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Normally
analogue-to-digital con-verter (ADC) needs interfacing through a
microprocessor to convert analogue data into digital format. This
requires hardware and necessary software, resulting in increased
complexity and hence the total 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
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