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Whether you are sensing temperature, humidity, distance, or any other input from a sensor, chances are you will need an ADC to convert the incoming analog signal to a digital signal. Digital signals are easier to process and transmit around a board. When you are designing a data acquisition system from scratch, choosing the right type of ADC is an important task as it will impact the system’s performance and budget. Here’s how to choose ADC ICs for your next PCB and what you need to know about their different operating parameters.
How to Choose ADC ICs
One might argue that platforms like Arduino have a built-in ADC, so why bother with another ADC in the system? The ADC in an Arduino is a 10-bit IC with 4.9 mV step size. For low level signals, you might need a higher/lower resolution or a different step size. In either case, your optimum solution may be to design your own board with an ADC and a microcontroller.
In today’s components market, there is a huge number of ADCs available, each with different topology, resolution, power consumption, price, and other performance aspects. The following figure should give you a good idea of the difference between bandwidth and resolution in different types of systems. This should give you a good idea of the frequencies you’ll be working with and the resolution required for accurate digital conversion in a number of applications.
Resolution vs bandwidth for different applications [Image source]
It is tempting to think that picking an ADC with the highest possible resolution is the best choice for your system, but this is not always the case. While it is true that you need to consider the resolution of your ADC, you also need to understand the range of frequencies it can reliably convert into a digital signal. This is your bandwidth. As an example, if you want to measure distance or proximity (shown in the teal box above), your sensors will likely produce signals in the 1 Hz to about 20 kHz range. The bandwidth of your ADC will need to cover some portion of this signal range if you want to gather accurate measurements.
Types of ADCs
There are many ADC architectures for high precision applications, including Flash, high-speed, and sigma-delta. The choice of architecture is usually determined by the resolution required and the rate at which the input signal is sampled. This actually brings up another important point regarding ADCs; the sampling rate for your ADC needs to be at least double the largest frequency you want to sample.
The most common architectures for imaging, measurement, and data acquisition systems are sigma-delta, successive-approximation register (SAR), and pipeline ADCs. A general distribution of resolution vs. sampling rate is given in the following figure, which provides an idea regarding which ADC best suits a given system.
Resolution vs sampling rate for various ADC architectures [Image source].
This figure tells us something important: there is always a trade-off between resolution and sampling rate. Hence, reaching high resolution is more challenging when you are trying to sample a higher frequency. Let’s look at each ADC topology to understand how this works:
These ADCs work well for high precision (16-24 bits) and sampling rates of a few hundred Hz. There are two important processes used in a sigma-delta ADC: oversampling and noise-shaping. Details on implementing each of these signal processing steps are outside the scope of this article.
Successive approximation register (SAR) ADCs are by far the most popular ADC used in data acquisition systems. They have resolution of 8 to 16 bits with sampling rates up to several MHz. They also have small form factor and low power consumption.
These ADCs are more popular for sampling high frequencies, which can range from a few MHz to hundreds of MHz with resolution of 8 to 16 bits. Some applications include imaging and video systems, high precision RF measurement systems, and Ethernet or wireless transceivers. Pipelined ADCs suffer from initially low latency (i.e., response time), but afterwards they have high throughput.
Placing an ADC in Your PCB
Once you’ve decided on the type of ADC you want to use in your board, you will need to carefully place it in your layout. If you properly designed the ground plane for your board, you’ll have separated your digital and analog portions into different sections. Note that, when working with low frequencies, it is appropriate to place a notch between the two ground sections, but make sure the entire ground plane remains an unseparated copper region.
You’ll need to place the ADC so that it overlaps the split between the digital and analog ground regions. This is appropriate as modern ADCs generally separate the analog and digital pins onto different sides of an ADC. This allows you to route your analog lines to one side of the ADC and the digital output on the other side. It is best to place the ground pins on the ADC as close as possible to the connection between the digital and analog sections of the ground plane. This helps prevent transient noise from the digital side of the chip from interfering with the analog side.
Now that you understand how to choose ADC ICs, you can start with your resolution requirements. One important thing to consider at this point is power consumption. If your design will run on a battery, then you might need to consider sacrificing some resolution to conserve battery life. The output will have some delay, so latency of your ADC should be tolerable. Some applications have severe area constraints, so you might want to use a single ADC in a time-shared fashion rather than multiple ADCs. This will mean a reduction in sampling frequency, but it cuts down on area and power. Also, check the voltage compatibility of the ADC and your microcontroller.
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