A to D Convertors work to convert analog (continuous, infinitely variable) signals to digital (discrete-time, discrete-amplitude) signals. In more practical terms, an ADC converts an analog input, such as a microphone collecting sound, into a digital signal.

1. Department of Mechanical Engineering
Shri Ramdeobaba college of Engineering and Management, Nagpur
Instrumentation and Control
Semester / Session – VI / 2020-2021
Topic – A to D Convertors
TA 3 - Group – 12
Group Members:
56 – Rishi Agrawal
57 – Ritik Singh
58 – Sagar Chaudhari
59 – Sahil Telani
60 – Sangeet Khule
Course Co-ordinator:
Dr. P.S. Deole

2. What is A to D convertor
Working of A to D convertor
Types of ADC’s
Successive Approximation ADC
Delta Sigma ADC
Dual Slope ADC
Flash ADC
Pipelined ADC
Applications of ADC’s
Testing
Calibration and Need for Calibration
References

3. •A data converter which allows digital circuits to interface with the real world by encoding an analogue signal into a binary
code.
•It allows micro-processor controlled circuits, Arduinos, Raspberry Pi, and other such digital logic circuits to communicate
with the real world.
•In the real world, analogue signals have continuously changing values which come from various sources and sensors which
can measure sound, light, temperature or movement, and many digital systems interact with their environment by
measuring the analogue signals from such transducers.
•Digital circuits on the other hand work with binary signal which have only two discrete states, a logic “1” (HIGH) or a logic
“0” (LOW).
•So it is necessary to have an electronic circuit which can convert between the two different domains of continuously
changing analogue signals and discrete digital signals, and this is where Analogue-to-Digital Converters (A/D) come in.

4. • Basically an analogue to digital converter takes a snapshot of an analogue voltage at one instant in time and
produces a digital output code which represents this analogue voltage.
• The number of binary digits, or bits used to represent this analogue voltage value depends on the resolution
of an A/D converter.
• ADCs follow a sequence when converting analog signals to digital. They first sample the signal, then quantify
it to determine the resolution of the signal, and finally set binary values and send it to the system to read the
digital signal.
• Two important aspects of the ADC are its sampling rate and resolution.
• Microcontrollers can’t read values unless it’s digital data. This is because microcontrollers can only see
“levels” of the voltage, which depends on the resolution of the ADC and the system voltage.

5. Fig 1: Working of ADC’s

6. There are basically five major
types of ADCs in use today:
Successive Approximation (SAR)
ADC
Delta-sigma (ΔΣ) ADC
Dual Slope ADC
Pipelined ADC
Flash ADC

8. The “bread and butter” ADC of the DAQ world is the SAR ADC (Successive Approximation Register). It
offers an excellent balance of speed and resolution and handles a wide variety of signals with
excellent fidelity.
Advantages: Simple circuit with only one comparator needed
Higher sample rates possible compared to delta-sigma ADCs
Handles natural and unnatural waveshapes well
Disadvantages Anti-aliasing filtering must be added externally
Bit resolution and dynamic range limited compared to delta-sigma ADCs
Applications Applications for SAR ADCs include DAQ systems, from low-end multiplexed ADC
systems to higher speed single ADC per channel systems, industrial control and
measurement, CMOS imaging.

9. A newer ADC design is the delta-sigma ADC (or delta converter), which takes advantage of DSP technology in order to
improve amplitude axis resolution and reduce the high-frequency quantization noise inherent in SAR designs. The complex
and powerful design of delta-sigma ADCs makes them ideal for dynamic applications that require as much amplitude axis
resolution as possible. This is why they are commonly found in audio, sound and vibration, and a wide range of high-end
data acquisition applications and in precision industrial measurement applications.
Advantages High-resolution output (24-bits)
Over-sampling reduces quantization noise
Inherent Anti-aliasing filtering
Disadvantages Limited to around 200 kS/s sample rate
Do not handle unnatural shape waveforms as well as SAR
Applications Applications for Delta-sigma ADCs include data acquisition, especially noise and vibration,
industrial balancing, torsional and rotational vibration, power quality monitoring, precision
industrial measurements, audio and voiceband, communications.

10. Fig 2: SAR Block Diagram
Fig 3: Delta Sigma ADC Block DIagram

11. Dual slope ADCs are accurate but not terribly fast. The principle way they convert analog to digital values is by using an
integrator.
The voltage is input and allowed to “run up” for a period of time. Then a known voltage of the opposite polarity is applied
and allowed to run back down to zero. When it reaches zero, the system calculates what the input voltage had been by
comparing the run-up time with the run-down time, and by knowing what the reference had been. The run-up and run-
down times are the two slopes for which this technique has been named.
• Very precise and accurate measurements
Advantages
• Slow conversion time due to the ramp-up and ramp-down iteration
Disadvantages
• Applications for dual slope ADCs include handheld and benchtop
multimeters.
Applications

12. • Flash ADCs are fast and operate virtually without latency, which is why they are the architecture of choice
when the highest possible sample rates are needed. They convert analog to a digital signal by comparing it
with known reference values. The more known references that are used in the conversion process, the more
accuracy can be achieved. For example, if we want a Flash ADC with a 10-bit resolution, we would need to
compare the incoming analog signal against 1024 known values. The 8-bit resolution would require 256
known values, and so on.
• Advantages
• Fastest ADC type
• Instant conversion without latency
• Disadvantages
• Circuit gets bigger and more power-consuming with each bit
• The resolution effectively limited to 8-bit
• Applications
• Applications for Flash ADCs include the fastest digital oscilloscopes, microwave measurements, fiber
optics, RADAR detection, and wideband radio

