New ASIC Makes Sense: Handling mA to kA

When measuring current with low-ohmic resistors, it's necessary to process µV range signals. Existing current sense converters do not meet these requirements due to offset and EMF constraints or measuring speed and resolution. However, a new ASIC provides the necessary characteristics for use with low-ohmic resistor current sensors. The device requires a single +5V supply and can handle ground-referring signals with positive and negative signs up to ±0.8V. Current consumption is below 5mA, while in sleep mode it's below 50 µA.

To understand how the system operates, we must look at an automobile's current sensing requirements. Accurate current sensing is a major requirement in various automotive applications. Depending on circuit requirements, several technologies can be used to sense current. Each technology has advantages and limitations. Limitations include physical, electrical, and cost issues. All technologies require the sensing device to be “sized” according to the worst-case (highest) current conditions to be measured or conducted across the device.

The automotive application is becoming more important and challenging in battery monitoring and management. Automotive power requirements place ever-greater demands on the automotive electrical system, which may have to support:

Power side and rear doors.
Electrical defrost and heating facilities.
Air conditioning.
Electrical power steering (EPS).
Electro-hydraulic braking (EHB).
Electrically actuated valves.
Hybrid electric vehicles (HEV).
Electric vehicles (EV).

Under certain conditions, these functions draw more current than the alternator system can provide. Thus, the battery must provide the additional current needed to operate at least the most important safety-related features.

Whether the electrical system is 12Vdc or 42Vdc, the battery sometimes puts more current into the system than the charging current it receives from the alternator. The battery system must be managed with accurate knowledge of the electrical load, state of charge, and expected life.

Battery Management

Existing battery management solutions rely on the ability to sense the amount of current flowing from the battery into the auto's electrical system. Low-ohmic shunts (sense resistors) are the ideal solution for sensing battery current. However, the low-ohmic shunt must be a precision resistor with respect to its stability, temperature coefficient-value, and, in particular, its low thermal EMF vs. copper. Fig. 1 shows a typical 100 µΩ shunt.

For example, using CuNi44 (Constantan) as the resistor element would be a disaster, because its EMF vs. copper is 40 µV/°C. This means that a 100 µΩ shunt with a temperature difference of only 1°C would produce an error voltage equivalent to a current of 400mA! The situation worsens under dc-load in the car because the Peltier Effect produces a temperature difference that could be as much as 10°C to 20°C. This could translate to 4A to 8A error voltage equivalent.

Another design consideration is that the shunt resistance should be kept as low as possible to avoid the high power generated at higher current levels. For example, a 250 µΩ shunt for sensing 1,000A would dissipate 250W. This is a very high thermal load on the shunt and on the evaluation circuit, which must be placed as close as possible to the shunt to minimize noise and EMI.

Peak current and power rating determine the optimum resistance value for low-ohmic shunts. For example, to measure the car's starting current, the shunt must be able to carry up to 1,000A for up to several seconds. With an ideal resistance value of 100 µΩ, the power would be 100W [(1,000)² × 0.0001 = 100W]. The resulting sense voltage is 100 mV (1,000 × 0.0001 = 0.100V), which is ideal for standard low-offset op amps.

In this example, noise interference from thermal EMF and the fluctuating temperature conditions common to “under the hood” automotive applications have little affect on the overall accuracy of the sense voltage. For example, a noise of 20 µV results in a 0.02% signal error. But when sensing currents of only 1A, the same circuit presents a bigger challenge. The sense voltage is now only 100 µV (1 × 0.0001 = 0.0001V). With the same 20µV noise, the signal error is 20% (20/100).

Amplifier Offset

The associated amplifier's dc offset also has a large influence on signal error. For example, in the 1A measurement range the required amplifier resolution in the idle mode is less than 10mA, so the dc offset must be below 1 µV — requiring an expensive chopper amplifier.

Also important is the measuring system's ability to resolve the huge range of the sense signal, from 1 µV to 100mV, which is more than five orders of magnitude.

Frustrated by these technical and cost limitations, designers of automotive battery management systems sometimes limit their expectations to monitor the full range of current (10mA to 1,500A). They use higher resistance values in the 100A to 200A range, switched in by a relay for only a limited time.

Presently, management systems rely on algorithms using the battery current and voltage, as provided by the low-ohmic shunt. The accuracy (stability, low-thermal EMF) of the shunt is the basis for the accuracy of the data processing and, ultimately, the management system. However, battery current and voltage are not sufficient to support an efficient management system. Additional information, such as battery temperature, recovery behavior, dynamic resistance of the battery, state of charge (SOC), and state of health (SOH), are necessary to assure total effectiveness of a management system.

