Picking The Right A/D Converter For Your Design

The analog to digital converter is a bridge between the analog and digital world, which is used in a wide range of applications where there is a need to convert the analog signal into the digital data. Based on fundamental architectures, there are several A/D converters. They are:

--> Successive Approximation
--> Sigma-Delta
--> Flash
--> Subranging (Pipelined)
--> Bit-per-stage (Ripple)

There are several factors to consider while selecting an A/D converter for your design. The most important parameters to consider are:

--> Resolution
--> Through-put (Speed)
--> Errors (offset, gain & linearity errors)
--> Monotonacity
--> AC Specifications: Such as Signal To Noise And Distortion (SINAD), Effective Number Of Bits (ENOB), Bandwidth

Let's now recapitulate the definitions of some of these factors as we studied during our engineering. Some of the important specifications of ADC are presented below:

Resolution is the smallest analog increment/decrement corresponding to a 1 LSB converter code change. We express resolution in number of bits.

Throughput Rate is the maximum continuous conversion rate of an ADC. This specification indicate the speed of an ADC. This is often expressed in KSPS (Killo samples per second), MSPS (Mega samples per second).

Offset Error is the difference between ideal LSB transition to the actual transition point. This specification is expressed in LSBs.

Gain Error is the difference between the input voltage just causing a transition to positive fullscale and (Vref - 1.5LSB). This specification is expressed in LSBs or % Full Scale.

Differential Non-Linearity (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB. DNL is commonly measured at the rated clock frequency with a ramp input and expressed in LSBs.

Intigral Non-Linearity (INL) is a measure of the deviation of each individual code from a line drawn from zero scale or negative full scale (1⁄2 LSB below the first code transition) through positive full scale (1⁄2 LSB above the last code transition). The deviation of any given code from this straight line is measured from the center of that code value. The end point test method is used. INL is commonly measured at rated clock frequency with a ramp input and expressed in LSBs.

Related to DNL are two critical figures of merit used in defining ADC operation They are:

Missing Code are those output codes that are skipped or will never appear at the ADC outputs. These codes cannot be reached by any input value.

Monotonicity - An ADC is monotonic if it continually increases conversion result with an increasing voltage (and vice versa). A nonmonotonic ADC may give a lower conversion result for a higher input voltage, which may also mean that the same conversion may result from two separate voltage ranges. Often, the transfer function will completely miss the lower code until after the higher code is converted (on an increasing input voltage).

Now, some of the important AC (dynamic) specifications of ADC are presented below:

Analog Input Bandwidth is a measure of the frequency at which the reconstructed output fundamental drops 3 dB below its low frequency value for a full scale input. The test is performed with fIN equal to 100 kHz plus integer multiples of fCLK. The input frequency at which the output is −3 dB relative to the low frequency input signal is the full power bandwidth.

Signal To Noise Plus Distortion (SINAD) is the ratio, expressed in dB, of the rms value of the input signal at the output to the rms value of all of the other spectral components below half the clock frequency, including harmonics but excluding dc.

Spurious Free Dynamic Range (SFDR) is the difference, expressed in dB, between the rms values of the input signal at the output and the peak spurious signal. where a spurious signal is any signal present in the output spectrum that is not present at the input.

Intermodulation Distortion (IMD) is the creation of additional spectral components as a result of two sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the power in the intermodulation products to the total power in the original frequencies. IMD is usually expressed in dB.

Keeping the above parameters in mind, we are ready to examine various applications for which ADCs are used and then we can sort out the most important parameters relevant to the application areas. Most of the A/D converter applications can be categorized into the following major application areas:

• Audio
• Communication (voice band)
• Data Acquisition & Control
• Precision Measurement (e.g. Oscilloscope)
• Image Processing

Now the question arises, how to choose the appropriate A/D converter for a particular application? We can find this out if we can map the ADC parameters relevant to each of the above application areas. The following table shows a relationship between the application areas and the A/D converter parameters important for the respective application areas.

From the Table 1, it can be observed that one parameter of ADC important for one particular application may not be important for other applications. For example, in some of the communication type applications, you're really interested in inter- modulation distortion, whereas in an industrial control loop, that might not factor in as much. You're more interested in the actual linearity or the monotonicity of the converter.

Based on their working principles, each type of ADC has certain advantages, i.e. one ADC category can be superior when compared to their peers based on certain specifications and can be inferior for other parameters. Based on the guidance provided in Table 1 we can concentrate only on the specific parameters important to the respective application area.

On a high level we can classify ADCs according to their suitability for the application categories based only on resolution and sampling rate as depicted in Figure 1. This can serve as a basic guideline for choosing the type of ADC architecture best suitable for the application. Next we can narrow down our choise based on the specifications mentioned in Table 1.

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