New-Tech Europe | March 2016 | Digital edition

Figure 1, A 250uOhm current sense resistor is mounted with RG58 coax soldered directly to the resistor for measurement the test setup. The DMM measured value reflects an error of 4% from the specified value. The measurement error includes the error of the two DC meters and the 1 Ohm calibration resistor. The expected error in the measurement due to the 1 Ohm calibration is: The measurement of the 250uOhm resistor is near perfect. Of course there are other errors, including the tolerance of the calibration resistor, the calibration of the two DMM’s and the gain accuracy of the VNA. There is also a cable shield resistance error,

power supply are connected in parallel and both are in current limit for this measurement. 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 power supply are connected in parallel

and both are in current limit for this measurement. The calculated DC resistance value is:

341uV/1.4184A=240uOhms

The noise floor of the measurement using the J2102A common mode transformer and a preamplifier is shown in Figure 6 below. Such a sample test is important and recommended as it confirms the validity and limits of

Figure 3 Simulation results of the 2-port shunt thru impedance measurement including cable resistance and 2mΩ DUT with and without the common mode transformer

Figure 4 Impedance measurement of the 1mΩ resistor with and without the J2102A common mode coaxial transformer using the OMICRON Lab Bode 100.

Figure 2 Schematic of the 2-Port impedance measurement including cable resistance, a 2mOhm DUT and the common mode transformer

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