Three Bit flash Analog to Digital Converter Circuit

Description: The flash type converter is the simplest and fastest type of analog-to-digital converter. The entire digital output word is available immediately after the propagation delay time of the comparators and the encoding logic gates. A typical conversion time for a three-bit flash analog-to-digital converter is 33 ns. The circuit of a three-bit flash analog-to-digital converter requires just three comparators and does not involve any combinational logic circuitry. In this configuration, the reference voltages at the inverting inputs of the comparators are dynamic rather than fixed. They change based on the outputs of other comparators. The non-inverting inputs of comparators A1, A2, and A3 receive an input voltage of up to 5 volts. The inverting terminal of A1 is supplied with VIN/7, which is 0.714 volts, where VIN is the full-scale voltage of 5 volts in the three-bit flash analog-to-digital converter. This is accomplished by providing 2 VIN/7 through a voltage divider formed by resistors R1, R2, R3, and R4 in parallel. Similarly, the inverting terminal of A2 receives 2 VIN/7, or 1.42 volts, achieved by dividing 4 VIN/7 into two parts using a voltage divider composed of resistors R5, R6, and R7 in parallel. When the input voltage is zero, the outputs of all comparators are zero, resulting in a binary output of 000. When the input voltage exceeds the VIN/7 threshold, the output of A1 becomes high, producing a binary output of 001. As the input rises above 2 VIN/7, the output of comparator A2 turns high, activating switch SW1, which connects the other end of R4 to +12 volts. This connection was previously grounded, causing the voltage at the inverting terminal of A1 to increase to 3 VIN/7, or 2.14 volts, resulting in a low output from A1, yielding a binary output of 010. When the input exceeds 3 VIN/7, the output of A1 also becomes high, leading to a binary output of 011. This process continues for higher input voltages, accurately producing a 3-bit binary output. The values of all resistors are calculated to ensure the reference voltage increments correspond to the outputs of the other comparators.

The flash analog-to-digital converter operates by comparing an input voltage against a set of reference voltages generated by the resistors. The comparators are arranged to detect specific voltage levels, which correspond to the binary output states. In this circuit, the dynamic nature of the reference voltages allows for a rapid response to changes in the input voltage, resulting in a quick conversion time of 33 ns for the three-bit configuration.

Resistors R1 to R7 are carefully selected to establish the desired voltage levels at the inverting terminals of the comparators. The use of voltage dividers ensures that the reference voltages are proportional to the full-scale input voltage (VIN), allowing for precise comparisons. Each comparator's output directly influences the subsequent comparator's reference voltage, creating a cascading effect that enables accurate binary representation of the input voltage.

The switch SW1 plays a crucial role in adjusting the reference voltage for comparator A1 when the input voltage crosses specific thresholds. By connecting R4 to +12 volts, the circuit effectively changes the reference point for the next comparison, allowing for a seamless transition between binary states as the input voltage varies.

Overall, the three-bit flash analog-to-digital converter exemplifies a highly efficient design that leverages simple components to achieve fast and accurate digital representation of analog signals. The dynamic reference voltages and minimal component count contribute to the circuit's speed and simplicity, making it an ideal choice for applications requiring rapid analog-to-digital conversion.The flash type converter is the simplest and the fastest type of analogue to digital converter. The entire digital output word is present just after the propagation delay time of comparators and the encoding logic gates. A typical conversion time of three bit flash analog to digital converter is 33 ns. The circuit of three bit flash analog to digi tal converter needs just three comparators and no combinational logic circuitry. In this circuit, the reference voltages on inverting inputs of the comparators are not fixed but dynamic. They change depending upon the state of output of other comparators. Non-inverting inputs of comparators A1, A2 and A3 are given input voltage of 5 volts maximum. Inverting terminal of A1 is given VIN, /7, i. e. 0. 714 volts where VIN is the full scale voltage which is 5 volts in three bit flash analog to digital converter.

This is achieved by providing 2 VIN/7 to voltage divided formed by R1 with R2, R3 and R4 in parallel. Similarly, inverting terminal of A2 gets 2 VIN/7, i. e. 1. 42 volts which is achieved by dividing 4 VIN/7 into two parts by using voltage divider comprising R5 with R6 and R7 in parallel.

When input voltage is zero, outputs of all comparators are zero, giving binary 000. When input voltage crosses VIN/7 level, output of A1 becomes high, giving us binary 001. When input goes slightly above 2 VIN/7, output of comparator A2 becomes high which makes switch SW1 on, thus connecting the other end of R4 to +12V volts. This end was previously connected to ground effectively. Now voltage at inverting terminal of A1 rises to 3 VIN/7, i. e. 2. 14 volts and output of A1 becomes low, giving us output in direct binary 010. When input goes above 3 VIN/7, output of A1 also becomes high giving output 011. Similar process is followed in higher input voltage, giving us exact 3-bit binary. Values of all resistors are calculated by considering the increment in reference voltage with respect to out-puts of other comparators.