Technical Overview

A data converter is the bridge between the real, physical world of analog signals like voltage or current, and the digital world of numbers represented by ones and zeros. An analog-to-digital converter (ADC) converts a voltage into a number; a digital-to-analog converter (DAC) converts a number into a voltage or current. An ADC might be used to measure a weight or the intensity of light, or allow an audio signal to be captured and stored as a digital file for playback later in a media player. Converting that digital file back into sound would require a DAC; a DAC could also be used to control a valve that affects the flow of chemicals into a chemical reaction, or the position of a cutting head on a system that machines mechanical parts.

Analog-to-Digital Converters

Figure 1 is a general representation of an ADC.

Every analog-to-digital converter (ADC) consists of three functional blocks: a sampler, a quantizer, and an encoder. In some architectures, some of the functions may actually be combined, but each function is there nonetheless.

The sampler is responsible for sampling the input signal at a certain time; it is implied that this function also “holds” the signal constant for the converter to operate on it during its conversion time.

The quantizer is responsible for measuring the input signal and determining an output code level that most closely represents the voltage of the analog input. It approximates the sampled voltage with a level from a fixed set of 2^N possible voltage levels, (where N is the number of bits of resolution) either via rounding or truncation.

The measurement of the input signal and creation of its corresponding output code is accomplished by comparing the input signal to a fixed reference voltage. The full-scale range (the maximum voltage that the converter can have on its input) is directly related to the reference voltage value. The minimum change in input voltage the converter can detect is called the least significant bit (LSB) value. For example, if the full-scale range is the same as the 5V reference voltage V_REF, and the converter has 12 bits of resolution, the LSB would be given by:

LSB = V_REF/2^N = 5V/2¹² = 5V/4096 = 1.22mV Equation 1

The best conversion accuracy generally is achieved by matching the input signal range closely to the converter's full-scale range, either through amplification before the ADC or by changing the reference voltage to adjust the full-scale range.

The encoder may turn the internal code used by the quantizer into a more usable code for a system (for example, turning a thermometer code into a 2's complement code) or may simply format the code into a serial data stream for easy communication to a host processor.

Figure 2 is an example of how an ADC is connected in a basic data acquisition system. The ADC is supplied power, a reference voltage, and the input signal voltage by analog circuitry; the digital interface to a host microcontroller or DSP is often a simple serial interface. High-speed applications may require more involved signal drive circuitry; the datasheet for most ADCs usually shows the recommended circuitry needed around the ADC.

Digital-to-Analog Converters

A digital-to-analog converter block diagram would be similar to Figure 1, viewed from right to left: the DAC accepts a digital input, which is then decoded into a form that the DAC circuitry can use; the quantizer then creates a voltage or current corresponding to the digital input value. The sampler is not always on the analog side because most DACs have a means to latch the digital value provided by the interface.

While ADCs generally accept only voltage inputs, the output of a DAC may be a voltage or a current. Figure 3 shows an example of a multiplying architecture, which usually has a current output, using an external op amp to convert the current to a voltage. Many DACs provide this conversion circuitry in the device itself.

How to read a data converter data sheet

As with any electronic component, the data converter datasheet is an essential resource. When deciding upon a converter to use for a design, much of what you need to know is usually prominently displayed on the first page of the datasheet. The most important parameters for a data converter are speed, resolution, and accuracy.

Speed

For an ADC, speed refers to the time it takes to convert an analog input into a digital value - the actual specifications are acquisition time and conversion time, which, when combined, limit how fast the ADC can output a conversion result. This can also be expressed as throughput, which is usually shown as the maximum sampling frequency.

For a DAC, the limiting factor on speed is usually the settling time, which is the time it takes for the output to settle to a new value from a previous value, within a specified error band. A 1µs settling time implies that the converter may be suitable for updating its output at a rate up to 1MHz.

Resolution

As described previously and with the example shown in Equation 1, resolution refers to the either the number of bits (N) the converter has as its input or output, the number of counts or codes (which is simply 2^N), or the value of the least-significant bit in volts or amps. Higher resolution means that the converter can discriminate between smaller changes on the input signal or provide more precise control over an output signal.

