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ALL ABOUT ELECTRONICS.
So, in this video, we will learn about the
Successive Approximation Type ADC.
So, it is one of the most commonly used types
of ADC and it is very suitable for the  general
purpose applications.
And it is very suitable for general purpose
applications.
So, first of all, let's understand the working
of this Successive Approximation type ADC.
So, this is the basic Schematic of this ADC.
And it consists of the comparator, the digital
to analog converter, and the Successive Approximation
Register along with the control circuit.
And to understand the working, let's take
the example of the four bit ADC.
so whenever the new conversion starts, then
this sample and hold circuit samples the input
signal.
And that signal is compared with the output
of the DAC.
So, here let's say the sampled input signal
is equal to 11.2V.
And the reference voltage of the DAC is equal
to 16V.
Now, whenever the new conversion starts then
the successive approximation register sets
the most significant bit to 1 and all other
bits to zero.
That means we can say that the input to the
DAC is equal to 1000.
And for a 16V of the reference voltage, if
you see the corresponding output voltage then
it will be equal to 8V.
which is just half of the reference voltage.
Now, this voltage will get compared with the
input voltage and based on the comparator
output the output of the successive approximation
register will get changed.
That means if Vin is greater than the DAC
output then the MSB will be kept as it is
and the next bit will be set 1 for the new
comparison.
On the other end, if Vin is less than the
DAC output then the MSB will be set to 0 and
then the next bit will be set to 1 for the
new comparison.
Now, here for the code of 1000, the output
of the DAC is equal to 8V.
And as it is less than the input voltage,
so the MSB will be kept as it is and the next
bit will be set to 1.
So, now the output of the DAC corresponding
to 1100 is equal to 12V.
And now once again, this voltage will get
compared with the input voltage.
So, if the input voltage is greater than the
DAC output voltage, then the second bit will
be kept as it is and the third bit will be
set to 1 for the new comparison.
On the other end, if the input is less than
the DAC output then the second bit will be
set to 0 and then the next bit will be set
to 1 for the new comparison.
And here, the output corresponds to 1100 is
equal to 12V.
And as it is greater than the input voltage,
so the second bit will be set to 0 and then
the next bit will be set to 1 for the new
comparison.
So, now the input to the DAC is equal to 1010.
And if you see the corresponding output voltage
then it will be equal to 10V.
So, once again this voltage will get compared
with the input voltage.
And here, as the input voltage is greater
than 10V, so the third bit will be kept as
it is and the next bit will be set to 1 for
the new comparison.
But if that is not the case, then the third
bit would be set to 0 and then the LSB would
be made 1 for the new comparison.
But anyway, here as the input voltage is greater
than the DAC output voltage, so that possibility
will get eliminated.
So, now the input to the DAC is equal to 1011.
And if you see the corresponding output voltage,
then it will be equal to 11V.
So, once again, this voltage will get compared
with the input voltage.
And as the input voltage is greater than 11V,
so the fourth bit will be kept 1.
And finally, this will be the output code
corresponding to the input voltage.
So, in this way, the output of the DAC is
compared with the input voltage and according
to the comparator output, the output of the
successive approximation register is changed
by 1 bit at a time.
And basically, it is started from the most
significant bit.
And as you can see over here, in the four
iterations, we will get the output digital
code corresponding to the input voltage.
And here is the list of all possibilities
which may occur during the four iterations.
So, according to the input voltage, only one
code will get selected at the end.
And if you see the same sequence in the time
domain, then it can be represented as follows.
So, initially, the output of the successive
approximation register is set to 1000.
And if you see the corresponding output of
the DAC, then it will be equal to 8V.
And as it is less than the input voltage,
so the first bit will be kept as it is and
the second bit will be set to 1.
And now if you see the corresponding output
voltage for 1100, then it is equal to 12V.
And as it is greater than the input voltage,
so the second bit will be set to 0 and the
third bit will be set to 1 for the new comparison.
And if you see the corresponding output voltage,
then it is equal to 10V.
Which is once again less than the input voltage.
So, the third bit will be kept as it is and
the fourth bit will be set to 1 for the new
comparison.
So, now corresponding to 1011, the output
of the DAC is equal to 11V.
And as it is less than the input voltage,
so the fourth bit will be kept 1.
So, in this way, after the four iterations,
we will get the digital code corresponding
to the input voltage.
So, here, for each iteration, this ADC will
take one clock cycle.
That means for the four bit ADC, it will take
4 clock cycles.
And in general, we can say that for N-bit
ADC it will take N- clock cycles.
That means the conversion time of this ADC
is equal to N times Tclk.
And unlike the other ADCs which we have discussed
earlier, the conversion time of this ADC is
independent of the input voltage.
Now, if we talk about the resolution of this
ADC, then this ADC provides the typical resolution
in the range of 8 to 16 bits.
But there is successive approximation type
of ADCs are available which can provide the
resolution up to 20 bits.
Now, if we talk about the conversion speed
of this ADC then the maximum speed supported
by this ADC is in the rage of 2 to 5 Mega
Samples per Second.
But some ADCs, for example, LTC2368 supports
up to 10 Mega Samples per Second.
So, this ADC provides good accuracy and requires
less power.
Apart from that, they are easy to use and
they have low latency time.
So, basically, this latency time is the time
between the beginning of the signal acquisition
and the time when the data is available to
download from the converter.
So, typically this latency is defined in the
seconds.
But is also defined in the number of data
or the conversion cycles.
So, if the sampled data is available within
the one conversion cycle, then we can say
that the ADC has zero cycle latency.
On the other end, if the data is available
after the N- conversion cycle, then we can
say that the ADC has N- cycle latency.
So, typically this successive approximation
type ADC has zero latency.
That means in the applications where the measured
signal information is required immediately
then this type of ADC can be used.
So, that's is for this video, and I hope in
this video you understood the working of the
successive approximation type ADC.
So, if you have any question or suggestion,
do let me know here in the comment section
below.
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