Current Monitoring Methods in Mixed-Signal PCBs

What do medical devices, sensors, and power electronics have in common? They all use current sensing, requiring a safe, precision approach to current measurements. Multiple design types and topologies can be used for this task, offering continuous monitoring and threshold detection solutions, or a combination of both. From current sense ICs to custom amplifier designs, there are many options for current sensing circuits, but not all in a sufficiently small footprint.

This article will cover some of the options for current sensing, as well as their advantages and disadvantages. Types range from standalone components with very little programmability to precision amplifier circuits and continuous monitoring solutions. Some component types, such as mixed signal processors, offer a greater level of programmability than you'll find with many standalone ICs.

Instrumentation Amplifiers

The most common way to perform a current sensing measurement is to convert the current to a voltage and amplify the voltage value. The output would then be read into an ADC and passed to an MCU, and the MCU would recover the original current value using a current sense resistor value and the instrumentation amplifier gain.

A block diagram and a calculation formula for the current is shown below. This type of design is suitable for monitoring large currents continuously as long as the current sense resistor value is low enough.

Basic instrumentation amplifier (part number: INA826)

These circuits are, at their most basic level, a set of three op-amps and some resistors, with the resistor values determining the feedback and gain provided to the output voltage.

These solutions are simple to design but they involve multiple ICs and therefore do not have a very small footprint. Some development effort is also required in the embedded application so that the ADC can be configured and messages can be received over a serial interface. In some current sense applications, current being sensed may be low frequency or DC, so proper segmentation among power domains and the digital/analog sections is required.

Current Sense Amplifiers

These components are more highly integrated than a set of instrumentation amplifier and ADC chips. They are specialized for current sensing and power electronics where the measurement is also taken across a shunt resistor. From a technical perspective, they offer certain advantages compared to an instrumentation amplifier approach:

• Capability of sensing broad voltage ranges (rail-to-rail)
• High common mode voltage/current withstand on the inputs
• High current sensing capability allows detection of shorts to ground
• High bandwidth allows for fast detection of changes in the measured current

These devices are essentially performing the same functions as an instrumentation amplifier by converting the current in a shunt resistor to a voltage drop. However, these components might have an I2C output that reports the output directly rather than requiring an ADC. Some devices marketed as current sense amplifiers are only cascaded op amp circuits which will require an external ADC.

ISL28006 used in a multi-output power supply. This component has adjustable feedback capability through an external voltage divider.

Because some current sense amplifiers have an integrated ADC, they may require an external clock or reference, which continues adding to the overall size of the circuitry. Finally, for high currents, physically large current sense resistors may be needed to measure high currents. In this final case, a different approach will be needed.

Current Transformers

For high current applications where there is risk of safety or damage to components, current transformers can be used to step down the current that needs to be measured to a safe level. These transformers have fixed ratios, such as 100:1 or 1000:1, which allows these to be used for high current measurements. Once the current is stepped down, the smaller current value can be measured with another method, and the step-down ratio needs to be accounted for in the final calculation of the current.

As these are magnetic components, they can be physically quite large. Typically these are through-hole components, or they may require a clip or flange to mount to the PCB. Sometimes, they are simply wire-wound components that will need external mounting, and the free ends of the wires will solder onto the PCB. An example is shown below.

The above current transformer works by wrapping a wire carrying the high current around the transformer core. The pins on the core then mount to the PCB and are connected across a current sense resistor. The current in the current sensor can then be measured as a voltage drop as described elsewhere in this article.

Current Mirror Circuits

Current mirrors are used to reproduce or scale a reference current using matched transistor pairs (BJTs or MOSFETs). One device sets the reference (the current we want to measure), and the other forces its current to track it. Accuracy relies on device matching, bias, and layout symmetry. Designers must also ensure sufficient voltage headroom to keep both devices in the active region.

