4-bit Flash Type Analog to Digital Convertor

Analog to Digital Converter (ADC) is one of the most important electronic blocks in electronic signal chain. The reason for the same is that, the physical quantities like temperature, pressure, sound etc are analog in nature and the electronic circuits used to process these physical quantities are digital in nature. For example if we want to make a system that measures temperature and display it on a LCD screen-we use temperature sensor that is analog in nature and a embedded processor to carry out Digital Signal Processing to display the sensed temperature on LCD. In order to do any digital signal processing the incoming data (from temperature sensor in this case) needs to be converted into digital form, and this makes ADC one of the most important electronic component in electronic signal chain.

Today ADCs are available in plenty in a form of a monolithic IC. Most of the modern day microcontrollers today have inbuilt ADCs, hence you can directly input your analog signal to microcontroller but still to understand the concept of Analog to Digital Conversion it is a good exercise to design an A-to-D converter using discrete ICs. This was a project carried out by 2nd year ECE students of our university: Varun Pratap Singh and Gangesh Kumar-Design of a 4-bit Flash Type Analog to Digital Convertor.

There are many ADC architectures that are being used in designing of ADC, some of the popular ones are successive approximation type, sigma-delta ADC, ramp-compare ADC and flash type ADC. The easiest to implement ADC is flash type, it is also the faster but the only limitation is that complexity of circuit increases as the resolution of ADC increases hence it is not possible to have more the 8-bit of resolution with flash ADC. Since these students took up a task of designing a 4-bit ADC, they decided to go for a flash type ADC.

No matter which ADC architecture you pick, there are a certain processes of conversion and characteristics that are common across architecture. A-to-D conversion involves quantization process which means conversion of large number of samples set (continuous wave) in smaller set of samples and sampling process which means periodically taking a samples of analog data and converting it into digital. So there is a term called sampling rate which means the rate at which digital data is generated for the continuous analog wave. According to Nyquist theorem sampling rate should be higher than twice of highest frequency of the analog signal. Resolution of ADC is defined as number of digital outputs we obtain, needless to say higher is a resolution more closely is the digital output related to analog input. Resolution is also defined in terms of analog voltage; it is a minimum voltage jump to cause 1 bit change in the digital output. All these characteristics and parameters discussed here can be easily derived mathematically.

4-bit ADC designed by students here consists of two 3-bit ADCs cascaded together logically. So here I am giving you an overview of 3-bit ADC. As shown in the circuit below 3-bit ADC consists of 7 comparator (23-1), comparator has two inputs, and their comparison is the output, if the voltage at +ve input is higher then -ve, the output of the comparator is higher and vice-versa. In this circuit, -ve terminal of all comparator is connected to reference voltage through a potential divider network (which means each -ve terminal has a fixed value of voltage on it), when we apply a analog signal to be converted into digital to this ADC it is applied to a positive terminal. Now the comparison happens and if +ve terminal voltage for a particular comparator is higher we get 1 at that comparator's output and 0 otherwise. This way we get a digital bit stream containing 8-bit data at the output. This data is fed to 8 to 3 encoder that converts the bit stream into 3 bits.

Now the four bit ADC circuit that is self explanatory is given below. It contains 15 comparators at the input (24-1) and two 8-to-3 encoder. Since the output of encoder we get 6 lines, you notice that encoder lines are logically connect through gates to obtain a 4-bit output. Also note here that value of each resistor is 1KOhms and reference voltage applied 2Volts, It means voltage at -ve terminal of first comparator is 130mV, for second is 2X130mV and so on, hence the voltage resolution for this ADC is 130mV. For each 130mV change in input voltage there is a change of one bit at the digital output containing 4 bits.

By Sagar Juneja-Research Associate, Chitkara University

This ADC was simulated in Multisim, here are the simulated results! Complicated isn't it! This is the only limitation of flash type!

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