General Grounding Guidelines for Most Mixed-Signal Devices

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General Grounding Guidelines for Most Mixed-Signal Devices [Copy link]

Grounding is an issue that cannot be ignored for all analog designs, and proper implementation of grounding is equally important in PCB-based circuits. Digital and analog design engineers tend to look at mixed-signal devices from different perspectives. This issue introduces a general grounding principle that applies to most mixed-signal devices.
　　Why do we need separate digital and analog grounds?
　　Digital circuits are noisy, and saturated logic (such as TTL and CMOS) briefly draws large currents from the power supply during switching. However, since the immunity of the logic level can reach hundreds of millivolts, the requirements for power supply decoupling are usually not high. In contrast, analog circuits are very susceptible to noise, including on the power rails and ground rails. Therefore, in order to prevent digital noise from affecting analog performance, analog and digital circuits should be separated. This separation involves the separation of ground loops and power rails, which can be troublesome for mixed-signal systems.
　　However, if high-precision mixed-signal systems are to fully perform, it is crucial to have separate analog and digital grounds and separate power supplies. The fact that some analog circuits can run from a single +5V supply does not mean that the circuit can share the same +5V noisy supply as a microprocessor, dynamic RAM, fan, or other high-current device. The analog portion must operate at peak performance using such a supply, not just to keep running. This distinction necessarily requires that we pay close attention to the power rails and ground interfaces.
　　Note that the analog and digital grounds in the system must be connected at some point so that the signals are all referenced to the same potential. This star point (also called the analog/digital common point) must be carefully chosen to ensure that digital currents do not flow into the ground of the analog portion of the system. It is usually convenient to set the common point at the power supply.
　　About AGND and DGND Pins
　　Many ADCs and DACs have separate "analog ground" (AGND) and "digital ground" (DGND) pins. On the device data sheet, it is usually recommended that the user connect these pins together at the device package. This may seem to conflict with the recommendation to connect the analog and digital grounds at the power supply, and if the system has multiple converters, it may seem to conflict with the recommendation to connect the analog and digital grounds at a single point.
　　In fact, there is no conflict. The "Analog Ground" and "Digital Ground" markings on these pins refer to the internal part of the converter to which the pins are connected, not to the system ground to which the pins must be connected. For ADCs, these two pins should usually be connected together and then connected to the analog ground of the system. They cannot be connected together inside the IC package because the analog portion of the converter cannot tolerate the voltage drop caused by the digital current flowing through the bond wires to the chip. However, they can be connected together externally.
　　Ground Connections for ADCs
　　The figure below shows the concept of ground connections for ADCs. This pin connection will reduce the digital noise immunity of the converter to a certain extent, by the amount of common-mode noise between the system digital ground and analog ground. However, since digital noise immunity is often in the hundreds or thousands of millivolts, it is generally unlikely to be a problem.

　　The AGND and DGND pins of the data converter should be returned to the system analog ground.
　　Analog noise immunity is only degraded by external digital currents flowing into the analog ground from the converter itself. These currents should be kept small and can be minimized by ensuring that the converter output is not heavily loaded. A good way to achieve this is to use a low input current buffer, such as a CMOS buffer-register IC, at the ADC output.
　　If the converter's logic supply is isolated with a small resistor and decoupled to the analog ground with a 0.1μF (100nF) capacitor, all of the converter's fast-edge digital currents will flow back to ground through this capacitor and will not appear in the external ground circuit. If a low impedance analog ground is maintained while analog performance is adequate, the additional noise generated by external digital ground currents will be of little concern.