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his
article
discusses
measurement
of
small
resistances, such as those encountered
during PDN measurement, using the
OMICRON Lab Bode 100, Keysight
E5061B, and the Picotest J2102A
common mode transformer.
The resistor being measured is a
250uOhm 1% resistor manufactured
by Stackpole. The datasheet is
not clear about how the device
measurement is performed, though
the resistor element is specified to be
within 1% or 2.5uOhms.
Both the OMICRON Lab Bode 100
and the Agilent E5061B are capable
displaying impedance in the 2-port
measurement. They support the
OPEN, SHORT and LOAD calibration
in the 2-port series and shunt
measurements for improved accuracy.
A common mode transformer is
necessary for all measurements
where the ground braid resistance
is significant, as shown in Figure
2, unless the VNA has a differential
or semi-floating 50Ω input. The
Keysight E5061B is unique in that its
low frequency gain-phase ports have
semi-floating inputs and do not need
the transformer. The higher frequency
Port 1/2 inputs, as well as the Bode
100 and most other VNAs for that
matter, do require the transformer
in order to make the 2-port shunt
measurement. The J2102A gets well
above 100MHz, which is sufficient
for general PDN use. Above that
frequency the transformer is not really
impactful.
By way of example, the simulation
result of the 2mOhm case is shown in
Figure 3, while sample measurements
of 1 and 2 mOhm parts are shown in
Figures 4 and 5, with and without the
J2102A common mode transformer
inserted. In order to quantify the
significance, the approximate error is
the parallel ground braid resistance
divided by the DUT. For instance, a
5% error would occur at a DUT value
of 1Ω as the parallel ground resistance
is 50mΩ or 5% of 1Ω.
Moving on to our 250uOhm
measurement, a DC measurement is
made using precision, low noise 6.5
Digit DMM, as a point of reference.
Figure 5 A DC measurement is made
using precision, low noise 6.5 Digit
DMMs (Picotest M3500A) to obtain a
second measurement for correlation.
The power supply does not have
current programming, which is the
reason the current is set to an odd
number. The two channels of the
T
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