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Guarding, what is it? why do we need it? And what does it actually do?
In a typical in-circuit test setup,
probes and wires connect the device under test to the measurement circuit on the tester.
To understand better about guarding,
let's start with the example of a resistor test setup using a constant voltage source.
The resister is connected
between the voltage source (Vs) on the drive or "s" bus
and the Measurement Op Amp on the sense or "i" bus.
In a perfect situation, 100% of the current flowing thru the DUT (IDUT) will
flow thru the reference resistor RREF.
Since IREF = IDUT, If we measure Vout, we can find that
IDUT = VOUT/RREF. Using ohms law, the resistance can be calculated by Vs/IDUT.
However, when testing a board of components, the components on the board other than the
DUT will form
a resultant parallel impedance as represented by Zsg and Zig, in the diagram.
Because of this parallel path, IREF becomes a combination of the
current flowing through the DUT and the parallel path, as represented by IRX and IZIG in the
diagram.
This results in an error in the calculated value of the resistor. Something needs to
be done to eliminate
IZIG so that IRX becomes equal to IREF,
thus making the calculation of the resistance correct again. The method to use is called
Guarding.
To eliminate IZIG, we need to make the potential between
point G & point I on the diagram to be the same. We can achieve this by tying point G
to ground.
Since the negative input to the MOA is virtual ground, the positive input will also be.
So, if both point G and I are ground, there will be no potential difference between the
2 points and therefore, no current IZIG.
This method to tie a point within the circuit on the board temporarily to ground, is called
Guarding.
The temporary ground point is called the Guard Point.
This method is implemented during test debug.
The test engineer will find in the schematics points across adjacent components to the DUT
that he can
place Guard Points and define them in his test.
The tester will automatically place the relevent resources to ground that point during the
test.
With some testers like the Agilent i1000D, a constant current source can be used instead
of the constant voltage source for the resistance test.
This time, the resistor is connected across the constant current source (Is) at points
"A" and "B.
The current (IDUT) flowing through the resistor creates a potential difference (VR) measured
by the voltmeter.
In this way,
assuming Is = IDUT, the resistance can be
calculated by VR/IDUT.
Considering the other components on the board , they form a resultant parallel impedance
as represented by Za and Zb in the diagram.
Similarly, as in the case of the constant voltage source, there is
a leakage (Ia) from the source current into the parallel path,
so Is is not equal to IRX, so the measured resistance is not the same as the actual resistor.
To eliminate the leakage current Ia, a guard point is added at point G.
This time, the guard point connects a buffer between point A and point G.
This makes the potential at points A and G equal, so no potential difference exist
to drive current Ia,
therefore, no current Ia.
With Ia = 0, the correct resistance R can be measured with VR/Is.
These methods are how guarding helps to make your measurements more accurate during in-circuit
test.
For more information on in-circuit test, go to www.agilent.com/find/ict