Sensors and ICs Simplify Current Measurement Technology

Today’s demand for energy efficiency at every level has made current a rapidly growing measurement metric. Designers can now choose from a dizzying array of hardware to best implement the technology with minimal impact on the main circuit.
Key Points
Coulomb Counters for Mobile Chargers
High-End Measurements Ensure Accuracy and Safety
Differential Amplifiers Attribute Small Sense Voltages to Ground
Sense FETs Enable Dynamic In-Line Measurements
Hall Effect Sensors Measure AC and DC
From small battery-powered tire pressure sensors to multi-megawatt wind turbines, the nature of today’s electronics and the ever-increasing demands for energy efficiency are making energy measurement a hot topic for every designer. At the smaller, low end, every portable electronic device requires better battery power management than ever before to increase operating time and provide more features to the user. And at the larger, nationwide power grid, fast, accurate, and rugged sensors are the basis for providing servo loop feedback to balance generator output under changing grid conditions. Most applications fall between these two extremes, from automotive power regulation to consumer electronics to industrial process control. In each of these cases, the ability to measure current is a key requirement for devices offered by semiconductor and sensor manufacturers. In this now separate market, home energy meters combine the ability to measure current and voltage in a cost-effective way in a conflicting environment (see sidebar, “Electricity meters require electronic measurements”).
Regardless of the power level, the measurement must always interface with supervisory logic such as an ADC. Although designers often think of voltage measurement as a piece of cake, today’s ICs and sensors often make current measurement easier—especially when isolation from the AC power source is an issue. But before delving into AC power measurements, since the concepts are essentially the same, if modified, it’s worth discussing DC applications and the various ways that they can make the designer’s job easier. Battery-powered devices have long used power measurements to report circuit status, of course. But perhaps ironically, advances in mechanical and electrical components are making the classic example—the automotive charging circuit—an increasingly rare sight in commodity vehicles. While the days of every car having a voltmeter and ammeter to tell the driver what’s about to go wrong are long gone, the analogy is more important than ever in automotive consumer electronics.
While a large number of automotive electronics use battery terminal voltage measurements to determine the operating time remaining before the next charge cycle, peak loads, such as the flash on a digital camera, require power measurements to manage energy resources and optimize overall device operation. For example, a microcontroller may choose to disable the flash if there is not enough power left to keep the camera running. In addition, battery voltage measurements are a rough approximation of battery capacity, which deteriorates over time as the battery's electrochemical properties degrade. For this reason, a technique called coulomb counting is gaining popularity. Here, current and voltage are used to monitor the increase and decrease of a timer during charge and discharge, and its full-scale value represents the battery's capacity. For example, in a battery-powered tire pressure monitoring system (TPMS), since there is no opportunity to charge the energy source and the device is safe if it is functioning properly, the monitoring circuit measures the battery discharge in nanoamps per second as the device periodically switches between standby and operating modes. The error signal indicates whether the remaining charge is low (Reference 1).
Although small devices such as TPMS require the use of an ASIC, coulomb counters are also easily implemented as ADC/timers in commercial microcontrollers. More complex applications such as smart battery packs can use dedicated battery energy monitor ICs that integrate peripheral power management functions. Chips such as Atmel's new ATmega406 surround the microcontroller with a voltage regulator and support circuits (including FET drivers for battery charging and two ADCs for current and voltage monitoring) to build an autonomous controller for lithium-ion battery pack chargers. Coupled with an 18-bit coulomb counter with 0.67mA resolution via a 5mΩ current bypass, the ±30A range of the device is also recommended for use in a wider range of control devices that can take advantage of 40kBytes of Flash, 2kBytes of RAM, and 512kBytes of EEPROM.

