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 digital 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.
To the third comparator 4 VIN/7, i.e. 2.857 volts is directly given.
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.
Analogue switches IC2 are used to provide exact 12 volts.
In this table outputs at expected and observed values are given, which clearly shows accuracy of this crude circuit idea.
The voltage divider used to get fixed reference voltage is shown in circuit diagram three bit flash analog to digital converter.
|Step Voltage||Binary Output||Desired Voltage||Observed voltage||% Error|
PARTS LIST OF THREE BIT FLASH ANALOG TO DIGITAL CONVERTER
Resistors (all ¼-watt, ± 5% Carbon)
R1, R5 = 33 KΩ
R2 = 115.6 KΩ
R3 = 69.2 KΩ
R4 = 138.6 KΩ
R6 = 64 KΩ
R7 = 68 KΩ
R8, R9 = 1 KΩ
R10, R11 = 100 Ω
R12 = 560 Ω
VR1 = 100 Ω
IC1 (A1 – A3) = LM324
IC2 (IC2a – IC2b) = CD4066