Tamara Schmitz, Senior Principal Applications Engineer and Global Technical Training Coordinator at Intersil Corporation introduces the basics of converters and their main performance parameters.
When trying to learn about converters, most resources only discuss one topology or are just too deep to grasp. But before getting started it is important to remember that analogue signals are continuous while digital signals are sampled representations of a sensor output or some other measured signal.
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 volts, 3.3 volts, 2.5 volts or even 1.8 volts) 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 2.5V reference has an initial accuracy of ±0.02% and a temperature coefficient of 7ppm/oC (parts per million per degree Celcius). The line regulation is the specification to reveal the output voltage change per supply voltage change. For this device it is 8ppm/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 percent and have temperature coefficients of 100ppm/ oC. (That’s more than 200 times worse in initial accuracy and 10 times worse over temperature!).
Topologies
There are many topologies for both analogue-to-digital converters and digital-to-analogue converters. If you are choosing a converter, the focus is more about the performance characteristics than the topology.
For the fastest conversion speeds, over 1GHz, 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. Note that multiple converters will burn more power.
On the opposite side of the spectrum, there are Delta Sigma topologies for both DAC and ADCs. 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.
R-2R (ratio of resistors) configurations for DACs are a popular solution for industrial applications. The R-2R used in a chain of op amps to create the output voltage. SAR (successive approximation registers) ADCs are common mid-range ADCs. These are very popular in the 12- to 16-bit range.
There is an abundance of information available about converters, their parameters and each topology. This overview aims to offer some background understanding needed to hone your search and more quickly find the right part for your system.