These circuits rely on the predictable relationship between gate/base voltage and drain/collector current for matched devices operating in the active/saturation region. Three major factors impact the measurement accuracy of these circuits:

• Mismatched devices: Causes a static error in the mirrored current, often due to variations in V(TH) (in MOSFETs) or V(BE) (in BJTs).
• Output Impedance: Ideally high; limited by the Early effect in BJTs or channel-length modulation in MOSFETs, typically requiring cascode structures.
• Voltage headroom: The minimum voltage required across the output transistor to keep it out of saturation. Critical in low-voltage integrated circuits.

Current mirrors are also sensitive to layout parasitics and temperature gradients, which can introduce mismatch between the reference and mirrored devices. In IC design, close physical placement and common-centroid layouts reduce these errors, but on PCBs these techniques are impractical. In a PCB layout, symmetrical routing is needed and the layout should not allow thermal hotspots near only one of the devices in the current mirror circuit.

Hall Effect Sensors

Hall effect sensors provide a method for measuring current by detecting the magnetic field generated by a current-carrying conductor (operating at DC). These devices can be fully integrated packages with an internal aperture or discrete Hall elements paired with external signal-conditioning (e.g., filtering) circuits. In either form, the conductor must be routed close to or through the sensor so that the magnetic field can be used to generate a Hall voltage.

Because the measurement path is magnetically rather than electrically coupled, Hall sensors avoid common-mode limitations and eliminate the need for a shunt current sense resistor. However, the accuracy of these circuits is strongly dependent on geometry, nearby magnetic materials, and exposure to external fields. In particular, proximity to high-dI/dt circuits will determine whether the measurement remains linear or whether there is AC noise injected into the measurement.

Although many Hall sensors integrate filtering and compensation, other sources of error often require system-level design decisions. This may include shielding against stray fields, adding external filtering, or allocating calibration steps to account for external fields. Still, Hall-based current measurements can be reliable and safe due to the fact that the sensor circuit is not directly exposed to the current being measured.

Mixed-Signal Matrix Processors

A mixed signal matrix can also be used for current measurements by building the analog front end into the processor. In the current sense resistor-based implementation, this also involves the use of a precision current sense resistor connected across two IOs on the chip. This will allow the voltage across the resistor to be sensed and processed by the internal circuitry.

The designer then needs to build that voltage measurement interface into the chip, either as a threshold-based detection or as continuous measurement (or a combination of both). The basic block diagram for sense resistor-based current measurements or monitoring is shown below.

• Threshold detection generally uses comparators or window comparators
• Continuous monitoring generally uses an ADC to track the current

In both approaches, the references in the chip are fully programmable, and the output is passed over an analog GPIO or I2C. For threshold-based detection, the GPIO output is normally used so that the current indicator can be used as a logic input to a system host or another IC. In continuous monitoring, the shunt voltage measurement value is streamed to a system host over a serial interface, or it can be used in the internal digital logic in the mixed-signal processor.

Current Monitoring With GreenPAK

The GreenPAK line of mixed-signal processors from Renesas brings CPLD-type functionality to mixed-signal devices, allowing designers to develop an analog front-end alongside custom logic processing on a single chip. Designers have the ability to reprogram settings in the analog front-end on the fly. In addition, programmable logic cells allow for implementation of a wide variety of digital logic functions.

• Real-time programmability of reference voltages and hysteresis windows
• Standard digital logic functions (clocks, LUTs, flip-flops, gates, etc.)
• Mixed-signal I/Os in a customizable pinout

The developer tools in Renesas GreenPAK give designers the ability to develop fully custom digital, analog, or mixed signal ICs. These programmable mixed-signal processors allow consolidation of functions found in clock and signal management circuitry, allowing for smaller, more efficient systems. To learn more, take a look at the GreenPAK components and reference examples.

• Learn more about GreenPAK components
• View dual-supply GreenPAK solutions

Whether you need to build reliable power electronics or advanced digital systems, use the complete set of PCB design features and world-class CAD tools offered by Altium to implement your GreenPAK solutions. Altium provides the world’s premier electronic product development platform, complete with the industry’s best PCB design tools and cross-disciplinary collaboration features for advanced design teams. Explore Altium solutions!