ADC Sampling Rate and Layout for Mixed Signal Boards
Whenever you’re selecting an ADC, whether it is built into an MCU or as an external component, the sampling rate is a prime consideration, as it will determine how well you can reproduce a measured signal. RF applications, analog sensor boards, and other mixed-signal devices will need at least one ADC with an appropriately chosen ADC sampling rate.
If you’re designing with a mixed-signal board, you’ll need to balance the required signal bandwidth with sampling rate and your ADC’s analog input bandwidth. The last point is seldom considered when working with harmonic frequencies, but it becomes quite important if you need to detect pulse streams, a broad range of harmonic frequencies, or any other wide bandwidth signal. If you select an ADC with the wrong sampling rate, you’ll end up in a situation where aliasing creates false signal artifacts.
You Can Thank Nyquist for Aliasing
Go back to your analog electronics class for a moment; you may remember something called the Nyquist frequency, named after the engineer Harry Nyquist. He was certainly a superstar in the telecommunications and signal processing community in the early 20th century. He is perhaps most remembered for his theorem relating the sampling rate of a signal sampling device to the bandwidth of the signal being sampled. According to Nyquist, the sampling rate should be at least double the bandwidth of the signal in order to capture all frequency components.
This has some important consequences in analog-to-digital converters (ADCs), and in oscilloscopes used for time-domain measurements. In an ADC, we generally work with a finite bandwidth signal and we need to produce a digital representation of it in the time domain. In oscilloscopes, we may need to reproduce any possible signal (including clocks or digital signals) in the time-domain. This all relies on setting the appropriate sampling rate.
Don’t Forget Your ADC Bandwidth
The ADC itself will also have some finite bandwidth, just like any other analog component and their signals. Just like a filter or amplifier, the analog bandwidth (or full-power bandwidth, FPBW) defines a -3 dB point, beyond which there is some roll-off beyond. Again, just like an amplifier, the ADC will not have distortionless output up to its bandwidth cutoff frequency.
Due to the finite input bandwidth, an ADC’s bandwidth is generally less than the Nyquist frequency, unless you set the sampling rate very low. Beyond the Nyquist frequency, all frequency components will be aliased. Two different types of responses are shown below, with the red area corresponding to the range of frequencies that will be aliased by the ADCs.
For the red curve, the ADC input frequency response is perfectly cut off exactly at the Nyquist frequency. This ideal behavior is impossible to reproduce physically, although you might be able to come close with the right set of filters. The real behavior of most ADCs is familiar to people who often work with oscilloscopes; the response is Gaussian or Gaussian-like. A better choice for working with wide bandwidth signals in your PCB is to select an ADC with bandwidth that is closer to the blue curve. Here, we have an “effective” Nyquist frequency that is equal to Fs/2.5.
ADC Sampling Rate vs. Input Bandwidth
If we look at the above graph, there are two points to understand when choosing an ADC for your mixed-signal board:
- Signal distortion already occurs before the aliasing frequency. This can be seen in the Gaussian and maximally-flat frequency responses for an ADC. Simply increasing the sampling frequency will not present this problem.
- Using a lower sampling frequency reduces cost, but it increases the chances that high frequency components create some artifacts on the signal output. Anyone that has seen a ghost trace, glitches, or artificial modulation on an oscilloscope trace is familiar with these artifacts in signal reproduction.
Use Your ADC Sampling Rate to Reduce PCB Noise
This gets back to the original question: what is the best sampling rate? The answer is…it depends! If your mixed-signal board has excessive broadband noise on an analog signal, you can use a higher sampling rate to reduce this noise. A good rule of thumb when sampling a wide bandwidth analog signal is to set the sampling rate anywhere from 2 to 5 times the fundamental frequency in your signal.
After sampling at a high rate, you can pass the output through an anti-aliasing filter. Sampling at a higher rate will spread the broadband noise power over a higher bandwidth, and passing the output through the anti-aliasing filter will cut off the un-needed higher frequency components, including high frequency noise.
Noise Reduction in an ADC Layout at High Sample Rates
After choosing an ADC with the desired sampling rate, you’ll have to think about your layout strategy. Perhaps the fundamental layout rule with ADCs in mixed-signal boards is to have the ADC straddle the digital and analog sections of the ground plane to ensure these different types of signals remain separated. Like other components with digital output, an ADC is susceptible to ground bounce, so be sure to use a bypass capacitor to suppress this noise source and provide accurate signal reproduction. Ground this bypass capacitor to the same plane as the ADC’s ground plane to provide the lowest possible loop inductance.
If the ADC is receiving an RF signal, consider using a coplanar waveguide configuration for routing the input line. This will help isolate the line from other analog components and reduce crosstalk. Be careful with routing clock signals close to an ADC’s input or output traces as you want to avoid crosstalk with your clock trace. Crosstalk between the clock and digital output lines produces a sinusoidal noise signal on the clock lines. This can then couple back to the analog input as phase modulation, producing a false reproduction of the signal at high sampling rates. Try to minimize broadside coupling between clock and signal lines when routing.
Once you’ve determined the appropriate ADC sampling rate for your system, you can identify the components you need and layout your board in Altium Designer®. The Manufacturer Part Search panel in Altium Designer gives you access to verified component libraries, complete with accurate footprints, symbols, and 3D models. You’ll also have access to a set of post-layout simulation tools for analyzing the behavior of your mixed-signal PCB.