ADC using 0808

Analogue-to-digital con-verter (ADC) using a microprocessor requires hardware and sofware, resulting in increased complexity and cost.

The circuit of A-to-D converter shown here is configured around ADC 0808, avoiding the use of a microprocessor. The ADC 0808 is an 8-bit A-to-D converter, having data lines D0-D7. It works on the principle of successive approximation. It has a total of eight analogue input channels, out of which any one can be selected using address lines A, B and C. Here, in this case, input channel IN0 is selected by grounding A, B and C address lines.

Usually the control signals EOC (end of conversion), SC (start conversion), ALE (address latch enable) and OE (output enable) are interfaced by means of a microprocessor. However, the circuit shown here is built to operate in its continuous mode without using any microprocessor. Therefore the input control signals ALE and OE, being active-high, are tied to Vcc (+5 volts). The input control signal SC, being active-low, initiates start of conversion at falling edge of the pulse, whereas the output signal EOC becomes high after completion of digitisation. This EOC output is coupled to SC input, where falling edge of EOC output acts as SC input to direct the ADC to start the conversion.

As the conversion starts, EOC signal goes high. At next clock pulse EOC output again goes low, and hence SC is enabled to start the next conversion. Thus, it provides continuous 8-bit digital output corresponding to instantaneous value of analogue input. The maximum level of analogue input voltage should be appropriately scaled down below positive reference (+5V) level.
The ADC 0808 IC requires clock signal of typically 550 kHz, which can be easily derived from an astable multivibrator constructed using 7404 inverter gates. In order to visualise the digital output, the row of eight LEDs (LED1 through LED8) have been used, wherein each LED is connected to respective data lines D0 through D7. Since ADC works in the continuous mode, it displays digital output as soon as analogue input is applied. The decimal equivalent digital output value D for a given analogue input voltage Vin can be calculated from the relationship.

Sawtooth Generator

Sawtooth wave signal can generated using several ways. One of the very popular method is by using constant current source to charge a capacitor. If you’ve seen many method that use an SCR or its bipolar transistor equivalent circuit to discharge the capacitor, here you can see an alternative method using 555 IC. Here is the schematic diagram of the circuit:

The output pin (pin 3) is connected to pin 5 through a diode, and this configuration make one input the internal comparator is forced down to almost zero level at the discharging cycle, forcing the capacitor to discharge below this level to switch the state to the charging cycle. The period of this sawtooth wave signal is equal to the value of C capacitor and the value of R resistor. You should keep the current through R fairly low, since the current will be shorted to ground at the discharging cycle of the capacitor. The current can be computeb by the formula: i=(Vz-Vbe)/R. Where Vz is the zener diode voltage (2.7V) and Vbe is the forward bias base-emitter voltage (0.7V) of the transistor. Keep the current below 20 mA for the best result.

8038 Signal Generator

ICL8038 signal generator chip is manufactured by Intersil. It can be used to produce three types of waveforms, sine, square and triangle. The frequency, amplitude and duty cycle can be varied, and selection of waveform is done digitally. To further reduce the complexity, a 3-to-1 switch may be used in place of the digital selection circuitry. Digital selection mechanism may be used as switches available on the market are prone to dirt accumulation and poor contact quality.