FROM EDN EUROPE: Sensors and ICs simplify current measurements

From tiny battery-powered tyre-pressure sensors to multi-megawatt wind turbines, the nature of today's electronics and the incessant drive to improve energy efficiency make energy measurement a hot topic for every designer. At the low end of the scale, portable electronics of every kind require ever better battery management to increase runtimes while offering users an increasing number of features. At the national grid power generation level, fast, accurate, and rugged sensors are essential to provide the servo-loop feedback that balances generator output against constantly changing grid conditions. In between these two extremes lies the vast majority of applications that span the gamut of automotive power conditioning to consumer electronics to industrial process control. In each case, the ability to measure current is a key requirement that semiconductor vendors and transducer manufacturers ease with a huge range of devices. In what has now become a separate market sector, domestic energy metering combines the ability to measure current and voltage in a hostile environment at very low cost (see sidebar "Electricity meters demand electronic measurements").

Regardless of power level, measurements almost invariably have to interface with supervisory logic via ADCs. While designers frequently assume that voltage measurements are a breeze, today's ICs and transducers often make it easier to measure current, especially where isolation from ac line supplies is an issue. But before delving into ac power measurements—and because the concepts translate with little if any modification—it's worth reflecting on dc applications and the various approaches that can simplify a designer's life. Of course, battery-powered applications have long used power measurements to report circuit status. Ironically perhaps, advances in mechanical and electrical components are making the classic example—a car's charging circuit—increasingly rare in production vehicles. Gone are the days when every saloon had a voltmeter and an ammeter to warn the driver of impending trouble, yet this analogy is ever more important within the context of mobile consumer electronics.

While a huge number of mobiles use a battery-terminal voltage measurement to derive the remaining life to the next charge cycle, peaky loads such as flash units in digital cameras demand power measurements to manage the energy resource and optimise overall device operation. For example, a microcontroller may choose not to enable a flash unit if there's likely to be insufficient remaining charge to permit the camera to continue working. Also, cell-voltage measurements are a crude approximation of capacity that worsens over battery lifetime as the electrochemistry degrades. For these reasons, so-called Coulomb counting techniques find increasing favour. Here, current and voltage monitors increment and decrement a timer during charge and discharge cycles, whose full-scale value represents the battery's capacity. In battery-powered tyre-pressure-monitor systems (TPMS), for instance, where there is no opportunity for recharging the energy source and where correct device operation has safety implications, monitor circuitry measures discharges in terms of nA/sec as the device periodically switches between standby and active modes; an error signal indicates when there's insufficient remaining charge (Reference 1).

While miniature applications such as TPMS demand ASICs, Coulomb counters are easy to implement as an ADC/timer task within a commodity microcontroller. More complex applications, such as smart battery packs, can take advantage of dedicated gas-gauge ICs integrating peripheral power-management functions. Chips such as Atmel's new ATmega406 surround the microcontroller's core with voltage regulators and support circuitry (including FET drivers for battery charging and dual ADCs for current and voltage monitoring) to construct a self-contained controller for lithium-ion battery-pack chargers. With its 18-bit Coulomb counter yielding 0.67 mA resolution using a 5 mΩ current shunt, the device's ±30A range also suggests use in wider control applications that can take advantage of 40 kB of Flash, 2 kB RAM, and 512 bytes of EEPROM.

Almost without exception, supervisory and control circuits require interfaces that refer measurement values to system ground, posing designers with the continual problem of how best to translate currents riding on arbitrary-level voltages to levels that suit off-the-shelf logic. Traditionally useful for high-sensitivity moving-coil dc ammeters, the classic low-side sensing technique inserts a current-sense resistor in the power-supply return path and measures the voltage it develops. This arrangement also has the advantage of referring the measurement to neutral potential in high-voltage ac circuits, avoiding high common-mode voltages and simplifying transient protection—albeit without the ability to detect short-circuits between, say, a motor's winding and its case. But to interface with logic, the necessity of tying the ADC's signal ground to circuit ground and for all other circuitry to float at dynamic sense-resistor potential creates offset problems between multiple circuits. It also makes it difficult to resolve the currents that individual circuits take—including the ADC—and introduces unwelcome impedances into the ground plane. Because an ADC's input sensitivity is much less than an ammeter's typical 75 mV full-scale value, an instrumentation amplifier that can handle common-mode voltages including ground must boost the sense voltage to a suitable level.

