Analog to Digital Conversion in PCB Design

Analog (A/D) converters have evolved from analog-centric origins to emphasize digital components in low-speed A/D converters. Despite this shift, PCB wiring criteria remain crucial for effective design.

Successive Approximation A/D Converters

• Available in resolutions of 8-bit to 18-bit
• Transitioned from bipolar to CMOS processes
• Routing strategies remain consistent

Successive approximation A/D converters utilize capacitive charge distribution topologies, with most energy consumed by internal analog circuits.

PCB Wiring Recommendations

• Connect AGND and DGND to analog ground plane
• Link analog and digital power pins to analog power plane
• Use bypass capacitance near power pins
• Isolate digital noise for higher resolution converters
• Employ external buffers for noiseless operation

For high-resolution successive approximation A/D converters, power supply and ground connections should be made to the analog plane, with the digital output buffered to isolate analog and digital sides effectively.

Σ-Δ Type A/D Converters

Emphasize digital silicon area, requiring careful cabling strategies to maintain signal integrity.

PCB Converter Best Practices

• In the past, PCB users were advised to use PCB planes to separate digital noise from analog noise.
• Modern A/D converters, like successive approximation types, come with multiple analog, digital, and power pins.
• Design engineers typically prefer segregating these pins onto different planes.
• However, separating pins may not always be the best solution, especially for 16-bit to 24-bit devices facing significant noise challenges.
• For high-resolution Σ-Δ A/D converters with a 10Hz data rate, the clock frequency can reach up to 10MHz or 20MHz.
• This clock signal drives the modulator and oversampling engine.
• In these setups, AGND and DGND pins are connected on the same ground plane, similar to successive approximation A/D converters.
• Analog and digital power pins are also linked on the same plane.
• The power plane requirements for analog and digital signals align with those of high-resolution successive approximation A/D converters.
• A well-designed floor plan is crucial and should span across at least two panels, covering 75% of the total area.
• The ground plane layer aids in reducing grounding impedance, inductive reactance, and shielding against EMI and RFI.
• If internal wiring is unavoidable on the ground plane, it should be kept short and perpendicular to the ground current loop.
• For lower-resolution A/D converters like six-bit or eight-bit devices, keeping analog and digital pins together is acceptable.
• However, as converter choices and resolutions increase, wiring guidelines become more stringent.
• High-resolution successive approximation and Σ-Δ A/D converters should be directly connected to low-noise analog ground and the power plane.