Protecting Measurement Accuracy
Almost without exception, monitoring and control circuits require an interface to refer measurements to system ground, presenting designers with the age-old problem of how best to convert current at arbitrary voltage levels to levels suitable for readily available logic. The traditional low-end sensing technique, traditionally suitable for high-sensitivity moving-coil DC ammeters, is to insert a current-sense resistor in the power supply return path and measure the voltage developed across it. This arrangement also has the advantage of referencing the measurement to neutral potential in high-voltage AC circuits, thereby avoiding high common-mode voltages and simplifying transient protection—although it cannot detect shorts between the motor coil and its housing. However, to interface with logic circuits, the ADC signal ground must be connected to circuit ground, and for all other circuits, leaving the sense resistor dynamically floating can cause deviations between multiple circuits. In addition, this arrangement makes it difficult to provide the required current to individual circuits (including the ADC) and tends to introduce objectionable impedance to the ground plane. Because the input sensitivity of the ADC is much less than the typical 75mV full-scale voltage of the ammeter, an instrumentation amplifier that can handle common-mode voltages (including ground) must be used to boost the sense voltage to a suitable level.
High-side measurements overcome these problems and are actually mandatory in applications that carry a large number of common ground-return paths, such as automotive. The problem now centers on ground referenced to a small sense voltage applied to the positive supply rail. Although a true differential amplifier or instrumentation amplifier approach can be used, well-matched resistors are required to maintain common-mode rejection ratio (CMRR) and to maintain accurate gain performance (Figure 1a). For example, a 0.1% imbalance between any two resistors will reduce the CMRR to 66 dB.
Chips such as Maxim's MAX4198/99 with fixed gains of 1 and 10 integrate these resistors and have better than 0.01% gain accuracy and greater than 110 dB CMRR. Packaging options include the company's small 8-pin mMAX package, which starts at about $1.25 (1,000 pieces). The company also offers a variety of components for current sensing applications. Analog Devices also offers a variety of instrumentation amplifiers for high common-mode sensing in its current-sense amplifier series. For example, the AD8205's 65V operating voltage limit makes it suitable for automotive 42V PowerNet monitoring. Flexible connections to the internal divider link make it easy to bias and scale the output voltage for unipolar and bipolar measurements. The chip, priced at $1.35 (1,000 pieces), is available in an 8-pin SOIC package with an operating temperature specified from -40°C to +125°C.

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Differential amplifier configurations are also suitable for high-voltage environments. For example, the LT1990 from Linear Technology can accommodate common-mode voltages up to ±250V when operating from ±15V supplies, with gains of 1 or 10 set by external links. It also features common-mode transient protection up to ±350V or ±500V, respectively, making it well suited for industrial applications. The LT1990 in an SO8 package has a minimum CMRR of 70dB and a maximum gain accuracy error of 0.28%, starting at $1.35 (1,000 pieces). A member of the same family, the LT1991, offers even higher accuracy over an input voltage range of ±60V. It includes eight on-chip precision-matched silicon-chromium resistors, allowing gains of -13 to +14 to be set with a gain error of less than 0.04% and a CMRR of more than 75dB. The op amp has a typical input offset voltage of 15mV at 3nA bias current, operates from a single supply of 2.7V to ±18V, and has minimal power consumption at about 100mA. In addition, the device is a 560kHz gain bandwidth product that maintains a -3dB unity gain response at 110kHz. It starts at $1.39 (1000 pieces) in a 10-pin MSOP package or leadless DFN package, and is only 3mm square.
Another differential amplifier method configures the device to use a full-swing input op amp to amplify the sense voltage directly on the supply voltage (Figure 1b). Since the P-channel MOSFET Q1 is used as a current source, the negative feedback will apply the differential voltage to the sense resistor R1, and the current in R1 will then flow to ground through R2. In this case, the CMRR depends only on the capability of the op amp, and the output voltage is directly grounded. But choose rail-to-rail output op amps carefully because they become nonlinear within a few volts of line voltage when a transistor operating in the midrange turns off and another set to operate close to the line voltage takes over (Reference 2). Alternatively, choose a device like the new LTC6101 from Linear Technology Corp. for resistor-shunt current measurements. This all-CMOS device integrates an op amp with a FET to provide a minimum PSRR (power-supply rejection ratio) of 110 dB. With a maximum input offset voltage of 450 mV and an input bias current of 170 nA at room temperature, the device is suitable for detection voltages up to 500 mV in a 4-V to 60-V environment, with response times in the 1-ms range. Erik
Soule, general manager of Linear Technology's signal conditioning division, points out that the chip can be powered from a battery or from the load side while measuring its own current consumption of about 250 mA at 14 V. “You can actually get better than 1% performance easily with 0.1% gain-setting resistors, because the resistors are the dominant error source,” he said. The LTC6101 chip, in a 5-pin, 1mm-high SOT-23 package, starts at $1.04 (1,000 pieces). Soule also recommends other current-sense components, including a higher-voltage LTC6101 and a bipolar device with an output buffer and four gain settings.

