Grounding is the most basic part of circuit design, but it is also the part that is most easily overlooked. Many circuit designers tend to be troubled and overwhelmed when debugging, but they don’t know that the problem often lies in the “ground”. Speaking of this, students may ask: “How does the ground affect the circuit itself?” Don’t worry, look down.

Grounding is the most basic part of circuit design, but it is also the part that is most easily overlooked. Many circuit designers tend to be troubled and overwhelmed when debugging, but they don’t know that the problem often lies in the “ground”. Speaking of this, students may ask: “How does the ground affect the circuit itself?” Don’t worry, look down.

Figure 1 shows that the signal source is separated from the load by a distance, and the grounds G1 and G2 are connected by a loop. Ideally, the ground impedance between G1 and G2 is 0, so ground loop currents do not create a differential voltage between G1 and G2.

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

Unfortunately, it is not possible to keep the return path with zero impedance, and the ground return impedance will generate an error voltage ΔV between G1 and G2 due to the ground current.

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

The connection between G1 and G2 is not only resistive, but also inductive. For the purpose of this article, the effect of stray capacitance is ignored here. But in the next article in the series, you’ll learn how capacitance between the power and ground planes can help with high-frequency decoupling.

Solderless breadboard, the resulting circuit may look similar to the circuit shown in Figure 3

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

The current flowing between G1 and G2 can be a signal current or an external current caused by other circuits.

You can see how the bus impedance in the breadboard in Figure 3 has both resistive and inductive components. Whether or not the ground bus impedance affects circuit operation depends not only on the DC accuracy requirements of the circuit, but also on the analog signal frequency and the frequency components generated by the digital switching elements in the circuit.

If the maximum signal frequency is 1 MHz, and the circuit requires only a few milliamps (mA) of current, the ground bus impedance may not be an issue. However, if the signal is 100 MHz, and the circuit drives a load that requires 100 mA, impedance is likely to be an issue.

In most cases, it is not acceptable to use a “buss wire” as a digital ground return due to its impedance at the equivalent frequency of most logic transitions.

take a chestnut

For example, #22 standard wire has an inductance of about 20 nH/inch and a resistance of 1 mΩ/inch. A transient current with a slew rate of 10 mA/ns resulting from a logic signal transition, flowing through 1 inch of this wire at this frequency, would create an unwanted voltage drop of 200 mV:

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

For a signal with a 2 V peak-to-peak range, this voltage drop translates to an error of about 10% (about 3.5 bits of accuracy). Even in an all-digital circuit, this error greatly reduces the logic noise margin.

For low frequency signals, this 1 mΩ/inch resistor also creates an error. For example, when 100 mA flows through 1 inch of #22 standard wire, the resulting voltage drop is approximately:

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

When a signal with a 2 V peak-to-peak range is digitized to 16-bit precision, its 1 LSB = 2 V/2 16 = 30.5 μV. Therefore, the 100 μV error due to wire resistance is approximately equal to 3.3 LSB error at the 16-bit level of accuracy.

Figure 4 shows how noisy digital currents flowing in analog ground loops can create errors in the voltage V IN input to the analog circuit. Connecting the analog circuit ground and the digital circuit ground at the same point (as shown in the correct circuit diagram below) can alleviate the above problems to some extent.

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

Ground planes are essential in today’s systems

In a solderless breadboard, even in a circuit board with a bus structure as shown in Figure 3, there are not many means that can be used to reduce the ground impedance. Solderless breadboards are very rare in industrial system design. Solid ground planes are the industry standard method of providing a low impedance return path. Production printed circuit boards typically have one or more layers dedicated to grounding. This approach is quite suitable for final production, but is more difficult to implement in a prototype system.

Figure 5 shows a typical grounding arrangement for a PCB that includes analog circuits, digital circuits, and a mixed-signal device (analog-to-digital converter or digital-to-analog converter, etc.).

Analog ground vs digital ground?  4 minutes to learn the “magic” of grounding

The analog and digital circuits are physically separated and located on their own ground planes. Mixed-signal devices span two ground planes, and the system single-point or star ground is the connection point between the two ground planes.

You should be aware that there are other proven grounding principles for analog and digital grounding. However, these principles are all based on the same concept – analyzing analog and digital current paths, and then taking steps to minimize their interaction.

Hopefully you’ve learned how important grounding is to your current and future designs.

The Links:   EG7500B-NS SKD110-16