Getting started with converters

The basics of converters and the main performance parameters (Part 2)

BY TAMARA SCHMITZ
Senior Principal Applications Engineer and Global Technical Training Coordinator
Intersil
www.intersil.com

When trying to learn about converters, most resources only discuss one topology or are just too deep to grasp. The purpose of this article is to introduce the basics of converters and the main performance parameters. Hopefully you will find the working knowledge you need to keep up with any signal chain conversation.

Before we dive in, remember that analog signals are continuous while digital signals are sampled representations of a sensor output or some other measured signal.

Once you are comfortable with the differences between analog and digital signals, the next step to understand the signal chain is to learn about the converters that translate from one type to another. First let’s examine the symbols used for analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC). See Fig. 1 .

Fig. 1: Symbols for ADCs and DACs

These symbols are made from the components that would process the signals. The most basic analog signal chain block is the op amp, represented by a triangle. Pictorial representations of digital flowcharts use boxes. Combining the two to represent an analog-to-digital block is then a triangle followed by a box. This pentagon has the point on the left, since the input is the analog side in an ADC. Naturally, the digital-to-analog block is also a pentagon, but with the point on the right. Again, this is the triangle side since the output is analog in a DAC.

The two definitive specifications of a converter are speed and resolution. Speed is how many evaluations that a converter can make in a second. Resolution is measured in the number of bits, or levels that the digital signal will have. These two parameters, together, determine the amount of processing that a converter will do. Really-high-speed converters can’t process two-dozen bits, and converters that process 24 bits or more can’t do it very quickly.

Beyond these two main criteria, there are many other parameters to consider. There are some parameters that are common to all electronic devices—like power dissipation and package size. When considering converters, another important consideration is the reference voltage.

Both the ADC and DAC need a reference, Vref. These limits are the boundaries of the supply, something like ground (zero volts) and the supply voltage (5, 3.3, 2.5, or even 1.8 V) in a single-supply design. These limits identify the absolute maximum and minimum values for the represented signal. However, the power supply also typically is noisy. It not only varies as the loads change and the current draw through the supply must adjust, but it also circulates any high-speed signals picked up like an antenna or signals coupled from circuits with clocks or other sharp edges. Therefore, most ADCs and DACs also have a reference input. This input is a more reliable and stable maximum value as a limit for the represented signal. Component manufacturers have responded to this need by offering devices specifically designed to provide a stable reference over temperature and voltage variations. An example is the ISL21090. The 2.5-V reference has an initial accuracy of ±0.02% and a temperature coefficient of 7 ppm/C (parts per million per degree Celcius). The line regulation is the specification to reveal the output voltage change per supply voltage change. For the ISL21090, this is 8 ppm/V. It is true that some manufacturers of converters have included internal references for easy system design. Beware of the trade-off of performance for simplicity. These references might be only accurate within 5% and have temperature coefficients of 100 ppm/C. (That’s more than 200 times worse in initial accuracy and 10 times worse over temperature!).

Topologies

There are many topologies for both analog-to-digital converters and digital-to-analog converters. If you are choosing a converter, you care more about the performance characteristics than the topology. However, knowledge of a few basic topologies can help streamline your search. Table 1 is a quick reference for different topologies of DACs and ADCs. While the fabrication technology limits the total possible performance, the topology and design set the trade-off of speed and resolution.

For the fastest conversion speeds, over 1 GHz, string topologies are popular for DACs and flash for ADCs. The highest resolutions are in the range of 14-bits and 16-bits. Many of these converters are used in 8- or 10-bit applications, while sometimes 4-bit resolution is enough. String DACs also have the benefit of being both linear and monotonic, so they are popular in instrumentation applications. They are low power, so they are popular in portable systems. Flash ADCs are composed of multiple paths in parallel, so they burn a lot of power to achieve that speed. They are used in applications like radar and wide band radio. There are other ways to help an ADC go fast—like pipelining and interleaving. These configurations use multiple converters and connect them internally to give the customer one package with extra performance. Notice that multiple converters will burn more power.

On the opposite side of the spectrum, there are delta sigma topologies for both DAC and ADCs. Don’t be afraid of the name “delta sigma.” Those are the names of two Greek letters: Delta stands for “difference” and sigma stands for “summation.” This particular topology has a loop in it where it measures a difference between the input and a running total. Then it adjusts (by summing in one bit) that running total up or down depending on the difference measurement. Delta sigmas are used when lots of resolution is needed, 22, 24, and even 32 bits. This level of resolution takes a while for the converter to converge with this accuracy, so they are slow. A good example of a slow, but accurate system is weighing. Customers don’t want to pay more postage than necessary for their package and are willing to wait a second or two for a precise measurement. Likewise, truck drivers want to carry as much cargo as possible without going an ounce over the legal limit and having to pay a penalty.

R-2R configurations for DACs are a popular solution for industrial applications. The R-2R stands for the ratio of resistors used in a chain of op amps to create the output voltage. SAR (successive approximation register) ADCs are common mid-range ADCs. These are very popular in the 12 to 16-bit range.

Of course, there is almost unlimited information available about converters, their parameters, and each topology. This overview hopes to give you the high-level understanding needed to hone your search and more quickly find the right part for your system. ■

Also see: “Don’t get your signals crossed: What you need to know about converters (Part 1)” http://www2.electronicproducts.com/Don_t_get_your_signals_crossed-article-facn_intersil_sep2011-html.aspx