For high-precision work, Linear Technology offers its LT1787 chip. The 40-mV input offset voltage of this 8-pin device allows 12-bit ADC accuracy at a 250-mV sense voltage. It operates from a 2.5-V to 36-V or 60-V (with HV subscript) supply, consumes only about 60 mA, and has a PSRR of about 120 dB. Two terminals, FIL+ and FIL-, provide additional differential and common-mode signal filtering by adding a capacitor in the middle of the chip's input divider chain (Figure 2). In operation, the op amp drives the potential between its inverting and noninverting inputs to zero, causing the current in the input resistors to flow into Q1 and Q2. The current mirror sums the currents (Figure 2) and converts them to a single-ended output with a fixed input-to-output gain of 8. The bias pin provides a reference level for the voltage output and is typically connected to the ADC's reference voltage. This connection ensures that the IC's current-to-voltage converter (ROUT) tracks changes in the ADC reference voltage over time and temperature. Positive current makes the output voltage positive relative to the bias level, while negative current does the opposite.
SOIC devices are available in industrial and automotive temperature ranges with a suggested price of $2.05 (1,000 pieces).
Another option suitable for many applications uses a current mirror built from a matched transistor pair to reflect a small portion of the load current to ground. In the first-generation ZDS1009 device, available from Zetex, any voltage across R2 induces a balancing current in R1 (Figure 3). If R3 equals R4, the transfer characteristic is (I × R2) × (R4/R1), making it easy to convert the ground-referenced output voltage to a level suitable for the ADC. Today, the company's ZXCT series uses an external resistor to set the circuit gain and is available in 3- or 5-pin SOT-23 packages, typically providing 1% accuracy for a 100mV sense voltage over a 2.5V to 20V supply range. Alan Buxton, marketing manager at Zetex, noted that the 5-pin device offers greater accuracy by including a ground pin for the IC's quiescent current, which is typically 4mA for the ZXCT1009 device optimized for operation at a 100mV sense voltage.
"The 3-pin device offers greater design flexibility by making its output float. Designers can accommodate any supply voltage by simply adding a suitable Zener diode between the chip output and a scaling resistor connected to ground," Buxton said. For automotive or industrial applications, a Zener diode between the input supply line and the IC's current output protects the chip from transients caused by relays and solenoids. During an overvoltage event, the Zener diode conducts to maintain a safe voltage across the device. The IC's current mirror design means that if there is a high enough reverse polarity, its transistor is forward biased, but the Zener diode provides a diode clamp to divert current away from the IC. Buxton said it is also possible to connect two devices back-to-back to form a bidirectional measurement circuit, and he added that the company will soon introduce an IC with this capability. The ZXCT1009 is priced at 45 cents in 1,000-unit quantities in a 3-pin SOT-23 package.

