An analog to digital converter (ADC) as its name indicates is an electronic device which converts continuous time-varying analog signals into discrete-time digital signals so that they can easily be read by the digital devices. It has many applications in electronics projects. ADC converts the physical quantities of a real-world phenomenon into a digital language which is used in control systems, data computing, data transmission, and information processing. The figure below shows the input/ output relationship of an ADC.
Analog to Digital Converter Introduction
Usually, transducers are also used to convert the input analog variables in the form of currents or voltages. Basically, the digital numbers used here are binary i,e ‘0’ and ‘1’. The ‘0’ indicates the ‘off’ state and ‘1’ represents the ‘on’ state. Hence all the analog values are converted into digital binary values by an ADC. For example, if we have to install an alarm in our house or at some facility, whose function is to set off in case of fire or overheating. Our whole alarm system will be electronic but the temperature sensor will give analog values at the output after sensing the temperature. Therefore to convert the varying values of temperature in digital or discrete values, we have to use an analog to digital converter.
A/D Conversion Process and How ADC works ?
Mainly there are two steps for the analog to digital conversion:
- S/H: Sampling and holding
- Q/E: Quantizing and Encoding
The ADC process is shown in figure below:
Sampling and Holding
An analog signal continuously changes with time, in order to measure the signal we have to keep it steady for a short duration so that it can be sampled. We could measure the signal repeatedly and very fast, and then find out the right time scale. or we could measure the signal at different timings and then average it. Or preferably we can hold the signal for a specific duration and then digitize the signal and sample the value. This is done by a sample and hold circuit. For, at least the time required for digitization, it keeps the value stable. Figure shows the circuit for sampling and holding of a signal.
We keep the switch normally open, and when we want to find a measurement, we close the switch momentarily.
Quantizing and Encoding
On the output of (S/H), a certain voltage level is present. We assign a numerical value to it. The nearest value, in correspondence with the amplitude of sampling and holding signal, is searched. And this value cannot be just any value, it should be from a limited set of possible values. It depends on the range of the quantizer and the range in given in a power of 2 i,e 2n (28 = 256, 210=1024 etc).
After identifying the closest value, a numerical value is assigned to it and it is encoded in the form of a binary number. The binary encoded numbers generated by quantizer are represented by ‘n’ bits. The resolution of an ADC can also be denoted by ‘n’ bit.The figure shows the whole conversion process:
The values achieved after the quantization and encoding process cannot be said to be thoroughly accurate. These are only the approximations of the real world values. The accuracy of the quantizer highly depends on the resolution of the qunatizer, greater the resolution, more accurate the values will be.The ADC resolution is limited by a number of constraints, out of which, time is a major issue. If the set of possible values, from which the closest value is to be searched, is greater, then it will surely take more time. But to accelerate this process, more techniques have been developed.
The following table shows the performance of different ‘n’ bit ADCs. If the number of bits is greater, then the frequency is lesser and time consumed is also greater. On the other hand, the error minimizes as the number of bits is increased. The maximum sampling rates have also been indicated in the table.
The most common types of analog to digital converters available are:
- Flash Analog to Digital converter.
- Dual slope Analog to Digital converter.
- Successive Approximation Analog to Digital Converter.
Flash ADC is one of the simplest ADCs. It is also known as the parallel ADC converter.It consists of a number of comparators. An encoder circuit is connected at the output of the comparators which gives us binary output. A flash ADC circuit of 3-bits is shown in the figure:
Vref is the reference voltage; if the analog value at the input becomes greater than the reference voltage then the comparator output will be high.Flash converter is the most efficient of all the converters in terms of speed. But the number of comparators increases as the number of bits increases. We would require 7 comparators for 3-bit and 15 comparators for 4-bits. This is the weakness of flash ADC.
But a flash converter can produce a non-linear output which is an additional advantage. The voltage divider network consists of equal-value resistors which provide a proportional response. But for special applications, the value of the resistors can be changed which will give a non linear response.
DUAL SLOPE ADC
A dual slope integrator first integrates and then disintegrates a voltage signal. It integrates an unknown voltage for a fixed time and disintegrates for variable time using a reference voltage. Figure 5 shows the graph of dual slope integration.
The main advantage is that the error occurring in a component during the integration is cancelled out during the phase of de-integration.Figure below shows a dual slope converter block diagram:
For example, if we want to obtain a resolution of 10 bits, we would integrate for 210= 1024 cycles and then de-integrate for 1024 cycles. By increasing the number of clock cycles, we can obtain more resolution.
SUCCESSIVE APPROXIMATION ADC
This ADC does not count in the binary sequence, this register starts with the most significant bit and finishes at the least significant bit. The comparator’s output is continuously monitored and compared with the analog signal input. This strategy gives much faster results. Figure below shows the working of this successive approximation register:
The operation of this ADC can be observed in the following graph:
So far we have discussed the three most common types of Analog to digital converters but there are other converters also such as sigma delta, digital ramp adc, tracking adc etc which are also widely used converters. You can read this article for in-depth guide on successive approximation ADC:
Either we notice or not, we are using hundred to thousands of ADCs and DACs in our daily life. Some of the popular applications are:
- Audio applications: For example, when listening to music on your mobile phone, music is stored in your mobile phone memory in digital form and a speaker accepts an electrical signal which is an analog signal. Therefore, we need an ADC to convert a digital bit-stream of music into an analog signal to play the music that we can hear through a mobile speaker. Hence, our mobile phone contains many ADCs for Audio and many other applications.
- Call Receiver and Transmitter on mobile phone
- Video Streaming
- Data Acquistion