Milli Ohm Measurements - Know how to Use Milli-ohm Meters Correctly

 

Processor power distribution network plays an important role in the proper working of a processor. A PDN is designed based on the impedance versus frequency characteristics. PDN design starts with the assessment of target impedance which is the ratio of maximum change is allowed voltage to the maximum change in current. The maximum change in current is usually taken as half the input current to a processor. To avoid ringing on the PDN, it is important to design the PDN as flat as possible. This reduces the radiated EMI issues. This also helps to improve the eye diagram for a transceiver chip. The most important issue in designing in the PDN is in measuring the PDN impedance.  Current industry standard chips are designed at the milli ohm impedance levels, and measuring impedance at these levels are a challenge.  High end processors are designed with micro ohm impedance levels. Another challenge is that they have to measured at the highest frequency of interest – usually many megahertz.
 
Vector network analysers (VNAs) are the main component used to measure ultra low impedances due to their superior sensitivities. VNA has sensitivities in the order of microvolts, and utilising two port shunt through measurement methods we can measure milli ohms and micro ohms. The two port shunt through measurements are an adaptation of four wire kelvin measurements used for DC resistance measurements. A two port shunt through measurement comes at the cost of ground loops, which can be remedied by a high quality common mode choke or a high quality differential amplifier.