User Manual

12
Calibration for Quantitative Measurements on PCB tracks
The relationship between the output voltage seen on the scope and the actual current in the PCB track
depends on a complex relationship between the width of the track, the width of the sensor inside the
current probe, and the thickness of the probe insulation between the track and that sensor. To obtain
quantitative measurements the sensitivity adjustment on the control box must set to suit the particular
track width being measured. The wider the track, the lower the field strength for a given current, so the
greater the gain required to obtain a sensitivity of 1A/V. At track widths above about 3.5mm the system
must be adjusted for a sensitivity of 2A/V.
This relationship between track width and the gain required is incorporated into the calibration graph
below. The calibration procedure involves placing the probe into the calibration hole in the control box
and applying the calibration current. The sensitivity control is then adjusted to obtain the particular output
voltage (as measured by the deflection of the scope trace) to suit the intended track width. The calibrator
can produce either a square wave or DC calibration current. To avoid difficulties caused by the influence
of the earth's magnetic field and the local magnetic environment, calibration is normally performed using
the square wave signal obtained by setting the switch to the inwards (
) position. The signal is a square
wave at about 1kHz. The amplitude setting refers to the peak-to-peak amplitude between the flat parts of
the square wave, ignoring any overshoot on the transitions.
Detailed Procedure
First, decide on the width of the track for which a calibrated measurement is required. Then, using
the graph, look up the calibration deflection setting for that track width and set the scope to a suitable
sensitivity (1 Volt per division for narrow tracks, or 0.5 Volt per division for wider tracks).
I-prober 520 PCB gain setting
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3 4 5 6 7
Track Width mm
Deflection pk-pk
1 Amp/div 2 Amp/div