Biomedical Engineering Reference
In-Depth Information
FIGURE 5-44
Successive
approximation ADC.
unit. Alternatively, it is possible to output clock signal at a specific frequency or with a
particular duty cycle to represent a number.
5.5.3 ADCs and DACs
There are many different ways of converting an analog signal to its digital equivalent. These
are selected depending on the number of digital bits required and the rate of conversion.
Two of the most common, used in commercial designs, are the successive approximation
and flash conversion, though others including single- and dual-slope integration, switched
capacitor, and delta-sigma conversion are implemented for specialist applications.
The successive approximation method is used in bioelectrical applications, for rela-
tively low conversion rates, because it is low cost. As can be seen in the block diagram
shown in Figure 5-44, it consists of a number of modules including a DAC in the feedback
path.
The process of conversion is as follows:
•
At the start of the conversion, the analog signal is latched (captured and held) using
a sample and hold (S&H).
•
The successive approximation register (SAR) sets its most significant bit (MSB),
which causes the DAC to output an analog value voltage equal to
V
max
/
2.
•
The DAC value is compared with the analog input value using a comparator.
•
If the analog signal is still larger than this value, then the output of the comparator
remains high and the bit remains set. If it is smaller, then the output of the comparator
goes low and the bit is reset.
•
The process is then repeated with the next most significant bit and then the one
after, until at last the least significant bit (LSB) has been output.
•
The successive approximation register then contains a digital equivalent of the
analog input signal, which is clocked into the output register and latched so that
the whole process can be repeated.
