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With a precision design, it is critical to have good circuit design and high-performance components. A precision system demands attention to detail, but the best system performance requires a good foundation at the PCB level. As you finish your circuit design concept, the next task is to evaluate phantom PCB resistances, inductors, and capacitors.

The PCB doesn’t show up on a schematic, however, it has very complex characteristics. In particular, the traces have resistance, inductance, and capacitance. Phantom voltage losses and dividers can be created with the PCB trace resistance.

The definition of precision depends on your signal range. Given a 16-bit system, a ±10V signal range gives a least significant bit (LSB) of 305 µV. A 5V signal only has a 76.2 µV LSB size. If a 24-bit resolution is needed, the LSB becomes 298 nV on the same 5V range. When adding gain to a system, the system LSB size can sink into the noise. For instance, consider a system with a sensor output of ±20 mV. If the system has a 16-bit converter with an analog gain of 256 V/V or a 24-bit converter with a 20 mV reference, the required noise level must be less than 2 nV.

With requirements to keep errors down in the nanovolt range, there are many enemies to your desired precision. We’ll start to investigate these different enemies in terms of the hidden PCB resistances, inductances, and capacitance. Today let’s look at PCB resistances. Understanding how these phantoms come about and how to minimize them is key to helping you optimize your PCB layout signal chain.

All conductors, except superconductors, have finite DC resistance. Depending on the conductor type and its orientation (Figure 1), this resistance can be calculated by using the following equations.