High-side measurements overcome these issues and are virtually mandatory in applications that share extensive common ground-return paths, such as cars. The problem now concerns ground-referencing a small sense voltage that's riding on the positive supply line. A true-differential or instrumentation amplifier approach works well but calls for well-matched resistors to preserve common-mode rejection ratio (CMRR) and maintain accurate gain performance (Figure 1a). For example, a 0.1% imbalance between any of the resistors degrades CMRR to 66 dB. Available with fixed gains of 1 and 10, chips such as Maxim's MAX4198/99 integrate these resistors and are capable of better-than-0.01% gain accuracy with more than 110 dB of common-mode rejection. Packaging options include the company's miniature 8-pin μMAX outline, and pricing starts at around $1.25/1k. The company also offers a wide range of parts that are specifically optimised for current-sensing applications. Analog Devices too offers a range of instrumentation amplifiers optimised for high common-mode sensing within its current-sense amplifier range. For example, its AD8205's 65V operating voltage limit suits uses such as automotive 42V PowerNet monitoring. Flexible connections to the internal divider chain make sit easy to bias and scale the output voltage to suit unipolar and bipolar measurements. The chip comes in an 8-pin SOIC that's specified for –40 to +125°C operation with a guide price of $1.35/1k.

The difference-amplifier configuration works well in high-voltage environments, too. For example, Linear Technology's LT1990 accommodates common-mode voltages as high as ±250V when operating from ±15V supplies, with a gain of 1 or 10 set by external links. It also enjoys protection against common-mode transients of up to ±350 or ±500V differentially, making it suitable for industrial use. With a minimum CMRR of 70 dB and maximum gain accuracy error of 0.28%, the LT1990 comes in an SO8 outline; pricing starts at $1.35/1k. A companion part, the LT1991, provides greater precision with input voltages of up to ±60V. It includes eight on-chip precisely-matched silicon-chromium resistors that allow gain settings from –13 to +14 with as little as 0.04% gain error and >75 dB of CMRR. The op-amp's input offset voltage is typically 15 μV with 3 nA of bias current. Operating from single 2.7V to ±18V supplies, power consumption is minimal at some 100 μA, yet the chip maintains a 560-kHz gain-bandwidth product that sites its unity-gain –3 dB response at 110 kHz. It costs from around $1.39/1k and comes in a 10-pin MSOP or a leadless DFN package measuring just 3mm².

Another take on the differential amplifier approach uses a rail-to-rail input op-amp to amplify the sense voltage directly at supply-rail voltages (Figure 1b). With p-channel MOSFET Q1 acting as current source, negative feedback impresses the differential across the sense resistor on R1. The current in R1 then flows to ground via R2, simplifying ground referencing and output-value scaling. In this case, CMRR depends solely on the op-amp's abilities, and the output voltage refers directly to ground. But choose rail-to-rail op-amps with care, as they can be non-linear within a few volts of the rail value when the transistors that work over the mid-range cut off and another set optimised for close-to-rail operation take over (Reference 2). Alternatively, choose a device such as Linear Technology's new LTC6101 that's optimised for shunt-resistor measurements. This all-CMOS part integrates the op-amp and FET to provide a minimum 110-dB power-supply rejection ratio. With a maximum input offset voltage of 450μV and 170 nA input bias current at room temperature, it suits sense voltages of up to 500 mV in 4 – 60V environments; response times lie within the 1-msec region. Erik Soule, general manager of the company's signal conditioning division, notes that it's possible to power the chip either from the battery side or from the load side, when it will also measure its own consumption of some 250 μA at 14V. "In practice, by using 0.1% gain setting resistors you can easily get better-than-1% performance since this is the dominant error source," he says. The guide price for the LTC6101 in a 5-pin, 1-mm profile SOT-23 package is $1.04/1k. Soule advises that additional current sense parts will follow, including a higher voltage version of the LTC6101 and a bipolar part with output buffering and four gain settings.

