Analog to Digital Conversion
analog: continuously valued signal, such as
temperature or speed, with infinite possible
values in between
digital: discretely valued signal, such as integers,
encoded in binary
analog-to-digital converter: ADC, A/D, A2D;
converts an analog signal to a digital signal
Analog signals – directly measurable quantities
in terms of some other quantity
• Thermometer – mercury height rises as
• Car Speedometer – Needle moves farther
right as you accelerate
Digital Signals – have only two states. For digital
computers, we refer to binary states, 0 and 1.
“1” can be on, “0” can be off.
• Light switch can be either on or off
• Door to a room is either open or closed
ADC Basic Principle:
• The basic principle of operation is to use the
comparator principle to determine whether or
not to turn on a particular bit of the binary
• It is typical for an ADC to use a digital-toanalog converter (DAC) to determine one of
the inputs to the comparator.
The number of possible states that the
converter can output is:
where n is the number of bits in the AD
Example: For a 3 bit A/D converter, N=23=8.
Analog quantization size:
Q=(V max -V min)/N = (10V – 0V)/8 = 1.25V
Analog Digital Conversion
• Quantizing - breaking down analog value is a
set of finite states
• Encoding - assigning a digital word or number to
each state and matching it to the input signal
Step 1: Quantizing
You have 0-10V signals.
Separate them into a set
of discrete states with
1.25V increments. (How
did we get 1.25V?
(Discussed in previous slide)
Step 2. Encoding
• Here we assign the
digital value (binary
number) to each state
for the computer to
Output Binary Equivalent
• It is a process of taking a sufficient number of
discrete values at point on a waveform that
will define the shape of waveform.
• The more samples you take, the more
accurately you will define the waveform.
• It converts analog signal into series of
impulses, each representing amplitude of the
signal at given point…….
• Consists of a series of comparators, each one
comparing the input signal to a unique
• The comparator outputs connect to the inputs
of a priority encoder circuit, which produces a
How Flash Works
• As the analog input voltage exceeds the
reference voltage at each comparator, the
comparator outputs will sequentially saturate
to a high state.
• The priority encoder generates a binary
number based on the highest-order active
input, ignoring all other active inputs.
• Simplest in terms of
• Most efficient in terms of
speed, very fast
limited only in terms of
comparator and gate
• Lower resolution
• For each additional
output bit, the number of
comparators is doubled
i.e. for 8 bits, 256
Dual Slope ADC
• Also known as Counter-Ramp or Digital Ramp ADC
• A dual slope ADC is commonly used in
measurement instruments (such as DVM’s).
Dual Slope ADC circuit
Dual Slope Function
• The Dual Slope ADC functions in this manner:
– When an analog value is applied the capacitor begins to
charge in a linear manner and the oscillator passes to
– The counter continues to count until it reaches a
predetermined value. Once this value is reached the
count stops and the counter is reset. The control logic
switches the input to the first comparator to a reference
voltage, providing a discharge path for the capacitor.
– As the capacitor discharges the counter counts.
– When the capacitor voltage reaches the reference
voltage the count stops and the value is stored in the
Successive approximation ADC
• Much faster than the
digital ramp ADC
because it uses digital
logic to converge on
the value closest to the
• A comparator and a
DAC are used in the
Successive Approximation ADC
• A Successive Approximation Register (SAR) is
added to the circuit
• Instead of counting up in binary sequence,
this register counts by trying all values of bits
starting with the MSB and finishing at the LSB.
• The register monitors the comparators output
to see if the binary count is greater or less
than the analog signal input and adjusts the
ADC Types Comparison
ADC Resolution Comparison
Medium – Fast
Examples of A/D Applications
• Microphones - take your voice varying pressure waves in the air
and convert them into varying electrical signals
• Strain Gages - determines the amount of strain (change in
dimensions) when a stress is applied
• Thermocouple – temperature measuring device converts
thermal energy to electric energy
• Digital Multimeters