Tips about PCB design: Part 2 - Single-card grounding for multicard systems
Most ADC, DAC, and other mixed-signal device data sheets discuss grounding relative to a single PCB, usually the manufacturer’s own evaluation board. This has been a source of confusion in trying to apply the principles described here to multicard or multi-ADC/DAC systems.
The recommendation is usually to split the PCB ground plane into an analog plane and a digital plane. It is then further recommended that the AGND and DGND pins of a converter be tied together and that the analog ground plane and digital ground planes be connected at that same point as shown in Figure C.14 below. This essentially creates the system “star” ground at the mixed-signal device.
Figure C.14: Grounding mixed-signal ICs: single PC board (typical evaluation/test board).
All noisy digital currents flow through the digital power supply to the digital ground plane and back to the digital supply; they are isolated from the sensitive analog portion of the board. The system star ground occurs where the analog and digital ground planes are joined together at the mixed-signal device.
This approach will generally work in a simple system with a single PCB and single ADC/DAC, but it is not optimum for multicard mixed-signal systems. In systems having several ADCs or DACs on different PCBs (or on the same PCB, for that matter), the analog and digital ground planes become connected at several points, creating the possibility of ground loops and making a single-point “star” ground system impossible.
For these reasons, this grounding approach is not recommended for multicard systems, and the approach previously discussed should be used for mixed-signal ICs with low digital currents.
Grounding Mixed-Signal Low Digital Currents
Figure C.15 below summarizes the approach previously described for grounding a mixed-signal device which has low digital currents. The analog ground plane is not corrupted because the small digital transient currents flow in the small loop between VD, the decoupling capacitor, and DGND (shown as a heavy line).
Figure C.15: Grounding mixed-signal ICs with low internal digital currents: multiple PC boards.
The mixed-signal device is for all intents and purposes treated as an analog component. The noise VN between the ground planes reduces the noise margin at the digital interface but is generally not harmful if kept less than 300 mV by using a low-impedance digital ground plane all the way back to the system star ground.
However, mixed-signal devices such as sigma-delta ADCs, codecs, and DSPs with on-chip analog functions are becoming more and more digitally intensive. Along with the additional digital circuitry come larger digital currents and noise. For example, a sigma-delta ADC or DAC contains a complex digital filter which adds considerably to the digital current in the device.
The method previously discussed depends on the decoupling capacitor between VD and DGND to keep the digital transient currents isolated in a small loop. However, if the digital currents are significant enough and have components at DC or low frequencies, the decoupling capacitor may have to be so large that it is impractical.
Any digital current that fl ows outside the loop between VD and DGND must flow through the analog ground plane. This may degrade performance, especially in high-resolution systems.
It is difficult to predict what level of digital current flowing into the analog ground plane will become unacceptable in a system. All we can do at this point is to suggest an alternative grounding method which may yield better performance.
Grounding Mixed-Signal Devices with High Digital Currents
An alternative grounding method for a mixed-signal device with high levels of digital currents is shown in Figure C.16 below. The AGND of the mixed-signal device is connected to the analog ground plane, and the DGND of the device is connected to the digital ground plane.
The digital currents are isolated from the analog ground plane, but the noise between the two ground planes is applied directly between the AGND and DGND pins of the device. For this method to be successful, the analog and digital circuits within the mixed signal device must be well isolated. The noise between AGND and DGND pins must not be large enough to reduce internal noise margins or cause corruption of the internal analog circuits.
Figure C.16: Grounding alternative for mixed-signal ICs with high digital currents: multiple PC boards
Figure C.16 shows optional Schottky diodes (back-to-back) or a ferrite bead connecting the analog and digital ground planes. The Schottky diodes prevent large DC voltages or low-fre-quency voltage spikes from developing across the two planes.
These voltages can potentially damage the mixed-signal IC if they exceed 300 mV because they appear directly between the AGND and DGND pins. As an alternative to the back-to-back Schottky diodes, a ferrite bead provides a DC connection between the two planes but isolates them at frequencies above a few MHz where the ferrite bead becomes resistive.
This protects the IC from DC voltages between AGND and DGND, but the DC connection provided by the ferrite bead can introduce unwanted DC ground loops and might not be suitable for high-resolution systems.
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