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Other companies with specialized ICs that simplify high-side DC current measurement include Ixys, National Semiconductor, and Texas Instruments. Ixys is a recent entrant into the market with its 40-cent (1,000-unit) IXI848 chip, which comes in an 8-pin SOIC package and operates from a 2.7-V to 40-V supply range. Characterized for a 150-mV sense voltage, the IC has a typical full-scale accuracy of 0.7%. Connections to internal precision resistors allow the user to set the gain to 10 or 50 V/V to accommodate a range of external sense resistors. The voltage output generally needs to be buffered because it is connected to a current source that can drive 33 kV at a gain of 10 or 165 kV at a gain of 50. National Semiconductor's LM3814 chip is unique in that it integrates a delta-sigma modulator that outputs a PWM waveform with a center frequency of 160 Hz and a resolution of 0.8%, respectively, suitable for direct connection to a microcontroller. The chip also integrates a sense resistor to accommodate a full-scale range of 61A or 67A, and can transmit positive and negative full-scale values ​​at duty cycles of 95.5% and 4.5%, respectively. The LM3814 chip is available in an 8-pin SOP package and costs about $1.50 (1,000 pieces in batches). Texas Instruments The 5-pin SOT-23 packaged INA138 and INA168 unipolar monitors offer the company's Burr-Brown heritage and are suitable for 2.7V to 36V or 60V operation. These current output devices feature a single resistor conversion for ease of use and cost about $1.25 (1,000 pieces in batches). The company also offers a variety of similar monitors with automotive AEC-Q100 qualifications (such as the INA169) and new voltage output devices. The INA193/198 series accommodates common-mode input voltages from -16V to +80V and offers fixed transfer ratios of 20, 50, and 100V/V for use with external current shunts. These 80-cent (1,000-piece) 5-pin SOT-23 packaged devices operate from 2.7V to 13.5V supplies over the -40°C to +125°C temperature range.

Sense FET Switching Current
Another approach to in-circuit current measurement uses a FET structure that uses a small current sensing pad on the same die as the main power switch to form the so-called sense FET. Here, the geometric match between the power switch and the sense element again reflects a small portion of the load current to the sense pin, allowing the ground resistor to generate a ground reference voltage. International Rectifier's N-channel HEXSense MOSFETs are generally suitable for switching applications and can handle currents up to 50A or voltages up to 500V with typical accuracy of ±2.5%. Its recently introduced lead-free product, the 60V-rated IRCZ44PBF device, has a maximum on-resistance of 0.028Ω (assuming adequate heatsinking), allowing the device to handle 50A in a TO-220 package. It senses the output current with a 1:2460~1:2720 ratio. Connecting the op amp's non-inverting input to the device's Kelvin ground pin and the inverting input to its sense pin allows the output to be scaled to any level using a resistor in the op amp's feedback loop. The device is priced at $1.45 in 1,000-unit quantities.

Infineon Technologies
' Sense-ProFET family of smart power switches has similar current sensing capabilities (Figure 4). Here, the transistor carrying the load may have 50,000 cells, while the sense transistor has about 10 cells. The op amp and P-channel FET hold the load transistor's potential across the sense transistor and reflect part of the current to ground. In theory, this current is equal to the load current divided by the ratio of the number of load transistors to the number of sense transistors. Built in chip-on-chip technology, these N-channel high-side switches have a charge pump driver and a range of protection and diagnostic features. Suitable for automotive and industrial applications, the switches come in a variety of TO-252 surface-mount and TO-220/218 through-hole plastic packages, switching currents from 17A to 165A. Its highest-current device, the BTS555, has an on-resistance of only 2.5 mΩ and an internal short-circuit current limit of 520A. Current sensing capabilities at this level include using external circuitry to reduce the short-circuit current limit to take advantage of the device's extremely low switching losses (Reference 3). The suggested retail price of BTS555 is US$4.50 (1,000 pieces).

