Using this parameter, we can define the Signal-to-Noise and Distortion
ratio or SINAD. SINAD can be considered as a parameter similar to SNR
but includes adding the effect of distortion and the noise in the output
of the ADC. SINAD is given by equation (7):
Click on image to enlarge.
The
term SINAD leads us to one of the most important parameters in the
dynamic characteristics of an ADC. It is known as the effective number
of bits (ENoB) of an ADC. ENoB is given by
equation (8) below:
Click on image to enlarge.
Note that
equation (8)
assumes that the test signal used to calculate the SINAD value spanned
over the full scale input voltage range of the ADC. In this condition,
it gives a true representation of effective number of bits in the output
of ADC.
SINAD and ENoB are very important parameters for
analysing the noise performance of an ADC. Other parameters such as THD
and SNR give the individual impact of harmonics and noise on the output
of an ADC. However, for all practical applications, both of these
factors are equally important. Therefore, ENoB or SINAD, which consider
the combined effect of noise and distortion together, is the parameter
to look for when comparing the noise performance of an ADC for
general-purpose applications.
As the name suggests, ENoB gives
the effective number of bits of the ADC output. This number is always
less than the resolution of ADC. The difference between ENoB and
resolution of ADC will represent the amount of noise and distortion
added by the ADC. The closer the ENoB is to the resolution of an ADC,
the better the noise performance of the ADC.
In this part, we
concentrated on the noise which is present inherently in an ADC. In
other words, an ADC would add this noise on its own to the analog signal
while converting it to a digital signal. Practically, the ADC would add
more noise than the noise figures indicate. A very common source of
such noise is the ADC reference. Overall, ADC noise cannot be less than
the reference noise. So, if an internal reference is being used in an
ADC, its specifications must be examined carefully to ensure that the
overall noise does not cause the required ENoB to go drop below the
system’s requirements.
In
upcoming parts of this series,
we will talk about single-ended vs differential ADCs, input impedance
specifications followed by various ADC architectures, and how to select
an ADC based on the application.
For convenience here are pdfs of
part 10 and
part 11 of the series dealing with
ADC noise issues.
About the authors:
Sachin Gupta
is working as Product Marketing Engineer 2 with Cypress Semiconductor.
He holds a Bachelor’s degree in Electronics and Communications from Guru
Gobind Singh Indraprastha University, Delhi. He has several years of
experience in mixed signal application development. He can be reached at
sgup@cypress.com.
Akshay Phatak is an Applications Engineer
with Cypress Semiconductor. He holds a Bachelor's degree in Electronics
and Telecommunications form College of Engineering, Pune (India). He
likes to work on mixed-signal embedded systems. He can be reached
at akay@cypress.com.
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