Data Sheet

Observe & measure currents in PCB tracks, captive component leads & ground planes
Current measurement techniques
True measurement of current requires the circuit to be broken and a current
measurement device inserted (e.g. a shunt that converts current to voltage).
However, breaking the circuit is impractical in many circumstances and, in the
case of PCB tracks, may be impossible.
Closed magnetic circuit current measurement
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.
If a conductor is surrounded by a closed magnetic circuit of high Mu material
the whole of the field is ‘captured’ by the magnetic circuit and the field density
can be scaled to represent current.
Conventional current probes achieve this by concentrating the field into a gap
within a loop of high Mu material. The field is then measured by a field sensor
inserted into the gap, often a Hall effect device.
Alternatively ac current can be
measured by transformer action
whereby the loop of magnetic
material creates a one turn
primary from the conductor that is
enclosed. Hybrid devices typically
use a field sensor for dc and low
frequencies plus a transformer for
higher frequencies.
Normally the probe provides a method
of mechanically splitting the magnetic
circuit to enable the conductor to be
inserted. The position of the conductor
within the loop has relatively little effect
upon the measurement .
PCB track current measurement
Measuring current in a PCB track presents particular difficulties because
it normally not possible either to break the track or to enclose it within a
magnetic circuit. Typically engineers have to guess at the current flowing in a
track from voltage measurements made in other parts of the circuit..
As electronic design moves towards ever higher densities, development omits
the “bread board” stage and goes straight to PCB design. The inability to
observe and measure currents in a circuit under development can pose a
serious problem for engineers.
Illustration shows high performance
current probes from Tektronix Inc.
Engineering a Solution
The only practical way to observe and measure the current in a PCB track is
by sensing the field in very close proximity to the track.
We refer to this type of current probe as a positional current probe .
To achieve a calibrated measurement, the field sensor must be capable of
maintaining a precise distance from the track. To achieve good sensitivity
this distance must be very small because field reduces with the square of
distance (to a first order approximation).
To create a practical current measurement probe, a very special type of
miniaturised sensor was needed. The requirements included very small size
with precision dimensions, dc sensing capability, wide ac bandwidth, and
low noise. None of the existing sensor technologies used within field and
current probes was suitable for this.
The Fluxgate Magnetometer
- updating an established principle
The I-prober 520 uses the well established principle of a fluxgate
magnetometer to measure field.
Conventional fluxgate magnetometers are relatively large with bandwidths
limited to a few kHz. They are typically used for precision measurement of
fields within geophysics and bio-electromagnetics.
By contrast, the sensor within the
I-prober 520 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.
The illustration shows the main part of
the magnetometer. The field sensing
element is of sub-millimetre dimensions
and is placed at the tip of the sensor.
Making PCB current measurement a reality
The concept of a positional
current probe is not entirely
new. However, previous
attempts have been physically
large and suitable only for
measuring high currents at low
bandwidth.
The high excitation frequency
miniature sensor within the
I-prober 520 provides it with
levels of positional accuracy,
sensitivity, bandwidth and
dynamic range that are superior
to anything previously achieved
by several orders of magnitude.
In consequence, the Aim I-prober 520 is the first and only probe
that can be used to measure currents from amps down to milliamps
at frequencies from DC up to MHz, making practical measurement
of PCB track currents a reality.
Modern high-density electronic PCB layouts make
current observation and measurement increasingly difficult