Measuring AC and DC Current with ICs
Traditional methods of nondestructively measuring AC current rely on current transformers that maintain a selective precision measurement technique. When used, the current conductor passes through the current transformer core to form a single-turn primary coil. The primary coil turns ratio can be increased by increasing the number of loops through the core to obtain higher sensitivity. With careful design and balanced coaxial load resistors, this technique can easily be used to obtain better than ±0.5% accuracy for everyday benchtop measurements with dedicated broadband equipment such as the Tektronix CT6 oscilloscope current probe that can operate at frequencies up to 2 GHz. Common benchtop applications include measuring the primary terminal current of a triac during device startup when the sensor output drives the oscilloscope's 50Ω termination resistor and another channel measures the trigger voltage. This application places a negligible burden on the current being measured and does not require a power supply. However, because it is a transformer technology, the frequency response lags significantly behind the power line frequency and waveforms with a DC component cannot be measured.
Current transducers using Hall-effect devices help overcome many of the application challenges in the DC to 100kHz bandwidth required by most industrial control applications. Companies such as Honeywell, LEM, and Sentron offer devices ranging from a few amperes to thousands of amperes suitable for use as monitoring and control logic in equipment such as wind turbine generators. Historically, Hall semiconductor devices have had poor sensitivity and practical temperature drift due to the nature of these devices and the ±12 or ±15V supply voltages required to power the signal conditioning circuitry. Today, ASICs often contain chopper-stabilized amplifiers that condition the output of the Hall device in a feedback loop. This loop can reduce the temperature drift by an order of magnitude or more to provide a stable, proportional output voltage that is typically centered around VCC/2 (where VCC can be a single 5V supply), thereby simplifying the interface to the ADC. Sensitivity improvements are generally achieved using magnetic field concentrators that sandwich the Hall-effect device in the gap between the ends of the circuit's magnetic core.
Examples include LEM's LTS series of unipolar, closed-loop, 5V-powered current transducers. The devices, with ±6A, ±15A and ±25A primary current ratings, measure responses from DC to 0.5 dB (100 kHz) or -1 dB (200 kHz). The devices weigh 10 grams and are common through-hole mounted, with package dimensions of approximately 24 mm long and 10 mm wide, with six pins on a 12.7mm grid, allowing three series/parallel connections to its three sets of internal coils. In this way, each device can provide pin-configurable gains of 1, 2 and 3 times greater than its corresponding rated primary current, respectively. In each case, the output voltage is 2.5V ± 0.625V, and the output voltage linearity is better than 0.1% within a 0.5V supply. This configuration allows the highest sensitivity LTS 6-NP devices to measure currents from 0 to ±19.2A. Another feature of these devices is a through-hole through the center of the magnetic core, thus providing another unity-gain connection that allows differential measurements when there is only one path for the current carrier. The device, priced at $10 (100 pieces), is now widely available, including from catalog distributors. Other series also provide current outputs suitable for 4mA to 20mA current loops in industrial instrumentation. Here, the ability to measure bidirectional DC current is suitable for isolated measurements in high-current battery packs.

