Generally used to describe transient noise coupling between digital and analog circuits, “Substrate Noise” is a widely misused term in mixed signal IC design. This term frequently gets used when the noise coupling paths are not readily determined. In reality, this is usually the combination of multiple things:
Electromagnetic Coupling – Coupling paths provided through magnetic fields associated with transient currents. This is very common between bonding wires, the pins in a package/lead-frame assembly, and metal traces with high transient currents.
Capacitive Coupling – Coupling paths between conductors created by parasitic capacitance. Common between metal connections, where a digital signal and it’s high frequency dv/dt content allows low impedance connections with minimal amounts of capacitance.
Bias Distribution Coupling – Rather than a particular method of coupling, this is a common path of noise introduction that can cause problems. Use of reference voltages (or currents) and their modulation by any coupling method, leads to problems.
Power Supply Instability – Variation in supply voltage due to current transients in the power supply interconnect. Current transients create voltage transients across the power connection and its inherent series impedance.
Ground Instability, a.k.a. “Ground Bounce” – Similar to power supply stability, but generally, interconnects associate with grounds are more tightly coupled to the substrate, than power connection are. Ground instability introduces its own problems with functionality and can also induce transient currents in the substrate through both resistive and capacitive charge/discharge paths.
Induced Substrate Currents – Current forced through the bulk, causing voltage variance between locations on the substrate.
In order to deal with these problems, a systematic approach to noise problems is needed. Please see our IC Help section, or consider our Show Stoppers training program.