Digital-to-Analog Converter
Appendix A
STDL130
A-1
Samsung ASIC
Digital-to-Analog
Converter
1. Resolution
- An n-bit binary converter should be able to provide 2
n
distinct and
different analog output values corresponding to the set of n-bit binary words. A
converter that satisfies this criterion is said to have resolution of n bits. The
smallestoutputchangethatcanberesolvedbyalinearDACis2
-n
ofthefull-scale
span.
2. Accuracy
- Error of a DAC is the difference between the actual analog output
and the output that is expected when a given digital code is applied to the
converter. Source of error include gain error, offset error, linearity error and noise.
Error is usually commensurate with resolution, less than 2
-(n+1)
, or 1/2 LSB of full
scale.
Figure 1-1.
Error of DAC
3. LSB (Least-Significant Bit)
- In a system in which a numerical magnitude is
representedbyaseriesofbinarydigits,theLSBisthatbitthatcarriesthesmallest
value or weight. It represents the smallest analog change that can be resolved by
an n-bit converter.
LSB (Analog Value) = FSR/2
n
FSR = Full-Scale Range, n = number of bits
4. MSB (Most-Significant Bit)
- The binary digit with the largest numerical
weighting. Normally, the MSB of a digital word has a weighting of 1/2 the full
range.
5. Compliance-Voltage Range
- For a current output DAC, the maximum range
of(output) terminal voltage for which the device will provide the specified current-
output characteristics.
6. Glitch
- A glitch is a switching transient appearing in the output during a code
transition. Its value is expressed as a product of voltage (V
×
ns) or current (mA
×
ns) and time duration or charge transferred.
7. Harmonic Distortion (and Total Harmonic Distortion)
- The DAC is driven
by the digitized representation of sine wave. The ratio of the RMS sum of the
harmonics of the DAC output to the fundamental value is the THD. Usually only
the lower order harmonics are included, such as second through fifth.
2
+
--V2
log
=
V1: RMS amplitude of the fundamental
8. Signal-to-Noise Ratio (SNR)
- This signal to noise ratio depends on the
resolution of the converter and automatically includes specifications of linearity,
distortion, sampling time uncertainty, glitches, noise, and settling time. Over half
Offset Error
Ideal
Actual
Analog
Output
Digital Input
Analog
Output
Digital Input
Ideal
Actual
Gain Error
THD
20
V3
2
V4
2
V1
V5
2
+
+
)
1 2