Several important knowledge points you need to know about mixed signal grounding
Time:2022-08-11
Views:1878
1. Source of confusion of mixed signal grounding
Most ADC, DAC, and other mixed signal device data manuals discuss grounding for a single PCB, usually the manufacturer‘s own evaluation board. When these principles are applied to multi card or multi ADC / DAC systems, people will feel confused. It is generally recommended to divide the PCB ground layer into analog layer and digital layer, connect the agnd and DGND pins of the converter together, and connect the analog ground layer and digital ground layer at the same point, as shown in Figure 1:
Figure 1. Mixed signal IC grounding: single PCB (typical evaluation / test board)
Thus, the system "Star" grounding is basically generated on the mixed signal device. All high noise digital currents flow into the digital ground layer through the digital power supply and then return to the digital power supply; Isolated from the sensitive analog part of the circuit board. The star grounding structure of the system appears at the position where the analog and digital grounding layers are connected in the mixed signal device.
This method is generally used in a simple system with a single PCB and a single ADC / DAC, and is not suitable for a multi card mixed signal system. In a system with several ADCs or DACs on different PCBs (even on the same PCB), the analog and digital grounding layers are connected at multiple points, making it possible to establish a grounding loop, while a single point "Star" grounding system is impossible.
In view of the above reasons, this grounding method is not applicable to a multi card system, and the above method should be applied to a mixed signal IC having a low digital current.
2. Grounding and decoupling of mixed signal IC with low digital current
Sensitive analog components, such as amplifiers and reference voltage sources, 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 as analog elements, similarly grounded and decoupled to the analog ground plane. At first glance, this requirement may seem contradictory, because converters have analog and digital interfaces and usually have pins designated as analog ground (agnd) and digital ground (DGND). Figure 2 helps explain this dilemma:
Figure 2. Proper grounding of mixed signal IC with low internal digital current
In an IC (e.g., ADC or DAC) with both analog and digital circuits, the ground is usually kept independent to avoid coupling the digital signal into the analog circuit.
Figure 2 shows a simple converter model. Connecting the chip pad to the package pin inevitably leads to wire bonding inductance and resistance. IC designers can do nothing about this. Just be clear. The rapidly changing digital current generates a voltage at point B and is bound to be coupled to point a of the analog circuit through the stray capacitor cstray. In addition, each pair of adjacent pins of the IC package has a stray capacitance of about 0.2pf, which is also unavoidable! The task of IC designers is to eliminate this effect and make the chip work normally.
However, in order to prevent further coupling, agnd and DGND should be externally connected by the shortest lead and connected to the analog ground plane. Any additional impedance within the DGND connection will generate more digital noise at point B; And then more digital noise is coupled to the analog circuit through the stray capacitance. Please note that connecting the DGND to the digital ground plane will apply vnoise at both ends of the agnd and DGND pins, causing serious problems!
The name "DGND" indicates that this pin is connected to the digital ground of the IC, but it does not mean that this pin must be connected to the digital ground of the system. It can be more accurately called the internal "digital circuit" of the IC.
This arrangement may indeed bring a small amount of digital noise to the analog ground plane, but these currents are very small and can be minimized as long as the converter output does not drive a large fan out (which is not usually designed in this way). The fan out on the digital port of the converter is minimized (which also means that the current is lower), and the logic conversion waveform of the converter is less affected by ringing, so as to reduce the digital switching current as much as possible, thereby reducing the coupling to the analog port of the converter.
By inserting small lossy ferrite beads, as shown in Fig. 2, the logic power pin pin (VD) can be further isolated from the analog power supply. The internal transient digital current of the converter will flow in the small loop from VD to DGND through the decoupling capacitor (this path is indicated by the red line in the figure). Therefore, the transient digital current does not appear on the external analog ground plane, but is confined to the loop. The decoupling capacitor of the VD pin should be installed as close to the converter as possible to minimize parasitic inductance. The decoupling capacitor shall be low inductance ceramic type, usually between 0.01 μ F (10nf) and 0.1 μ F (100nF).
Finally, I would like to emphasize that no single grounding scheme is applicable to all applications. However, problems can be minimized by understanding the various options and the advance rules.
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