General grounding principles for most mixed-signal devices

“For all analog designs, grounding is an issue that cannot be ignored, and in PCB-based circuits, proper grounding is equally important. 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.

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For all analog designs, grounding is an issue that cannot be ignored, and in PCB-based circuits, proper grounding is equally important. 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 ground and analog ground?

Digital circuits are noisy, and saturated logic (such as TTL and CMOS) will briefly draw a large current from the power supply during the switching process. However, since the immunity of the logic level can reach hundreds of millivolts or more, the requirements for power decoupling are usually not high. In contrast, analog circuits are very susceptible to noise, including on power rails and ground rails. Therefore, in order to prevent digital noise from affecting analog performance, analog circuits 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 a high-precision mixed-signal system is to give full play to its performance, it must have separate analog and digital grounds and a separate power supply, which is very important. In fact, although some analog circuits operate with a single +5V power supply, it does not mean that the circuit can share the same +5V high-noise power supply with microprocessors, dynamic RAMs, electric fans, or other high-current devices. The analog part must use this type of power supply to run at maximum performance, not just keep it running. This difference inevitably requires us to pay great attention to the power rail and ground interface.

Please note that the analog ground and digital ground in the system must be connected at a certain point so that the signals are all referenced to the same potential. This star point (also called the analog/digital common point) should be carefully selected to ensure that the digital current does not flow into the ground of the analog part of the system. It is usually convenient to set up a common point at the power source.

About AGND and DGND pins

Many ADCs and DACs have separate “analog ground” (AGND) and “digital ground” (DGND) pins. In the device data sheet, it is usually recommended that users connect these pins together at the device package. This seems to be in conflict with the suggestion to connect analog and digital grounds at the power supply; if the system has multiple converters, this seems to conflict with the suggestion to connect the analog and digital grounds at a single point.

In fact, there is no conflict. The “analog ground” and “digital ground” marks of these pins refer to the internal part of the converter to which the pins are connected, not the system ground to which the pins must be connected. For ADC, these two pins should usually be connected together and then connected to the analog ground of the system. Since the analog part of the converter cannot withstand the voltage drop generated when the digital current flows to the chip via the bonding wire, it is impossible to connect the two inside the IC package. But they can be connected together externally.

ADC ground connection

The figure below shows the concept of ADC ground connection. Such a pin connection will reduce the digital noise immunity of the converter to a certain extent, and the reduction is equal to the amount of common mode noise between the digital ground and analog ground of the system. However, because digital noise immunity is often at the level of 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.

The analog noise immunity is only reduced by the external digital current of the converter itself flowing into the analog ground. These currents should be kept small, and the current can be minimized by ensuring that there is no high load on the converter output. A good way to achieve this goal is to use a low input current buffer at the ADC output, such as a CMOS buffer C register IC.

If the logic power supply of the converter is isolated by a small resistor and decoupled to the analog ground through a 0.1μF (100nF) capacitor, then all the fast edge digital currents of the converter will flow back to the ground through the capacitor instead of appearing outside. In the ground circuit. If the analog ground of low impedance is maintained and the analog performance can be fully guaranteed, then the additional noise generated by the external digital ground current will basically not constitute a problem.

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