One of the most interesting and controversial aspects of PCB layout for mixed-signal systems happens to be placement, routing, and layout of ADCs. Mixed-signal PCB design guidelines generally tend to have many incorrect layout recommendations attached to them, but ADCs tend to see these recommendations applied in the extreme. This should not be surprising as this is where the analog and digital worlds meet in a mixed-signal PCB.
ADC placement and routing in a mixed-signal PCB layout generally has two goals:
- Prevent digital signals from injecting noise into the analog signal to be sampled by the ADC
- Ensure the sampled signal is measured against the correct reference level
Both of these goals are related to stackup design, grounding, and routing in the PCB layout. Follow these guidelines to ensure your ADC functions properly and is noise-free.
ADC Layout in a PCB
Successful design with an ADC first requires selecting the appropriate component. At a high level, there are some guidelines to understand and follow in order to achieve the two goals outlined above:
- Make sure you understand the return paths for the digital interface on your ADC
- Use a solid ground plane; do not split up the GND net into regions unless your sensor interface has low SNR and is at low frequency
- Implement best power system design practices for the digital interface
- If needed, slow down the digital interface on the ADC (usually SPI or I2C) with an RC circuit (for inputs) or series resistor (for outputs); see the routing guidelines below for more details
The first point to look at is placement and routing, followed by grounding. The correct grounding design will support noise suppression in your routing.
Place and Route Digital and Analog in Different Board Regions
Like any mixed-signal system, most of the noise can be prevented by thinking about placement and routing, and in particular where return paths will exist in the PCB reference planes. If you look at a typical pinout for an ADC, you will see that the pinout attempts to separate the digital and analog interfaces on different sides of the component. The image below shows an example for a real ADC with multiple input analog channels and an SPI interface output interface.
ADC pinout example. Placement of these various component should be confined on each side of the ADC package.
Based on this separation, you can plan for different regions of the board to carry only digital or only analog components and signals. This is the best way to prevent digital signals from injecting noise into analog signals.
Some points to note in the above example:
- SHDN (active low) pin: This pin is an enable pin and is essentially a control signal. It can be hooked up to a slow interface (GPIO) on the system processor (on the digital side)
- COM pin: Not all ADCs have this pin. This pin is the common reference against which the analog input channels are measured. As an example, the voltage measurement for CH1 is equal to [V(CH1) - COM]. Usually, COM is connected to GND, so COM = 0 V (more on this below).
- REFIN and REFOUT: ADCs that allow an external reference will connect this to REFIN. Not all ADCs will have a REFOUT pin; this allows the internal reference of the component to be accessed and used in another component, or connected to REFIN.
Connect AGND and DGND
Assigning grounds can be a confusing subject generally, and it can be even more confusing when ADCs are considered. There seems to be a perception about the need for physically disconnected ground regions for referencing analog signals and digital signals.
- Leave DGND and AGND totally disconnected (should only be followed in galvanically isolated systems)
- Connect them at one single point, usually below the ADC (usually recommended in datasheets but should be used very carefully)
- Use a single ground plane for both the DGND and AGND nets (best recommendation)
Point #1 stating that the AGND and DGND regions should be split and never connected are generally wrong and should not be followed in most cases. There are exceptions to this, such as when a low-SNR low-frequency signal needs to be sampled in a dense layout, or when the analog section and digital section must use separate isolated power supplies that are floating (in which case they are connected with safety caps and not copper).
The matrix below matches up board stackup and grounding strategies with low frequency and high frequency analog signals.
Only in isolated systems
(Multilayer, thin dielectric below components)
(Multilayer, thin dielectric below components)
Make sure the ground region is on the layer adjacent to the signal + ADC layer, and it is a good idea to use at least a 4 layer board to ensure best results in terms of EMI. If you must use a 2-layer board, use grounded copper fill around digital and analog signals to provide clear return paths that confine signals.
Higher Speed Serial Data Interfaces
While not the most common situation in ADCs, there are components that use a differential digital interface to access digital data for the converted signal. LVDS is one differential interface that is accessible in some components. However, there is another standardized interface used in data converters generally: JESD204C.
The most recent revision to the JESD204C standard, published by Joint Electron Devices Engineering Council (JEDEC), is a high-speed serial interface (up to 32.5 Gbps) that standardizes interconnects between ADCs/DACs and FPGAs, ASICs, and other high-speed processors. Not all controllers or ADCs include a JESD204C interface, but those that do are targeting particular applications where higher sample and conversion rates are demanded:
- High-speed data acquisition cards
- 5G cellular equipment and base station gear
- High-speed sensor interfaces aggregated across multiple links
- Software-defined radio (SDR)
Advanced systems like satellites rely on JESD204C for high-speed serial data transfer from data converters and ADCs.
These more advanced systems will make use of this higher speed standard, so it is important to learn its routing requirements as your mixed-signal systems require higher speeds and higher frequencies.
When you need to select your ADC, place components, and route signals in your PCB layout, make sure you use Allegro PCB Designer, the industry’s best PCB design and analysis software from Cadence. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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