For high-precision work, Linear Technology offers its LT1787. This 8-pin device's input offset voltage of around 40 μV allows 12-bit ADC precision with sense voltages of 250 mV. It operates from supplies of 2.5 to 36V or up to 60V for the HV suffix, consuming some 60 μA while presenting around 120 dB of power-supply noise rejection. Two terminals, FIL+ and FIL–, provide for additional differential and common-mode signal filtering by adding a capacitor halfway across the chip's input divider chain (Figure 2). In operation, the op-amp drives the potential between its inverting and non-inverting inputs to zero, so the currents flowing in the input resistors flow in Q1 and Q2. The current mirror sums and converts these currents to a single-ended output that has a fixed gain of 8 from input to output. A bias pin provides a reference level for the voltage output and typically connects to an ADC's reference voltage. This connection ensures that the IC's current-to-voltage converter tracks ADC reference voltage changes over time and temperature. Positive currents make the output voltage positive with respect to the bias level; negative currents do the opposite. The guide price for the SOIC version is $2.05/1k; both industrial and automotive temperature ranges are available.

One solution that suits many applications exploits a current mirror built from matched transistor pairs that reflects a tiny proportion of the load current to ground. In the first-generation ZDS1009 from Zetex Semiconductors, any voltage across sense resistor R2 impresses a balance current in R1 (Figure 3). If R3 equals R4, the transfer characteristic is (I×R2) times (R4/R1), making it easy to scale the ground-referenced output voltage to levels that suit ADCs. Today, the company's ZXCT series uses a single external resistor to set circuit gain and comes in a choice of three- or five-pin SOT-23 outlines that typically offer 1% accuracy for 100-mV sense voltages within 2.5 – 20V supply lines. Alan Buxton, marketing manager at Zetex, notes that the five-pin versions offer enhanced accuracy by including a ground pin for the IC's quiescent current—which is typically 4 μA for versions, such as the ZXCT1009, that are optimised for operation at the 100-mV sense-voltage level. Other versions suit sense voltages from 10 mV to 2.5V: "The three-pin versions offer greater design flexibility by floating their outputs. Designers can then accommodate arbitrary power-supply levels simply by adding a suitable Zener between the chip's output and the scaling resistor that connects to ground," Buxton says. For automotive or industrial use, a Zener between the input supply line and the IC's current output terminal protects the chip from the transients that accompany relays and solenoids. During an over-voltage, the Zener conducts to maintain a safe voltage across the device. The IC's current-mirror design means that its transistors will be forward-biased if subject to a sufficiently high reverse polarity, but the Zener then provides a diode clamp to divert current away from the IC. Buxton says that it's also possible to connect two devices back-to-back to create a bi-directional measurement circuit, and he adds that the company will soon release this capability within a single IC. Guide pricing for the three-pin SOT-23 packaged ZXCT1009F is $0.45/1k.

