Construct a circuit to verify the operation of a flash type A\D converter

Experiment Name: Construct a circuit to verify the operation of a flash type A\D converter.

In this experiment we have to construct a circuit of flash type A/D converter which produces 3-bit binary codes.

Equipments:

1. Digital multimeter,  2. Triple output programmable power supply (HP E3631A): 5V, -5V, 3. Protoboard,  4. Cables and connectors,  5. Resistors: 1kOhm, 6. Potentiometer, 7. Ten LEDs, 8. Quad Comparator LM 339 (quad=four devices in one package) and 9. 47148 priority encoder.

Introduction:

           Analog-to-digital (A/D) converter is a device that for a given analog input voltage Vin produces an n-bit digital word at the output. Also called the parallel A/D converter, this circuit is the simplest to understand.

It is formed of a series of comparators, each one comparing the input signal to a unique reference voltage. The comparator outputs connect to the inputs of a priority encoder circuit, which then produces a binary output.

 Vref is a stable reference voltage provided by a precision voltage regulator as part of the converter circuit, not shown in the schematic. As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state.

The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs. Flash analog-to-digital converters, also known as parallel ADCs, are the fastest way to convert an analog signal to a digital signal.

           Flash ADCs are ideal for applications requiring very large bandwidth; however, they typically consume more than other ADC architectures and are generally limited to 8-bits resolution.

Figure: A typical flash ADC block diagram

For an “N” bit converter, the circuit employs 2N-1 comparators. A resistive driver with 2N resistors provides the reference voltage. The reference voltage for each comparator is one least significant bit (LSB) greater than the reference voltage for the comparator immediately below it. Each comparator produces a “1” when its analog input voltage is higher than the reference voltage applied to it. Otherwise, the comparator in Fig. 3 output is “0”. Comparator design specification is shown in Table I. As showing fig, the flash ADC is Composed of three major components: resistors string, comparators and encoder. The analog input voltage is concurrently compared to the reference voltage levels generated from resistors string and the speed of A/D conversion is therefore maximized. The outputs of comparators from a thermometer code (TC) which is a combination of a series of zeros and a series of ones, e.g., 000…011…111. Because binary code is usually needed for digital signal processing, a encoder, where k is the resolution (bits) of ADCs.

Circuit diagram:

Procedure:

1. Build the flash ADC as shown in Figure 7. Use two LM339 comparators and 74148 priority encoder for building the circuit. LM339 is a quad comparator the needs pull-up resistors to enable output voltages (Pull-up resistors are used in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance is introduced. They may also be used at the interface between two different types of logic devices, possibly operation at different power supply voltages.

When the switched is open the voltage of the gate input is pulled up to the level of Vin. When the switch is closed, the input voltage at the gate goes to ground.

A pull-up resistor weakly “Pulls” the voltage of the wire it is connected to towards its voltage source level when the other components on the line are inactive.)

Figure: Flash ADC circuit diagram

2. Note the voltage and value of resistors required to build the circuit. Vary the input voltage using a potentiometer R9 and make Vref=4V.

3. Record the values of the input voltage when each LED switches on. To do this, connect the U8002 power supply instead of the input signal and vary the input slowly to 4V. Note down the value of the voltage when each of the LED lights up.

4. Disconnect the power supply and connect the signal generator and set the signal output as shown in figure.