All mixed-signal systems use analog-to-digital converters to interface with the analog world and collect important signals for processing. ADCs give designers a way to collect and digitize signals, but they rely on carefully matching the signal with the particular ADC that will be used to collect the signal. In addition, there is a challenge with analog front-end design, where the signal may be pre-filtered and conditioned before being collected by the ADC and processed accordingly.
If you’re looking to brush up on your analog design skills and you want to ensure noise-free signal collection with ADCs, we’ve compiled a comprehensive set of resources to help you successfully select and use ADCs in your design. Each step in the process relies on understanding the types of ADCs and how continuous signals can be quantized as discrete samples of data. Eventually, a systems designer will need to place and route one or more ADCs in a PCB without compromising system performance.
What You Need to Know to Successfully Use ADCs
Successful selection and implementation of ADCs in an electronic device centers around four main areas: component selection, placement in an assembly, sampling, and noise suppression.
Types of ADCs
There are different types of ADCs that serve different measurement applications. The internal architecture of an ADC will determine the measurement capabilities in terms of resolution, output data format (serial or parallel), and latency. For some time-critical applications, throughput and latency may be more important than resolution, while bandwidth and sample rate may be more important in some other applications.
The main types of ADCs available to designers are:
- Successive approximation (SAR)
Make sure you understand the capabilities of each type of ADC architecture before you select components for your design.
Selecting an ADC
The ADC sampling rate, bandwidth, and resolution need to be carefully selected to match the dynamic range and frequency of the signal that will be measured. These aspects of ADC selection will determine the accuracy of the reproduced signal, as well as the potential for quantization errors due to noise. In the end, the designer may have to rely on an analog front-end with filtering and amplification to condition the signal such that it can be successfully captured and converted to a digital bitstream.
All ADCs need to have a voltage reference that provides the baseline against which the input analog signal is compared. While there may be an instinct to route a standard voltage regulator output into the reference port on an ADC, this is not the best approach if power supply noise is present or if there is significant temperature rise in the system. Instead, a specialized reference or a heavily-filtered dedicated voltage source will be needed to provide a stable reference voltage for an ADC. This will help ensure the most accurate possible measurements.
Clocking ADCs in Mixed-Signal Systems
ADCs need to have a clock that sets the sampling rate and the data output rate. An external oscillator can be used to time the system, or a system clock can be used as long as it is routed correctly and the clock has low jitter. In addition to the right type of oscillator, data streams from multiple ADCs may need to be synchronized, especially in time-critical applications with near-real-time latency.
Guide to PCB Layout For an ADC
Placing an ADC in a PCB layout is one area of design that is fraught with bad advice. Older application notes recommend practices that span as far back as 30 years, and these practices have been thoroughly debunked by EMI/EMC experts as well as digital designers. Practices like physically separated grounds and using ferrites for isolating power rails are as bad in ADC design as they are in high-speed PCB design.
Instead of relying on outdated application notes that are known to result in EMC failures, follow the guidelines in the link below. Many ADC noise problems and EMI failures in general are prevented with appropriate PCB stackup design, component placement, floorplanning, and routing.
We hope that the resources we’ve compiled above will help you understand everything needed to successfully select and use an ADC in your system. Whether you select an MCU with integrated ADC, an ADC module, or an integrated circuit, these guidelines should help you ensure accurate signal acquisition.
Once an ADC selection has been made, designers need simulation and evaluation tools to help them qualify their system before creating the physical layout. The complete set of system analysis tools from Cadence gives electronics designers everything needed to simulate system functionality, including analog systems with multiple ADCs. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.