivalents. In head-to-head benchmarking, Novosense often shows marginal advantages in EMI immunity, particularly in high-noise industrial environments.

Conclusion: Future-Proofing Your Isolated Current Sensing Design

Motor drive engineers face a fundamental trade-off: accuracy, cost, and reliability. For too long, designers have accepted the premise that achieving all three requires accepting complexity and risk.

Novosense isolated amplifiers dissolve this trade-off. The NSI1300D series delivers automotive-grade reliability with precision tight enough for field-oriented control algorithms. The NSI1400 provides industrial-grade performance at consumer-friendly prices. Both offer the 310kHz bandwidth required for modern PWM-based motor control and the low offset drift necessary for temperature-stable, long-term accuracy.

By choosing Novosense over traditional multi-chip isolation chains, you reduce design complexity, lower BOM cost, and improve product reliability. By selecting between NSI1300D (automotive-qualified) and NSI1400 (cost-optimised), you optimise for your specific market and supply-chain requirements.

The result is a motor drive that commutates smoothly, protects reliably, and scales efficiently from low-power wearables to high-power industrial and EV applications.

Ready to implement isolated current sensing in your next motor drive? Contact Ineltek's applications engineering team for NSI1300D/NSI1400 datasheets, shunt resistor selection guides, and reference designs for BLDC commutation and FOC algorithms.

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Frequently Asked Questions

Q. What's the difference between ±50mV and ±250mV input versions?

A. NSI1300D05 (±50mV) requires lower shunt resistor value for the same current measurement, reducing shunt power dissipation significantly. NSI1300D25 (±250mV) accepts higher shunt voltages, useful for high-current applications or where low shunt resistance creates thermal challenges. Both offer identical 310kHz bandwidth and ±0.3% gain accuracy. Choose 05 if power dissipation is critical (high-current, battery-powered systems); choose 25 for standard industrial and automotive applications. The 25 variant also offers better signal-to-noise ratio and simpler PCB layout.

Q. How do I implement software calibration for isolated current sensing?

A. Perform two-point calibration at manufacturing: (1) measure zero-current output voltage with no load applied, recording V_offset; (2) apply known current (50% of full-scale operating range) and record V_measured. Store both values in firmware non-volatile memory. During operation, apply linear correction: calibrated_current = (measured_voltage − V_offset) / (slope × gain_factor). This compensates for component tolerances and temperature drift, achieving sub-0.1% accuracy. Repeat calibration annually or when changing shunt resistors.

Q. Can I parallel multiple NSI1300D for higher current measurement?

A. Not recommended without careful matched-pair selection. Offset voltage variations between devices (typically ±0.2mV across production tolerance) will cause unequal current sharing, with larger offsets drawing more current. Better approach: use multiple shunt resistors (one per phase in three-phase systems) with dedicated NSI1300D for each phase. This is standard industrial practice and eliminates current-sharing complexity. For applications requiring true parallel measurement (e.g., redundant current sensing for safety), select devices from the same production batch and perform offset matching during assembly.

Q. What's the thermal drift impact on long-term current monitoring in UPS systems?

A. NSI1300D05 has offset drift of -0.8–1.0µV/°C. Over 50°C temperature change (from cold startup to warm steady-state), offset can shift by 40–50 mV. For UPS systems operating 24/7, this drift is problematic. Recommended solution: implement periodic re-calibration during low-load periods (e.g., when battery charge current drops below 10% of rated value). Alternatively, integrate an on-chip temperature sensor (available on some MCU platforms) and apply firmware-based software compensation: offset_correction = (T_current − T_calibration) × drift_coefficient.

Q. How do I select between NSI1300D25 and NSI1400 for solar inverter grid current sensing?

A. Both are electrically suitable for solar inverter applications. Choose NSI1300D25 if your product targets utility-scale or commercial rooftop systems where supply-chain traceability and long-term reliability are mandated by grid operators. Choose NSI1400 if you're targeting residential or emerging-market applications where cost is primary concern and reliability documentation is less critical. Novosense NSI1400 offers 15–20% cost savings with identical ±0.3% gain accuracy. Pin-compatible, so design choice can be deferred until production phase.

Q. What's the minimum shunt resistor value I can safely use?

A. The practical minimum depends on three factors: (1) input range of the IC—aim to use 50–80% of full scale at maximum current to allow headroom for transients; (2) shunt power budget—verify that I²R dissipation doesn't exceed thermal design limits; (3) PCB trace resistance—ensure that shunt resistance is at least 10× trace resistance to avoid Kelvin connection errors. For a 100-amp application with NSI1300D25 (±250mV input), minimum shunt would be 0.250 V / 100 A = 2.5 mΩ, providing 12.5 mV at full scale (5% of input range). In practice, 1–2 mΩ is typical. For very low-voltage designs, use Kelvin connections (four-wire measurement) to eliminate lead resistance errors.

Q. Can I use NSI1300D for AC current measurement?

A. Yes, with proper biasing. NSI1300D and NSI1400 measure differential input voltage; AC current measurement requires biasing the common-mode input to approximately half the supply voltage. This allows the differential input to swing both positive and negative around the bias point. Recommended circuit: AC-couple the shunt signal through a capacitor (100 µF for 50/60 Hz grid current), set VREF (amplifier reference pin) to VCC/2, and apply gain settings normally. The capacitor blocks DC offset, allowing AC current to swing around the biased level. This approach is standard in three-phase grid-connected inverters and is well-documented in application notes.

Q. Is there a ratiometric version of NSI1300D?

A. NSI1400 offers optional ratiometric output (output voltage scales with input supply voltage), simplifying integration with ratiometric ADCs. NSI1300D is fixed-output only, requiring a precision voltage reference for accurate ADC conversion. If your microcontroller has ratiometric ADC capability, NSI1400's ratiometric option simplifies the design by eliminating the need for external precision voltage references. Check your MCU datasheet for ratiometric ADC support; if available, NSI1400 ratiometric variant reduces component count and cost.

Q. How does the 310kHz bandwidth benefit motor commutation?

A. Motor control algorithms generate PWM signals typically at 16–20 kHz switching frequency. Each PWM pulse creates a fast dV/dt transient on the phase voltage and corresponding current ripple. The dI/dt can be extremely fast—on the order of 1,000 amperes per microsecond in high-power systems. The NSI1300D's 310kHz bandwidth allows the amplifier to capture these fast transients without attenuation. In field-oriented control (FOC), the controller calculates instantaneous current magnitude and phase angle relative to the rotor position. The amplifier's 310kHz bandwidth ensures that current samples faithfully represent the true instantaneous current. Without sufficient bandwidth, current samples appear smeared across time, and commutation becomes jerky. With 310kHz bandwidth, commutation is smooth and efficient.

Q. Can I use NSI1306M modulators instead of amplifiers?

A. NSI1306M modulators use a different architecture than NSI1300D amplifiers. Rather than directly amplifying the input signal, they modulate it onto a high-frequency carrier (5–21 MHz, user-selectable). Modulators offer higher bandwidth (5–21 MHz) and lower power consumption than amplifiers, but they introduce additional complexity. You must provide an external clock signal and manage the modulation/demodulation timing. They're appropriate for ultra-high-bandwidth applications (sub-microsecond measurement precision) but overkill for typical motor drives. For BLDC commutation and industrial motor drives, NSI1300D amplifiers are the right choice—simpler integration, lower cost, and sufficient bandwidth for virtually all applications.