High-Performance, Stand-Alone ADCs for a Variety of Embedded Systems Applications
2 ADC Converter Function Pack Design Guide
ADC Converter Function Pack Design Guide 3
High-Performance, Stand-Alone ADCs for a Variety of Embedded Systems Applications
ADC BY ARCHITECTURE – SAR CONVERTERS
SAR (Successive Approximation Register) applies to the
converter that uses approximation to convert the analog input
signal into a digital output code. SAR converters typically lie
in the 8- to 16-bit range and can have sample speeds up to 1
MSPS.
One major benefit of a SAR converter is its ability to be
connected to multiplexed inputs at a high data acquisition
rate. The input is sampled and held on an internal capacitor,
and this charge is converted to a digital output code using the
successive approximation routine. Since this charge is held
throughout the conversion time, only the initial sample and
hold period or acquisition time is of concern to a fast-changing
input. The conversion time is the same for all conversions. This
makes the SAR converter ideal for many real-time applications,
including motor control, touch-screen sensing, medical and
other data acquisition systems.
When designing a system with a SAR converter, the following
specifications and terms should be understood:
Acquisition Time – The time required for the sampling
mechanism to capture the voltage after the sample command
is given for the hold capacitor to charge.
Conversion Time – The time required for the A/D converter
to complete a single conversion once the signal has been
sampled.
Throughput Rate or Samples Per Second (SPS) – The time
required for the converter to sample, acquire, digitize, prepare,
and output a conversion.
Integral Non-Linearity (INL) – Specification most relevant
to the overall accuracy of the converter. INL is the maximum
deviation of a transition point of a conversion to the
corresponding transition point of an ideal conversion. INL
represents cumulative DNL errors.
Differential Non-Linearity (DNL) – The error in width between
output conversion codes. The maximum deviation in code
width from the ideal 1 LSB code width (FSR/2^n). DNL errors
of less than –1 correspond to a missing code.
Missing Code – The situation where an A/D converter will
never output a specified code regardless of the input voltage.
Monotonic – Implies that an increase (or decrease) in the
analog input voltage will always produce no change or an
increase (or decrease) in output code. Monotonicity does not
imply that there are no missing codes.
Bipolar vs. Unipolar Output – Differential converters give a
bipolar output corresponding to positive and negative numbers.
The binary output scheme is usually two’s complement. A
unipolar output corresponding to a positive output, from 0 to
V
REF
.
Successive Approximation Register (SAR) ADC
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