Mark Edwards, Sales Director at Aim-TTi explores how a new solution of miniaturised field sensors enables measurement of PCB track currents – A new type of current probe has been introduced by Aim-TTi, which is capable of directly observing and measuring current in a PCB track simply by placing the insulated tip onto it.
Known as a ‘positional’ current probe, the Aim I-prober 520 has a bandwidth of DC to 5MHz and a dynamic range of 10mA to 20A.
Conventional dc capable current probes do not measure current, they measure field density. Current flowing through a conductor creates an H field, which is directly proportional to the current range.
If a conductor is surrounded by a closed magnetic circuit the whole of the field is ‘captured’ by the magnetic circuit and the field density can be scaled to represent current. The problem with a PCB track is that there is no way of surrounding it.
To make a quantitative measurement of current from the field that it generates requires that a known proportion of that field is measured.
Conventional current probes achieve this by concentrating the whole of the field within a loop of high Mu material, and measuring using transformer action or a field sensor within a gap.
By contrast, the positional current probe, measures a known proportion of the field by positioning the sensor at a known distance from the conductor. This has the potential to provide a calibrated measurement of current.
However, to be useable with a PCB track, the size of the sensor must be very small as must its distance from the conductor.
Previous positional current probes have been physically large, capable only of measuring high currents at low bandwidth, and totally unsuited for use in the high densities of modern electronic circuits.
By contrast, the sensor within the new instrument uses a patented miniature fluxgate magnetometer of sub-millimetre size incorporating a highly advanced core material.
This enables it to use an excitation frequency of several tens of MHz resulting in a sensor with a bandwidth of dc to 5MHz combined with low noise and wide dynamic range. This miniature sensor is fitted within a double insulated probe tip, which is just 1.8mm wide with the sensor spaced 0.7mm from the surface of the tip.
Because the field reduces with the square of the distance (to a first order approximation), this spacing is critical to the operation of the probe giving it both high sensitivity and reduced susceptibility to fields from any adjacent conductors.
In consequence, the prober is one of the first probers that can be used to measure currents from amps down to milliamps at frequencies from dc up to 5MHz, making practical measurement of PCB track currents a reality.
The magnitude of the signal from a positional current probe is critically related to its position relative to the conductor. The size of the conductor (e.g. the width of a PCB track) also has a significant effect.
This means that the sensitivity of the prober has to be adjusted to match the track width when quantitative measurements are required. A calibrator within the control box enables sensitivity adjustment in conjunction with a calibration graph. The output from the control box is scaled to one volt per amp and is intended for connection to a conventional oscilloscope.
The measurement result will also include other field effects present at the tip of the probe and not just that coming from the current through the conductor. This may include DC effects from adjacent magnetised components and from the earths magnetic field, plus AC effects from transformers and other field radiating sources. Current in adjacent tracks, or tracks on the opposite side of the PCB will also affect the measurement.
Consequently, the use of the prober requires interpretation based upon a proper understanding of circuits and systems, and is a tool for the professional engineer.
The probe bandwidth of DC to 5MHz encompasses a wide range of applications including most power switching circuits. The maximum current that can be measured is 20A pk-pk in a track of up to 3.5mm, or 40A pk-pk in a wider track. The minimum visible current is limited by the noise figure which, for a 0.5mm track, is equivalent to around 6mA rms at full bandwidth reducing to 1.5mA at lower bandwidths. In practise this enables ac signals as low as 10mA pk-pk to be usefully observed.
It should be understood, however, that accurate measurement of very small dc currents is likely to be impracticable in most situations. The effect of the earths magnetic field is equivalent to up to 180mA flowing through a 0.5mm track.
Whereas this can be nulled out within the control box, the effects of small changes in orientation can cause variations that would invalidate mA level measurements. Similarly the effect of magnetised components and high Mu materials close to the probe can cause significant offsets.