Besides the low-ohmic shunt, today's battery management systems rely on circuits comprised of discrete components to accommodate the conditions mentioned above. At this time, these discrete circuits can't accurately measure all “dream” functions. They are complex, expensive, and require a significant amount of “real estate.”

There are significant challenges in these current sensor systems and limited solutions for communicating accurate measurement data to the management computer. In response to this measurement problem, we developed a unique ASIC, the IHM-A-1500 (patent pending), spending more than two years of R&D. The ASIC is a complete data acquisition system for low-level current sense signals contained in a standard SOIC-16 package. Fig. 2, on page 22, shows the ASIC and its associated shunt resistor.

The IHM-A-1500 replaces discrete circuits now used for signal conditioning and communication to the battery management system's computer. It's intended for use with a precision 100 µΩ resistor. Both ASIC and resistor are available as an integrated package (Fig. 3, on page 22).

Requiring only a single +5V supply, the IHM-A-1500 provides 16-bit resolution of battery current, voltage, and temperature over an SDI bus to the battery management computer (Fig. 4). Current consumption is below 5mA; in sleep mode, it is below 50 µA. When coupled with the 100 µW shunt, the IHM-A-1500 accurately measures the starter current of up to 1,500A as well as the very low idle current of just a few mA in standby mode. This ASIC has four ground reference inputs that can be switched separately to the internal programmable gain amplifier (PGA). Two input channels can also be operated as a fully differential ground-free input. The system measures both positive and negative input signals.

The PGA amplification ranges from 1 to 100, which enables the IHM-A-1500 to measure signals from 7 mV to 800 mV full-scale with high accuracy, linearity, and speed. The sampling frequency for all input channels and gain settings can be programmed from 2 Hz up to 16 kHz and with the active offset compensation switched off — even up to 64 kHz.

Sensor Flexibility

A proprietary internal chopping/dechopping feature performs continuous offset suppression. The high chopping frequency and a special filter makes the system free of 1/f noise. The 0 Hz to 10 Hz noise is typically below 1µV, which is comparable to the best chopper amplifier on the market. For external temperature measurement, the IHM-A-1500 uses a variety of temperature sensors, such as RTD, PTC, NTC, thermocouples, and diodes or transistors. You can switch a built-in programmable current source to any input and activate these sensors without needing external components. The internal chip temperature sensor allows the temperature compensation of sensitive parameters, enhancing overall system accuracy.

You can store sensor-specific data in the ASIC's internal zener-ZAP memory. This data calibrated each measurement by the internal data processing unit before transmission to the battery management computer via the serial SDI interface. The IHM-A-1500's flexibility is further increased by a digital comparator that can be assigned to any measured property (current, voltage, or temperature) as well as to activate wake-up while in sleep mode. Analog input terminals can be checked for wire brake via the SPI interface. The curves in Fig. 5 demonstrate the measurement of starter and generator current in a typical automobile.

As shown in Fig. 6, the IHM-A-1500 can measure the starter current of the battery, the charging and discharging current during normal operation, and the idle current in the standby mode of the car.

Voltage divider R3/R2 provides the battery voltage measurement. The low impedance of R2 does not cause additional resistance noise. A special, fast double mode allows the IHM-A-1500 to measure the current and voltage simultaneously. This mode is ideal for diagnostic evaluations, such as the determination of the dynamic resistance during the starting cycle or in the normal operation where current and voltage fluctuations can be used for the calculations, which is only possible due to the high speed and resolution of the measurement.

Battery temperature measurement involves use of a Pt-100 resistor. However, you can use any NTC (negative temperature coefficient) or PTC (positive temperature coefficient) resistor. The IHM-A-1500 supplies the sensing current. In Fig. 6, metal film resistor R4 is a reference resistor that eliminates any inaccuracy, drift, and the TC-value of the integrated current source. Measuring both voltages ETS and ETR with the same current and gain settings immediately after one another ensures that the ratio ETS/ETR is free of any drift, TC and gain error is proportional to R_t/R_o, which precisely defines the temperature in accordance with IEC 751.

Additional Applications

As a universal low-noise and offset-free data acquisition system for low-level signals, the IHM-A-1500 is ideal for applications in industrial and consumer electronics. With its four inputs and the internal temperature measurement, the IHM-A-1500 is a highly sensitive 4-channel temperature measuring system for thermocouples, including the cold junction compensation. Several IHM-A-1500s can be paralleled by multiplexing the clock line of the serial SPI-bus resulting in a low-cost, high-precision multichannel temperature measurement system that eliminates the need for expensive relay multiplexers.

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