The Nodal impedance analysis is a method which can be used to troubleshoot those hard to locate PCB faults down to component level without having any detailed circuit knowledge and without applying any power to the circuit board.
There are a wide variety of equipment and techniques to locate faults. Sometimes faults are just too time-consuming to locate and sometimes we may simply discard some PCBs as “uneconomical to repair”. Conventional test equipment such as logic analysers and oscilloscopes can only be used on powered boards which requires skill and circuit knowledge and would still be unlikely to uncover, say, a zener diode with a soft knee. This sort of problem can be easily uncovered by probing a circuit node and displaying its dynamic impedance.
Nodal Analysis:
If a potential difference is applied across a conductor the resultant current will depend on the voltage applied across the conductor and its electrical characteristics. This voltage-current plot (the dynamic impedance of the component) is distinct for each component and can quickly identify the type of a component. For this reason the V/I characteristics are often referred to as impedance signatures. Fault finding by inspecting or comparing impedance signatures is known as nodal impedance.
Nodal analysis is possible when all the circuit elements’ branch constitutive relations have an admittance representation. Nodal analysis produces a compact set of equations for the network, which can be solved by hand if small, or can be quickly solved using linear algebra by computer. Because of the compact system of equations, many circuit simulation programs use nodal analysis as a basis. When elements do not have admittance representations, a more general extension of nodal analysis, modified nodal analysis, can be used.
Locating faults with nodal impedance analysis:
Using nodal impedance analysis pcbs are always tested with no power applied so nodal impedance analysis is absolutely safe and components cannot be damaged. The Fault Locator applies a small sinusoidal voltage across the components and the resulting voltage-current graph is displayed as an x-y plot on a CRT or PC screen. In many cases fault finding is reduced to simple pattern recognition.
For example the signature for a pure resistance is a straight line, so a change of slope from that of a device on a known good board indicates an obvious wrong value resistance. A capacitor exhibits an elliptical signature whose shape depends on the value of the capacitor.
A wrong value capacitance would alter the roundness of the ellipse, leakage would cause the ellipse to appear tilted – both elusive faults, but easily detected using nodal impedance analysis.
Comparison techniques:
The most effective method of trouble-shooting will be to simultaneously display good and faulty signatures where good boards are available. Some Fault Locators allow you to “learn” a set of PCB’s signatures and send them to service centres – you can even store signatures for components from each of your device vendors.
Testing digital devices with nodal impedance analysis
Even when you test a digital IC using nodal impedance analysis you are actually displaying the analog behaviour of the input protection circuitry. Many failures in digital devices in service are due to damage in the input/output region of the device (e.g. caused by lightning strikes, etc. in telecommunication equipment).
| Damage to the I/O region of a digital device will be easily revealed using nodal impedance analysis. Consider the typical digital ic input protection circuit shown below. |
The positive and negative excursions of the Fault Locator drive voltage will cause both diodes to conduct so the signature will appear as below.
| Suppose a large transient severely damages the protection diodes so that the circuit appears as a simple resistor; the signature is dramatically altered and very easy to see – the device is an obvious candidate for replacement. |
Nodal impedance analysis is a widely used fault finding technique, independent of the PCB technology employed. It’s safe, simple to use and highly effective.