Other vendors with dedicated ICs that simplify high-side dc current measurements include Ixys, National Semiconductor, and Texas Instruments. Ixys is a recent entrant into this sector with its IXI848, an 8-pin SOIC device that operates with supplies in the range from 2.7 to 40V. Characterised for sense voltages of 150 mV, the chip has a typical full-scale accuracy of 0.7%. Connections to internal precision resistors allow users to set gain at 10 or 50V/V to suit a wide range of external sense resistors. The voltage output typically requires buffering as it connects to a current source driving either 33Ω (gain = 10) or 165 kΩ (gain = 50). The guide price is $0.40/1k. National's LM3814 is unusual in that it integrates a delta-sigma modulator that outputs a PWM waveform with a centre frequency of 160 Hz and 0.8% resolution, thereby suiting direct connection to a microcontroller. The chip also integrates sense resistors to accommodate ±1 or ±7A full-scale ranges, with 95.5% and 4.5% duty cycles respectively signalling positive and negative full-scale values. The LM3814 comes in an 8-pin SOP outline and costs around $1.50/1k. And reflecting its Burr-Brown inheritance, Texas Instruments offers the five-pin SOT-23 packaged INA138 and INA168 unipolar monitors that suit operation from 2.7 to either 36 or 60V. These current-output devices feature single-resistor scaling for ease of use and cost around $0.99/1k and $1.25/1k. The company also offers a range of similar monitors with automotive AEC-Q100 qualification, such as the INA169, as well as a new range of voltage-output devices. This INA193/198 range tolerates input common-mode voltages from –16 to +80V and offers fixed transfer ratios of 20, 50, and 100V/V for use with external current shunts. Available now, these 5-pin SOT-23 parts operate from 2.7 to 13.5V supplies over the extended –40 to +125°C range and cost from $0.80/1k.

A quite different approach to in-circuit current measurements uses a FET structure that has a small current-sense pad on the same die as the main power switch to build the so-called "sense FET". Here, geometric matching between the power switch and the sense element again reflects a small proportion of load current to the sense pin, allowing a resistor to ground to generate a ground-referenced voltage. Typically suiting switching applications, International Rectifier's N-channel HEXSense MOSFETs handle as much as 50A or up to 500V with typical ±2.5% accuracy. Newly available in a lead (Pb)-free version, the 60V-rated IRCZ44PBF has a maximum on-resistance of 0.028Ω that—given adequate heat-sinking—permits the device to handle 50A in a TO-220 package. It outputs a current with a sense ratio of between 1:2460 and 1:2720. Connecting an op-amp's noninverting input to the device's Kelvin-ground pin and the inverting input to its sense pin allows a single resistor in the op-amp's feedback loop to scale the output to arbitrary levels. The guide price is $1.45/1k.

Infineon Technologies offers its Sense-ProFET family of smartpower switches that include similar current-sense abilities (Figure 4). Here, the load-carrying transistor (LTR) has maybe 50,000 cells while the sense transistor (STR) has around 10 cells. The op-amp and the p-channel FET maintain the load transistor's potential across the sense transistor and reflect a proportional current to ground; ideally, this current is equal to the load current divided by the ratio of the load-carrying and sense transistor's cell counts. Built in a chip-on-chip technology, these N-channel high-side switches also include charge-pump drivers and a range of protection and diagnostic functions. Suiting automotive and industrial use, the family switches between 17 and 165A in a variety of TO-252 surface-mount and TO-220/218 through-hole plastic packages. The highest-current BTS555 has an on-resistance of just 2.5 mΩ and an internal short-circuit current limit of 520A. Uses for the current-sense function at this level include lowering the short-circuit limit with external circuitry to take advantage of the device's extremely low switching losses (Reference 3). Guide pricing for the BTS555 lies around $4.50/1k.

The traditional non-intrusive method for measuring ac current relies on current transformers that remain the technique of choice for precision measurements. In use, the current-carrying conductor passes through the current transformer's core to form a single-turn primary winding; increasing the number of loops through the core increases the primary turns ratio to provide greater sensitivity. With careful design and using balanced coaxial load resistors, the technique easily betters ±0.5% accuracy for everyday benchtop measurements, with special-purpose wideband devices—such as the CT6 oscilloscope current probe from Tektronix—working at frequencies up to about 2 GHz. Common benchtop uses include, for instance, measuring main-terminal current in triacs during device turn-on, when the transformer's output can drive a scope's 50Ω¸ termination resistor while another channel measures trigger voltage. This application presents negligible burden to the circuit-under-test and requires no power supply. But being a transformer technique, frequency response tails off significantly below power line frequencies, and it's not possible to measure waveforms that include a dc component.