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Other general-purpose devices, especially for automotive and industrial applications, include Allegro's ACS current sensor family. The latest addition to the growing family of Hall-effect sensors from ±5A to ±200A is the ±50A-rated ACS754-050. Like the rest of the family, this leadless chip operates from a 5V supply but replaces one resistor, which typically reduces impedance, power dissipation, and voltage drop by two to three orders of magnitude. This approach makes some new packages suitable for high-current applications. In this case, the package is like adding two more traditional three-terminal power devices with minimal insertion loss and true wings that pass power through the package, with an internal resistance of just 100mΩ. The ratiometric voltage output comes from a chopper-stabilized Hall-effect IC built with ICBiCMOS (factory trimmed for minimum gain and offset errors) (Figure 5). The result is a total output error of ±1% at 25°C and ±5% over the -20°C to +85°C temperature range. Extended applications show
less than ±10% error over the extreme automotive temperature range of -40°C to +150°C. Its 35kHz bandwidth and 3 kV rms input-output isolation also make it suitable for use in low-frequency AC applications such as motor control. The ACS754 is currently priced at approximately $3.23 (1,000 units).
Bob Christie, European applications manager at Allegro, points out that the company's new 8-pin SOIC packaged ACS704 device, which has two varieties of ±5A and ±15A, "The ACS704 brings the current into the SOIC package as close as possible to the Hall plate on the die to achieve higher accuracy and sensitivity, while still keeping its internal resistance at a low level of 1.5 mΩ." He adds that by bringing the current path into the package, the ACS704 can also control leakage and the spacing distance that makes its 800V-rms voltage isolation specification possible. The device can operate from a single 5V supply, and its output sensitivity is typically 133 mV/A (5A device) and 100 mV/A (15A device). The output voltage is centered at VCC/2 and has a positive slope representing positive current. Both devices have a bandwidth from DC to 50kHz, making them suitable for a variety of low-current and space-limited applications. The device is available now with a suggested retail price of $1.61 (1,000 pieces).

Sidebar: Electricity
Meters
Need Electronic Measurements
... The current market leader is Analog Devices, which has deployed 50 million meters. Its latest product is the ADE7757A, an improved version of its popular ADE7755 with a reduced pin count. The new chip reduces costs by using an oscillator to drive the digital signal processing block and providing a direct interface for current bypass (Figure A). Its two 16-bit Σ-Δ ADCs include the main analog circuits and digitize the voltage and current channel signals at an oversampling rate of 450 kHz. By digitally processing the instantaneous power signal, the chip can calculate the actual power. In other words, it infers the actual power from the real-time product of its voltage and current measurements. The high-pass filter in the current channel removes any DC components that would otherwise introduce fixed errors into the product operation.

The analog input bandwidth is approximately 7 kHz, allowing the FFT algorithm to maintain accuracy when processing non-sinusoidal signals. Within the chip's 45Hz to 65Hz measurement bandwidth, accuracy is better than 0.1% over a 500-to-1 dynamic range, exceeding the accuracy requirements of the IEC-61036 standard. The output signal includes a pair of low-frequency pulse signals to match an electromechanical counter or microcontroller. Independent high-frequency logic outputs reflect instantaneous real power measurements, suitable for equipment calibration. The chip is packaged in a 16-lead narrow-body SOIC package, and samples can be obtained by contacting Analog Devices.
STMicroelectronics, which introduced its first product at about the time of this article's publication, is another company entering the electricity meter market. Based on the company's 0.35-micron BiCMOS process, the STPM01 chip targets standalone Class 0.5 meter applications in single-phase circuits, using the output of its stepper motor to drive an electromechanical meter. (Note: "Classification" terminology will be 0.5% measurement accuracy attributed to unity power factor circuits)
In addition, the chip can also work as a peripheral device for microcontrollers and provide active, reactive and apparent power measurement in single-phase and three-phase environments. Using two sets of current inputs IIP1/N1 and IIP2/N2, the STPM01 can be arbitrarily adapted to live and neutral current measurement (Figure B). This feature enables it to detect 20 forms of tampering, thereby ensuring that the meter will not be stolen (a common phenomenon in some areas).

The analog front end is suitable for current shunts, current transformers or Rogowski
coils, and device settings and calibration can be completed by programming its 48-bit OTP (one-time programmable) memory using the chip's SPI interface. This SPI link can also communicate with a microcontroller.
Calibration adjustments include voltage and current calibration, phase correction and temperature compensation. Special outputs include LED drivers that report instant visual status, and a zero-crossing AC detector that can control external load switching to avoid arcing and interference. The STPM01 is packaged in a 20-pin TSSOP. You can visit the company's website for pricing and ordering information.
Both ADI and STMicroelectronics provide reference designs and low-cost evaluation kits that simplify instrument development.