Measuring the current in a system is a fundamental yet powerful tool for monitoring its state. Thanks to advanced technologies, the physical size of electronic or electrical systems has been greatly reduced, along with lower power consumption and costs, with minimal compromises in performance. Every electronic device monitors its own health and status, and these diagnostics provide crucial information needed to manage the system and even determine its future design upgrades.
There is a growing need to measure a wide range of currents in systems, from minute current levels to several amperes. For example, a high dynamic range of the current flowing or consumed in a system can be determined in the following cases:
1. In addition to normal operation, sleep/inactive current can also determine overall load performance and battery/power supply estimation.
2. ATE/test environments need to handle currents from micro/low microamp levels to ampere levels for R&D or production-grade testing.
3. The production workshop environment is used to identify production problems (flux issues under ICs, unnecessary solder short circuits or open circuits) and to test normal operational functions.
4. Industrial equipment monitoring: Power dissipation during startup and shutdown provides information about the health of the equipment. For example, monitoring normal current and leakage current in the equipment can help determine its wear over time.
In applications with higher voltage levels (common-mode level) up to 80V, an external simple current sense amplifier (CSA) (although employing a complex integrated circuit design with an architecture catering to precision and accuracy) and a sense resistor are the solution to most problems when measuring current. Current sense amplifiers currently offer best-in-class accuracy and precision to meet the requirements for achieving microamp current levels, while still maintaining better signal-to-noise ratio (SNR) performance to provide the measurement resolution required by the system design.
However, choosing an optimized CSA is not an easy task for designers. Several trade-offs need to be considered:
1. Available supply
2. Minimum detectable current (indicating how low the device's input offset voltage (VOS) is)
3. Maximum detectable current (converted to maximum input sense voltage (V SENSE))
4. Permissible power dissipation on R SENSE
Since the differential voltage range is determined by the selection of the current sense amplifier, increasing the R<sub>SENSE</sub> value can improve the accuracy of measurements at lower current values, but the power consumption at higher currents is higher, which may be unacceptable. Furthermore, the range of the sensed current will also decrease (I<sub>MIN</sub>: I<sub>MAX</sub>).
Lowering the R SENSE value is more advantageous because it reduces power dissipation from the resistor and increases the sensing current range. Lowering the R SENSE value will decrease the SNR (which can be improved by averaging the input noise). It should be noted that in this case, device offset will affect measurement accuracy. Typically, calibration at room temperature can improve system accuracy and eliminate offset voltage, but it will increase the testing cost for some systems.
For most CSAs, V SENSE-RANGE is typically 100mV, with an input offset voltage of approximately 10μV. Note that if V SENSE_MIN is chosen as a 10xV OS factor, this provides up to three decibels for ±10% error in an uncalibrated system. Similarly, choosing 100xV OS achieves a ±1% error range, but the dynamic range shrinks to two decibels. Therefore, there is a trade-off between dynamic range and accuracy: tightening the accuracy budget reduces the dynamic range specified by V SENSE_MIN, and vice versa.
One important point to note is that in a CSA + R SENSE system, R SENSE (tolerance and temperature coefficient) is often the bottleneck for the overall system accuracy. Compared to other alternatives (such as galvanometers, CSAs with integrated chip resistors, and discrete implementations using operational amplifiers and differential amplifiers), this remains an effective practice in the industry for monitoring/measuring system current due to its simplicity, reliability, and reasonable cost. Higher tolerance and temperature coefficient sense resistors can be found, but they are more expensive. The total error budget for the application over the temperature range must equal the error introduced by R SENSE.
Resistanceless sensing solution:
For applications requiring the measurement of currents with a higher dynamic range (from hundreds of microamps to several amps), the integrated current sensing device (U1) is a very useful and effective solution. This solution meets the following criteria:
1. Integrated sensing element (resistorless)
2. Dynamic range of current sensing greater than 4 decimal units.
3. Current output function (provides 0-1V V OUT with 160Ω LOAD, compatible with all ADC/microcontroller inputs for current output).
An integrated sensing unit replaces the external sensing resistor. Located between the VDD input and the load (LD) output, it measures system load current (ILOAD) from 100µA to 3.3A. An internal gain module with a gain of 1/500 provides the output current during ISH. A 160Ω resistor connects the ISH current output to GND, converting the VISH voltage output to 0V to 1V.
At a 3A load current, the voltage drop between VDD and LD on the sensing element is approximately 60mV, equivalent to a power consumption of only 180mW. At lower current values, the total error observed when sensing a range of 100μA is around 10%. This approach outperforms conventional sensing circuits due to its low power consumption at higher current loads and its ability to maintain an improved error budget at lower current levels. Therefore, applications requiring a wider current sensing range (up to 3A sensing) can benefit from this approach.
Resistanceless sensing solution with extended line/input voltage:
The input voltage range has been expanded, allowing the supply voltage of U1 to now accept higher line voltages, up to 6V to 36V. A Zener diode (D1) maintains the voltage between VDD and the gate of the PFET (M1) at 5.6V. Most of the high-voltage line is absorbed by M1, whose source is clamped to approximately 4V-4.5V from the VDD input voltage, thus maintaining the U1 operating voltage (VDD - VSS) within its normal operating range. This source voltage of M1 then biases the gate voltage of the M2 PFET. The source of the M2 PFET at VSS(U1) + VTH(M2) ensures that the U1 ISH output is within an acceptable voltage level range. The ISH current output and R1 produce a 0 to 1V output relative to GND.