Current transducers that use Hall-effect devices help overcome many application difficulties in the dc-to-100 kHz bandwidth that most industrial-control applications require. Vendors such as Honeywell, LEM, and Sentron offer device ranges that span a few A to the kA levels that suit supervisory and control logic in applications such as wind turbines. Historically, such devices have been plagued with poor sensitivity and substantial temperature-related drift due to the nature of Hall-effect semiconductors, as well as the requirement for ±12 or ±15V power rails to supply the signal-conditioning circuitry. Today, ASICs that now often contain chopper-stabilised amplifiers condition the Hall-effect device's output within a feedback loop. The latter reduces temperature-related effects by an order of magnitude or more to provide a stable ratiometric output voltage that typically centres on Vcc/2—where Vcc can be a single 5V rail, simplifying the interface to ADCs. Sensitivity improvements typically employ magnetic field concentrators that sandwich the Hall-effect device in the gap between the ends of a circular magnetic core.

Contemporary examples include LEM's LTS-series of single-pole closed-loop current transducers that operate from a 5V rail. Available with nominal primary current levels of ±6, ±15, and ±25A, these devices measure from dc to a response that's 0.5 dB down at 100 kHz or –1 dB at 200 kHz. Weighing 10 grams and occupying a common through-hole mounting package that's about 24 mm square and 10 mm wide; six pins on a 12.7-mm grid allow three serial/parallel connections to three internal sets of coils. In this way, each device offers pin-configurable gains of x1, x2, and x3 relative to its respective nominal primary current level. In each case, the output voltage is 2.5V ±0.625V at this level, and the output voltage is linear to <0.1% to within 0.5V of the supply rails. This allows, for example, the highest-sensitivity LTS 6-NP version to measure 0 to ±19.2A. Another feature of these devices is a hole through the core's centre that—given a single pass of the current-carrying conductor—provides an alternative unity-gain connection that permits differential measurements. The devices are widely available, including from catalogue distribution sources for about $8.20/1K. Other series provide current-output that suit, for example, 4 to 20mA current-loops within industrial instrumentation. Here, the ability to measure bi-directional dc currents suits isolated measurements in high-current battery arrays.

Other versatile devices that especially suit automotive and industrial applications include Allegro's ACS current sensor family. Recent additions to this growing range of Hall-effect devices that span ±5A to ±200A include the ACS754-050, a ±50A-rated sensor. Like other family members, this Pb-free chip operates from a +5V supply and replaces a sense resistor, usually with a 2-to-3 order-of-magnitude reduction in resistance, power dissipation, and voltage drop. This approach gives rise to some novel packaging solutions for high current use. In this case, the package resembles a conventional three-terminal power device with the addition of two substantial "wings" that route power through the package with minimal insertion loss; the internal resistance is just 100 μΩ. The ratiometric voltage output comes from a chopper-stabilised Hall-effect IC built in BiCMOS that's factory-trimmed to minimise gain and offset errors (Figure 5). The result is a total output error figure of ±1% at 25°C that's held to ±5% over –20 to +85°C; extended specifications show error below ±10% at the –40 to +150°C automotive extremes. Its 35-kHz bandwidth and input-to-output isolation of 3 kV rms also suggest use in low-frequency ac applications such as motor control. The ACS754 is available now for around $3.23/1k.

Bob Christie, Allegro's European applications manager, points to the company's new 8-pin SOIC-packaged ACS704 that's available in ±5A and ±15A versions: "The ACS704 brings the current into the SOIC package in order to get it as close to the Hall plate on the die as possible for greater accuracy and sensitivity, while still keeping its internal resistance down to 1.5 mΩ," he says. He adds that by bringing the current path into the package, the ACS704 also controls the creepage and clearance distances that make its 800V rms isolation voltage rating possible. Operating from a single 5V supply, output sensitivity is nominally 133 mV/A at the 5A level and 100 mV/A for the 15A device. The output voltage centres on Vcc/2 with a positive slope representing positive currents. Both versions have dc-to-50 kHz bandwidth that makes them useful in a wide range of low-current, space-constricted applications. Available now, the guide price is $1.61/1k.