1 A/D Converter Figures of Merit
The number of output bits from an analog-to-digital converter do not fully specify its behavior.
Real A/D converters can differ from ideal behavior in many ways. While static imperfections,
such as gain and offset, are easy to quantify, the success of many signal-processing applications
depends on the dynamic behavior of the A/D converter. Ultimately, the application determines
the requirements, and A/D converter resolution may not be either necessary or sufficient to specify
the required performance. In many cases, the quality of the A/D converter must be tested for the
specific application.
The wide variety of analog-to-digital converter applications leads to a large number of figures
of merit for specifying performance. These figures of merit include accuracy, resolution, dynamic
range, offset, gain, differential nonlinearity, integral nonlinearity, signal-to-noise ratio, signal-to-
noise-and-distortion ratio, effective number of bits, spurious-free dynamic range, intermodulation
distortion, total harmonic distortion, effective resolution bandwidth, full-power bandwidth, full-
linear bandwidth, aperture delay, aperture jitter, transient response, and overvoltage recovery.
These specifications can be loosely divided into three categories — static parameters, frequency-
domain dynamic parameters, and time-domain dynamic parameters — and are defined in this
section.
1.1 Static Parameters
Static parameters are the A/D converter specifications that can be tested at low speed, or even
with constant voltages. These specifications include accuracy, resolution, dynamic range, offset,
gain, differential nonlinearity, and integral nonlinearity.
1.1.1 Accuracy
Accuracy is the total error with which the A/D converter can convert a known voltage, including
the effects of quantization error, gain error, offset error, and nonlinearities. Technically, accuracy
should be traceable to known standards (for example, NIST), and is generally a “catch-all” term
for all static errors.
1.1.2 Resolution
Resolution is the number of bits, N, out of the A/D converter. The characteristic in Figure 1
shows a 3-bit A/D converter. Probably the most noticeable specification, resolution determines the
size of the least significant bit, and thus determines the dynamic range, the code widths, and the
quantization error.
1.1.3 Dynamic Range
Dynamic range is the ratio of the smallest possible output (the least significant bit or quantum
voltage) to the largest possible output (full-scale voltage), mathematically 20 log
10
2
N
≈ 6N.
1.1.4 Offset Error
Offset error is the deviation in the A/D converter’s behavior at zero. The first transition voltage
should be 1/2 LSB above analog ground. Offset error is the deviation of the actual transition voltage
from the ideal 1/2 LSB. Offset error is easily trimmed by calibration. Compare the location of the
first transitions in Figures 1 and 2.
3
- 1
- 2
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