TTI i-Prober 520 Manual De Instrucciones página 11

Insert the probe into the calibration recess and turn on the calibration signal by setting the CALIBRATOR
switch to the AC position (marked
obtain the maximum signal amplitude on the scope, then adjust the Trace Position control to centre the
trace on the screen.
Adjust the PCB Sensitivity control so that the peak-to-peak voltage of the scope trace (ignoring
overshoot) equals the value obtained from the calibration graph for the required track width. The probe
sensitivity is now set to give either 1A/V or 2A/V (depending on which calibration curve is used) for
measurements on the actual PCB track.
Switch off the calibration signal by returning the CALIBRATOR switch to its centre position.
For tracks wider than 6.5mm, it can be assumed that the reading is inversely proportional to the track
width plus 2.2mm. Alternatively, switch to Magnetic Field mode and measure the field in Amps per metre.
This can be converted to Amps, by assuming that the path length is twice the width of the track plus
4.4mm (this figure accounts for the fact that the sensor is 0.7mm above the track surface). This
relationship assumes that the track is long in relation to its width and reasonably uniform in layout.
Practical Aspects of Quantitative Measurements
To accurately measure the current in a track the probe must be
precisely positioned vertically in two dimensions, placed exactly
above the centre of the track and aligned with the long dimension of
the tip at right angles to the track.
The diagram shows the proper orientation. If the probe is not aligned
at right angles across the track (yaw), the output varies according to a
sine law, so small errors are tolerable; similarly, rotating the probe
forwards or backwards around its rounded tip (pitch) does not cause
major errors. The most critical position requirements are the centring
over the track, and being vertical side to side (roll), so that the probe
is flat on the track, not canted up on the corner of its tip.
Avoid the temptation to manipulate the probe tip with the
fingers, unless it is certain that there are no dangerous
voltages present.
In the absence of uneven extraneous fields the correct position is the one giving the greatest output but it
will be found that, in an environment containing many magnetic materials, the localised magnetic field can
vary substantially in both magnitude and direction from the earth's North-South field that exists in free
space. As a result, even small movements in the position of the probe can have a significant effect on the
residual output voltage. The best way to minimise the effect of this is to place the probe in the required
location on the track to be measured, switch the current off and set the zero position, then switch the
current on and note its magnitude. Many of these difficulties can be avoided if an AC measurement is
possible.
When measuring small currents (small in relation to the effect of the magnetic field of the local
environment) it is helpful if the probe can be held fixed in space (by using a retort stand or similar device)
and moving the signal being tested under the fixed probe.
Note that the calibration procedure given above only gives accurate results when the measurement is on
an isolated track some distance from any other current. Adjacent tracks carrying currents, including those
on the other side of the PCB, will have a significant effect on the measurement. Obtaining a quantitative
result in such circumstances requires mathematical analysis from first principles.
). Optimise the orientation of the probe within the calibrator recess to
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