Download `Flash` Analog-to-Digital Conversion

’Flash’ Analog-to-Digital Conversion
A circuit that accepts an analog voltage (or current) and produces a numerical value (in binary) that
represents the value of the voltage (or current) is known as an analog-to-digital converter (ADC).
There are a number of different circuit topologies that perform ADC, each with its own set of
advantages and disadvantages. The ‘flash’ ADC topology has the advantage that when a new
analog voltage is applied to its input, an updated digital result occurs very quickly (it has a very low
‘conversion time’ (i.e., the amount of time to ‘convert’ an analog voltage to its corresponding digital
value)). The flash ADC topology has a very low conversion time because it only takes two
propagation delays from a new analog input voltage to a new digital result – whereas most of the
other ADC topologies require a clock signal and a circuit that must go through some number of states
before an updated digital result occurs. The disadvantage of the flash ADC topology is that it requires
a large number of ‘comparators’ if the digital result is to have a moderate number of bits (e.g., 255
comparators are required to get a full 8 bits of digital result (known as 8-bit ‘resolution’)).
The flash ADC topology requires a number of ‘comparators’ where a comparator compares two
voltages (e.g., VA and VB) and reports, on its output, if VA > VB, or not. Note that the ‘+’ input to the
comparator is known as the ‘non-inverting input’ (not the ‘positive’ input) and the ‘–‘ input is known
as the ‘inverting input’ (not the ‘minus’ input).
The symbol used for a comparator is the same symbol used for an ‘op-amp’ (‘operational amplifier’).
Op-amps can be used as comparators – but most comparators cannot be used as op-amps since
most comparators have ‘open-collector’ outputs. Also, op-amps are generally used to create a circuit
with ‘negative feedback’, which comparator circuits would not have.
’Flash’ Analog-to-Digital Conversion
The following truth table shows the values on the W, X, Y, and Z comparator outputs for various
Analog Input Signal (AIS) voltage ranges for the flash ADC circuit on the prior slide:
Analog Input Signal
W
X
Y
Z
Digital result
AIS < 1.0v
0
0
0
0
000 = 010
1.0v < AIS < 2.0v
1
0
0
0
001 = 110
2.0v < AIS < 3.0v
1
1
0
0
010 = 210
3.0v < AIS < 4.0v
1
1
1
0
011 = 310
AIS > 4.0v
1
1
1
1
100 = 410
Note that the priority encoder ‘looks’ for the highest numbered input with a ‘1’ on it, and, as such, it
will react to the highest-lettered comparator output with a ‘1’ on it (e.g., when the AIS is between 3.0
volts and 4.0 volts (4th line in the truth table from the top), the priority encoder will react to the ‘1’ on
the Y comparator output).
The four-bit code on the WXYZ outputs is not considered a true binary code since there are only five
possibilities on the four bits (a normal binary code would have 16 possibilities on a 4-bit code). The
sequence of codes on the WXYZ outputs is often called a ‘thermometer code’ since the 1’s go up
and down in response to the Analog Input Signal’s voltage.
Note that the flash-type ADC on the prior slide would get a full three bits of resolution for its digital
result (results 0 – 7 instead of just 0 – 4) if there were seven comparators. This can be extended:
Number of bits of resolution (n)
3
4
8
Number of comparators needed (2n – 1)
7
15
255