Signal Chain Basics #21: Understand and configure analog and digital grounds

As we wish Bill all the best in his retirement, we look forward to continuing his work by providing the quality material you have come to expect from this column. Look for Signal Chain Basics to be published on a monthly basis, where a different author will share their ideas on how to make the most of your analog signal chain.

Now, let's get started:

Ground is a reference point for all of the signals in a system. Ideally, it has a potential of zero volts everywhere. In the real world, ground is nothing more than another signal carried by real conductors with some impedance. Currents flowing through this ground cause the potential across the conductor to be different at different points. A "good" ground is realized by keeping the conductor impedance low, which minimizes the potential difference. This is done by dedicating a layer of the printed circuit board to being a ground plane, where a large area of copper lowers the impedance.

Many people separate the ground plane into analog and digital sections with a split through the plane, as shown in Figure 1. The analog circuitry is over the analog part of the plane, and the digital portions of the system are over the digital part. The two grounds eventually come together back at the system power supply.

The data converter, either digital-to-analog (D/A) or analog-to-digital (A/D), generally straddles this split and has two ground pins: one called analog ground (AGND); and the other digital ground (DGND). However, it is not as simple as connecting the converter's pins to the corresponding portion of the ground plane.

Figure 1: Split ground plane, tied together at power supply
(Click on image to enlarge)

Because AGND and DGND are not connected inside the data converter IC (the internal impedances would be too high (Reference 1), the converter relies on an external, low-impedance connection between the AGND and DGND pins to make it operate properly.

The AGND and DGND pins of the converter must be tied together at the converter or performance will suffer (References 2 and 3). In terms of how these grounds are connected together, there are three possibilities:

1) Connect the analog and digital ground pins of the converter to the respective plane, and tie the planes together underneath the converter, as shown in Figure 2.

Figure 2: Split ground plane with connection under ADC
(Click on image to enlarge)

Ideally, this point would also be where the power supply ground connects to create a true "star" ground, but in most systems this is impractical. Technically, this creates a ground loop, but if the only currents flowing under the converter are its own return currents, this is usually not a problem.

2) Connect the converter analog and digital ground pins to the analog ground plane exclusively, as shown in Figure 3.

Figure 3: Data converter grounds both returned to analog plane
(Click on image to enlarge)

This puts the converter ground pins entirely over the analog plane, routing the split around the DGND pin. This approach works better for systems with multiple converters, because using the technique shown in Figure 2 with multiple converters would result in multiple ground loops. This is asking for trouble.

3) Don't put a split in the ground plane at all, and tie the two data converter ground pins to the common plane, as shown in Figure 4.

Figure 4: Solid ground plane, partitioned into analog and digital sections
(Click on image to enlarge)

With a board that is properly partitioned into analog and digital sections, designed with a split in mind, it is quite possible to not even put the physical split in the ground plane. With lower power devices, a solid ground plane can be quite effective (Reference 4). This approach also helps avoid the inadvertent routing of traces across the split, which results in higher impedance return paths and potential EMI problems.

With these approaches in mind, you can find one that will fit your system needs. Just remember that a data converter is analog!

References

1. Kester, W., "Grounding (Again)," Analog Dialogue--Ask the Application Engineer: http://www.analog.com/library/analogDialogue/Anniversary/12.html
2. Hu, B.; See, K.Y., "Impact of analog/digital ground design on circuit functionality and radiated EMI," Electronic Packaging Technology Conference, 2005. EPTC 2005. Proceedings of 7th , vol.1, 4 pp, 7-9 Dec. 2005: http://ieeexplore.ieee.org/iel5/10751/33891/01614363.pdf
3. Downs, R., "Analog-to-Digital Converter Grounding Practices Affect System Performance," Texas Instruments (sbaa052): http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=sbaa052
4. Ott, H. W., "Partitioning and Layout of a Mixed-Signal PCB," Printed Circuit Design, June 2001, pp. 8-11: http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed", www.planetanalog.com/features/showArticle.jhtml;?articleID= 210004183, click here
• "SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators", www.planetanalog.com/features/showArticle.jhtml;?articleID=209900450, click here
• "SIGNAL CHAIN BASICS (Part 18): The op amp as integrator", www.planetanalog.com/features/showArticle.jhtml;?articleID=209101070, click here
• "SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208802817, click here
• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208403856, click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter—dynamic parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=208401183, click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter—static parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=207800114, click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", www.planetanalog.com/features/showArticle.jhtml;?articleID=207001505, click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504089, click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here