Accuracy

Many people confuse resolution with accuracy - just because you have a converter with 16 bits of resolution doesn’t mean you always have 16 bits, or 15ppm, of accuracy. Recall that a data converter requires a reference, and accuracy is the degree to which the result conforms to the correct value, measured against a standard or reference. If you put exactly 1V into an ADC, and could resolve exactly that value with 16 bits, but the ADC tells you that it’s 1.2V, that’s only 20% accuracy - a far cry from 15ppm!

At a system level, you can see that the accuracy of the reference voltage itself will be one of the primary factors in the overall accuracy of the system. But the data converter itself contributes some errors. Some data converters will express an overall accuracy specification, often called Total Unadjusted Error, but it is more common to see specifications for offset error, gain error, and linearity.

The ideal transfer function for a data converter is shown in Figure 4, as a blue line. It appears as a staircase because the quantizer can only represent a range of voltages with a single code. The location of the transition point from one code to another is key to describing the accuracy of the converter.

Offset error is the difference between the actual transition point and the ideal transition point. Although this error is measured around a near zero voltage input or output, the error continues throughout the entire transfer function from zero to a full-scale (FS) voltage. Consequently, the offset error near zero is the same as the offset error with a near FS voltage.

Gain error is the difference between the ideal slope of the transfer function and the actual slope between the measured zero point to FS, minus the converter’s offset error. This is actually measured by looking at where the last transition point, at full-scale, occurs, as shown in Figure 5.

As you might imagine from the use of the term “error”, smaller gain and offset errors imply a more accurate converter. It should be pointed out that gain and offset errors can often be corrected for in the digital domain, using the system microcontroller or DSP.

Linearity errors, on the other hand, are much more difficult to correct. While the transfer functions in Figure 4 and Figure 5 show steps that are exactly the same size, in actual converters the width of these steps will vary. Figure 6 shows how differential non-linearity (DNL) is measured - looking at how closely each code corresponds to the ideal 1LSB width.

A DNL error less than –1LSB can cause an entire code to “disappear”, resulting in what is called a missing code. The smaller the range of the DNL specification, the better.

Integral nonlinearity (Figure 7) is the cumulative effect of all of the differential nonlinearity errors, and is the maximum deviation between the actual code transition points and the corresponding ideal transition points, after gain and offset error have been removed. The smaller the INL specification, the more accurate the data converter will be.

Speed, resolution and accuracy are important considerations in selecting a data converter, and you will often find what you need to know about these on the front page of the converter datasheet. But other important factors, such as the details of the digital interface, and mechanical drawings of the package, are further inside the datasheet. You need to read the entire datasheet to effectively use a data converter.

Choosing the Right Data Converter

There is a multitude of data converters available for your design. While the three main specifications of speed, resolution and accuracy will help in the final selection process, narrowing down the options to choose from is the first step.

When selecting an Analog-to-Digital Converter (ADC) for a particular application, a good first selection criteria is to look at the topology of the ADC. Figure 8 shows the how delta-sigma converters are most suitable for higher-resolution tasks, while SAR and pipeline architectures are the ones to look to for higher-speed applications.

Digital-to-analog converters have similar tradeoffs in architecture: delta-sigma DACs tend to be slower, while R-2R, string, and multiplying DACs all provide good general-purpose performance. High frequency, fast settling DACs are generally current-steering architectures.

Knowing the speed and resolution requirements of your application, you can then anticipate which architecture of converter will be most likely to suit your needs. Then, you can look more closely for the converter that matches the accuracy requirements and other features that the application demands.

Table 1 lists some applications where data converters are used, and lists the selection criteria and some examples of actual converter products suitable for those applications.

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This work is licensed by Rick Downs and Tom Hendrick under a Creative Commons Attribution License (CC-BY 3.0), and is an Open Educational Resource.

Last edited by Gene Frantz on Aug 30, 2012 8:45 am -0500.