13. Fig 4: Dual Slope ADC
Fig 5: Flash ADC

14. • For applications that require higher sample rates than SAR and delta-sigma ADCs can provide, but
which do not require the ultra-high-speed of the Flash ADCs, we have Pipelined ADCs.
• As discussed in the previous section, in a Flash ADC, the comparators are all latched simultaneously,
hence the lack of latency. But this requires a lot of energy - especially when more and more
comparators are used to achieve higher bit resolution. However, in a Pipelined ADC, the analog
signal is not latched by all comparators at the same time, spreading out the energy required to
convert the analog to a digital value. Hence the flash comparators are “pipelined” into a quasi-serial
process of 2-3 cycles. This has the benefit of allowing higher resolutions to be achieved without
huge energy, but it imposes two penalties: sample rates cannot be as high as a pure Flash
approach, and there is a latency of typically of 3 cycles. This can be mitigated somewhat, but can
never be completely eliminated.
• Advantages
• Almost as fast as a pure Flash ADC type (faster than SAR and Delta-sigma)
• Cons
• Latency due to serial “pipelined” conversion process
• Maximum sample rate limited by bit resolution
• Applications
• Applications for Pipelined ADCs include digital oscilloscopes, RADAR, software radios, spectrum
analyzers, HD video, ultrasonic imaging, digital receivers, cable modems, and Ethernet.

15. Fig 6: Pipelined ADC

16. Each ADC technology has its place. And because applications are so different, it is impossible to say one is better than
another overall. However, it is absolutely possible to say one of them is better than another with respect to one or more
of today’s DAQ applications requirements:

17. 1) Used together with the Transducer :- Ideally, the span of the amplified sensor
signal should fill this range. The voltage needed to power this device is a single 5-V supply.
The amplified pressure transducer signal is connected to an ADC. Since the ADC connects
to a microprocessor or DSP, final calibrations can be done in software.

18. 2) Used in Computer to convert the analog signal to digital signal :- if an
analog input is sent to a computer, an analog-to-digital converter (ADC) is required. This
device can take an analog signal, such as an electrical current, and digitize it into a
binary format that the computer can understand. A common use for an ADC is to
convert analog video to a digital format.

19. 3) Used in Microcontrollers :- An analog-to-digital converter (ADC) is used to convert
an analog signal such as voltage to a digital form so that it can be read and processed by a
microcontroller. Most microcontrollers nowadays have built-in ADC converters. It is also
possible to connect an external ADC converter to any type of microcontroller.

20. 4) Used in digital storage oscilloscopes :- The input signal from the attenuator and Y
preamplifier is digitized by an analog to digital converter (ADC or A/D), so that it can be
stored in the acquisition memory. There are two main types of ADC used in digital storage
oscilloscopes: the successive approximation register (SAR) type and the flash converter.

21. 5) Used in scientific instruments :- Since the real world is analog, but the computer
world is binary, we need to be able to convert signals between the two. Devices that change
an analog signal to a digital signal are called analog to digital converters (ADC).

22. 6) Used in music reproduction technology :- ADCs may also be used to convert analog
audio streams. The accuracy of the audio conversion depends on the sampling rate used in
the conversion process. Higher sampling rates provide a better estimation of the analog
signal, and therefore produce a higher-quality sound.

23. Testing an Analog to Digital Converter requires an analog input source and hardware to send control
signals and capture digital data output. Some ADCs also require an accurate source of reference
signal.
The key parameters to test an ADC are:
• DC offset error
• DC gain error
• Signal to noise ratio (SNR)
• Total harmonic distortion (THD)
• Integral nonlinearity (INL)
• Differential nonlinearity (DNL)
• Spurious free dynamic range
• Power dissipation

24. • The fundamental 1/2-bit quantization uncertainty associated with analog-to-digital conversion, ADC testing
is more difficult than DAC testing.
• Owing to the need for determing both the output code and the transition point, referred to the input, rather
than simply measuring an output response to a predetermined code.
• The effects of noise (occurring in either the signal or the converter, or picked up in the wiring) are to
introduce an uncertainty in the precise determination of the analog input values at which the output code
transitions take place, and to, in effect, increase the quantization band.
• The nature of these quantization and noise uncertainty errors is shown in Figure .

25. • A flowchart of a calibration firmware in the AVR is given. It uses the
external DAC through the test fixture and runs its own calibration
algorithm.
• There is no need for using several ADC channels, only switching
between single ended and differential conversion. The ADC parameters
are the same regardless of which channel is used. The multiplexer does
not introduce any errors.
• This piece of firmware is programmed into the AVR prior to calibration,
and is replaced by the actual application firmware afterward. Once
again, the EESAVE fuse must be programmed to preserve the
calibration parameters in EEPROM during Flash reprogramming.

26. • The total error of the actual ADC comes from more than just quantization error.
• For most applications, the ADC needs no calibration when using single ended conversion. The
typical accuracy is 1-2 LSB, and it is often neither necessary nor practical to calibrate for better
accuracies.
• However, when using differential conversion the situation changes, especially with high gain
settings. Minor process variations are scaled with the gain stage and give large parameter
differences from part to part. The uncompensated error is typically above 20 LSB. These variations
must be characterized for every device and compensated for in software.
• At first sight 20 LSB seems to be a large value, but using simple calibration algorithms, accuracies of
typically 1-2 LSB can be achieved.

27. • https://en.wikipedia.org/wiki/Analog-to-digital_converter
• https://www.analog.com/media/en/training-seminars/design-handbooks/Analog-Digital-
Conversion-1977/Chapter2-4.pdf
• http://ww1.microchip.com/downloads/en/appnotes/atmel-2559-characterization-and-
calibration-of-the-adc-on-an-avr_applicationnote_avr120.pdf
• Measurement System, Application and Design, E.O. Doebelin, Mc Graw Hill Education.