There are several important things to know about mixed-signal grounding!
“Most ADC, DAC, and other mixed-signal device data sheets discuss grounding for a single PCB, usually the manufacturer’s own evaluation board. Applying these principles to a multi-card or multi-ADC/DAC system can be confusing. It is generally recommended to separate the PCB ground plane into analog and digital layers, and connect the converter’s AGND and DGND pins together, and connect the analog and digital ground planes at the same point, as shown in Figure 1.
The source of confusion about mixed-signal grounding
Most ADC, DAC, and other mixed-signal device data sheets discuss grounding for a single PCB, usually the manufacturer’s own evaluation board. Applying these principles to a multi-card or multi-ADC/DAC system can be confusing. It is generally recommended to separate the PCB ground plane into analog and digital layers, and connect the converter’s AGND and DGND pins together, and connect the analog and digital ground planes at the same point, as shown in Figure 1.
Figure 1. Mixed-Signal IC Grounding: Single PCB (Typical Evaluation/Test Board)
This essentially creates a system “star” ground on mixed-signal devices. All noisy digital currents flow through the digital power supply to the digital ground plane and back to the digital power supply; isolated from the sensitive analog parts of the board. System star ground structures occur in mixed-signal devices where the analog and digital ground planes are connected together.
This method is generally used for simple systems with a single PCB and a single ADC/DAC, and is not suitable for multi-card mixed-signal systems. In systems with several ADCs or DACs on different PCBs (even on the same PCB), the analog and digital ground planes are connected at multiple points, making it possible to create ground loops, whereas single-point “star” ground systems impossible. For the above reasons, this grounding method is not suitable for multi-card systems and should be used for mixed-signal ICs with low digital currents.
Grounding and Decoupling for Mixed-Signal ICs with Low Digital Currents
Sensitive analog components, such as amplifiers and references, must be referenced and decoupled to the analog ground plane. ADCs and DACs (and other mixed-signal ICs) with low digital currents should generally be considered analog components, also grounded and decoupled to the analog ground plane. At first glance, this requirement may seem contradictory, since converters have analog and digital interfaces, and often have pins designated as analog ground (AGND) and digital ground (DGND). Figure 2 helps explain this dilemma.
Figure 2. Proper Grounding for Mixed-Signal ICs with Low Internal Digital Currents
Inside ICs that have both analog and digital circuits, such as ADCs or DACs, grounds are usually kept separate to avoid coupling digital signals into the analog circuits. Figure 2 shows a simple converter model. Connecting die pads to package pins inevitably creates wire bond inductance and resistance, which IC designers can’t do anything about, just be aware of it. The rapidly changing digital current produces a voltage at point B and is bound to couple to point A of the analog circuit through the stray capacitance CSTRAY. In addition, the IC package has about 0.2 pF of stray capacitance between each pair of adjacent pins, which is also unavoidable! The IC designer’s task is to rule out this effect and make the chip work properly.
However, to prevent further coupling, AGND and DGND should be connected together externally with the shortest possible trace and to the analog ground plane. Any additional impedance within the DGND connection will generate more digital noise at point B; in turn, more digital noise will be coupled into the analog circuitry through stray capacitance. Note that connecting DGND to the digital ground plane imposes VNOISE across the AGND and DGND pins, causing serious problems!
The “DGND” designation indicates that this pin is connected to the IC’s digital ground, but it does not mean that this pin must be connected to the system’s digital ground. It can be more accurately called the internal “digital loop” of the IC.
This arrangement does potentially introduce a small amount of digital noise to the analog ground plane, but these currents are very small and can be minimized by ensuring that the converter output does not drive a large fanout (which is usually not the case). Minimizing fanout on the converter’s digital ports (which also means lower current) also makes the converter’s logic transition waveform less susceptible to ringing and minimizes digital switching currents, which in turn reduces the amount of power to the converter’s analog ports. coupling. By inserting a small lossy ferrite bead, as shown in Figure 2, the logic power pin (VD) can be further isolated from the analog power supply. The converter’s internal transient digital current will flow in a small loop from VD through the decoupling capacitor to DGND (this path is indicated by the red line in the diagram). Therefore, transient digital currents do not appear on the external analog ground plane, but are confined within the loop. The VD pin decoupling capacitor should be mounted as close as possible to the converter to minimize parasitic inductance. Decoupling capacitors should be low inductance ceramic types, typically between 0.01 μF (10 nF) and 0.1 μF (100 nF).
Again, no one grounding scheme is suitable for all applications. However, problems can be minimized by understanding the options and making the rules ahead of time.