Building circuits whether on a PC (Printed Circuit) board or a breadboard, grounding have always been important. Careful grounding becomes critical on applications where performance and integrity of the signals are of prime importance. Examples are low-level application such as in audio, in precision sensing and measurement circuits, and in mixed analog and digital circuits .
When do we use star ground and ground plane? What is the difference between the two? Which is actually better? We will try to answer first what the difference between the two is.
Star Ground
Also sometimes called mecca ground, this is based on the concept of creating a single return point in the circuit as a common reference in order to avoid ground loops and thus circuit errors (See figure 1 [1]). In the figure, we could see the input signal return path is shared, assuming very high input impedance and no significant current flowing in it. Those in the path of significant current flow, such as the power supply returns, and high frequency signal could be sources of errors, as we have explained in the first part of this series. The input signal and power supply return meet at the star point, where it doesn't introduce an error to the input signal.
This would be very beneficial in applications where small error in signals matters and pinpoint accuracy is desired. And as mentioned, it prevents ground loops by avoiding to cross with current return paths that could interfere with the integrity of the signals. In order to be effective with star grounding, the circuit designer has to fully understand the current and signal flows in the circuit.
Ground Plane
A more practical approach to multi-layer PCB is the use of ground plane, where the ground pins of the components can connect directly to ground without using longer traces. The idea behind is that ground planes have wider areas for current to travel through and therefore less resistance or impedance (inductance). The ground plane provides direct return paths for AC signals on the adjoining layers because of the capacitance created between the signal trace and the ground plane, with the insulator in between acting as a dielectric. Remember that Direct Current (DC) signals seek the least resistance and shortest path, while Alternating Current (AC) signals take on the least impedance path which means not necessarily the path the DC signals take.This is also particularly beneficial with bypass capacitors.
Figure 3 shows two points A and B, with B providing a connection for ground return [2]. Signal travels from point A to B, with the final ground return underneath point A on the ground plane. The DC takes a direct path towards the return, while the AC signal takes the least impedance path (higher capacitance and less inductance on the ground plane), which means the area directly underneath the top layer signal trace.
Continue on to the last part of the series where we will continue to discuss more on the practical applications of the grounding schemes, and when to use star ground or ground plane:
Part III - Grounding Series
References:
[1] Analog Devices Application Note AN345, Paul Brokaw
[2] Staying Well Grounded by Hank Zumbahlen, Analog Dialogue Volume 46
When do we use star ground and ground plane? What is the difference between the two? Which is actually better? We will try to answer first what the difference between the two is.
Star Ground
Also sometimes called mecca ground, this is based on the concept of creating a single return point in the circuit as a common reference in order to avoid ground loops and thus circuit errors (See figure 1 [1]). In the figure, we could see the input signal return path is shared, assuming very high input impedance and no significant current flowing in it. Those in the path of significant current flow, such as the power supply returns, and high frequency signal could be sources of errors, as we have explained in the first part of this series. The input signal and power supply return meet at the star point, where it doesn't introduce an error to the input signal.
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| Figure 1 Star Ground |
Ground Plane
A more practical approach to multi-layer PCB is the use of ground plane, where the ground pins of the components can connect directly to ground without using longer traces. The idea behind is that ground planes have wider areas for current to travel through and therefore less resistance or impedance (inductance). The ground plane provides direct return paths for AC signals on the adjoining layers because of the capacitance created between the signal trace and the ground plane, with the insulator in between acting as a dielectric. Remember that Direct Current (DC) signals seek the least resistance and shortest path, while Alternating Current (AC) signals take on the least impedance path which means not necessarily the path the DC signals take.This is also particularly beneficial with bypass capacitors.
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| Figure 2 Ground Plane |
Figure 3 shows two points A and B, with B providing a connection for ground return [2]. Signal travels from point A to B, with the final ground return underneath point A on the ground plane. The DC takes a direct path towards the return, while the AC signal takes the least impedance path (higher capacitance and less inductance on the ground plane), which means the area directly underneath the top layer signal trace.
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| Figure 3 |
Continue on to the last part of the series where we will continue to discuss more on the practical applications of the grounding schemes, and when to use star ground or ground plane:
Part III - Grounding Series
References:
[1] Analog Devices Application Note AN345, Paul Brokaw
[2] Staying Well Grounded by Hank Zumbahlen, Analog Dialogue Volume 46
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