Simple Analog to Digital Converter

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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  • Mark Penrice

    OK, so that’s shunting out 8 bits of unsynchronised data at 550khz (though effectively about 63.75khz as that’s the rate at which it strobes through the 8 bits being compared) with no real clear reading on which of the great many output states is the “actual” sample…

    Which is, like, cool and all, but I’m having trouble figuring out the point? Isn’t the idea of connecting it to some kind of processor or at least microcontroller (or, heck, a small 8×4-bit ROM and a 3-bit stepped address generator tied to the same clock that runs through a sequence of outputs which assert Start Conversion at cycle 0, and then End Conversion and Output Enable (and maybe the Address Latch as well, but I’m not 100% sure what it’s even for as I don’t remember it in the last description I saw of a successive approximator) at cycle 7 (in fact, don’t even need the ROM, you could do it all with a pair of ORs and a clocked flip-flop or just a NOR in parallel with the second OR (same difference)) before looping endlessly.

    Which then gives you something that runs at 550khz but only presents the output data (which you can then use for … something … IDK … input to a microcontroller that doesn’t have its own ADCs?) to the actual output lines at 63.75khz, and on a known and determinable clock cycle which makes it easy to interface with some external 8-bit bus without having a hideous amount of noise and general uncertainty.

    In fact, as I’ve been wondering how to put together something that can turn audio (…which I know isn’t the primary focus of this, but there’s not THAT many other inputs which need such temporal resolution; you could use it with a couple of trimpots and an 8-bit-to-3x 7-seg chip and matching display to make the worlds most insanely responsive digital thermometer?) into 8bit data for feeding into a simple parallel IO port on an old computer (lol, chiptunes), this could do the job quite nicely. If it responds well to underclocking, anyway, that is… (I feel that simply taking every other sample, or every third, fourth, etc in order to reduce the input rate might cause quite a lot of aliasing noise which would otherwise be smoothed out by the converter itself if it simply ran more slowly and the clock was divided instead… and 63.8khz is pretty extreme for the intended use; in fact, 31.9 would be somewhat unnecessary…)

    Question is, if we’re after something period-sympathetic, how long ago were chips like this first made?