ADC interfacing with Microcontrollers: Introduction
►Introduction In our daily life, anything we deal like sound, prassure, voltage or any measurable quantity, are usually in analog form So what if we want to interface any analog sensor with our digital controllers? There must be something that translate the analog inputs to digital output, and so Analog to digital convertors come to play. Usually we call them ADC (Analog to digital convertor). Before going to learn how to interface an ADC with a controller we first take a look at basic methods of analog to digital conversion.
This is a sample of the large number of analog-to-digital conversion methods. The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.
Following are the most used converion methods:
- Digital-Ramp ADC
- Successive Approximation ADC
- Flash ADC
►Digital-Ramp ADCConversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard. A common way to do that is to apply the analog voltage to one terminal of a comparator and trigger a binary counter which drives a DAC. The output of the DAC is applied to the other terminal of the comparator. Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when its voltage exceeds the analog input. The transition of the comparator stops the binary counter, which at that point holds the digital value corresponding to the analog voltage.
►Successive Approximation ADCIllustration of 4-bit SAC with 1 volt step size
The successive approximation ADC is much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage. A comparator and a DAC are used in the process. A flowchart explaning the working is shown in the figure below.
►Flash ADC
Illustrated is a 3-bit flash ADC with resolution 1 volt (after Tocci). The resistor net and comparators provide an input to the combinational logic circuit, so the conversion time is just the propagation delay through the network - it is not limited by the clock rate or some convergence sequence. It is the fastest type of ADC available, but requires a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. The encoder logic executes a truth table to convert the ladder of inputs to the binary number output.
Now we lets take a look at the various Analog to Digital convertors that are most commonly used with our controllers
| Name | Description |
| ADC0800 | 8-bit ADC |
| ADC0801 | 8-bit ADC 100us 0.25 LSB |
| ADC0802 | 8-bit ADC 100us 0.5 LSB |
| ADC0804 | 8-bit ADC 100us 1.0 LSB |
| ADC0808 | 8-bit 8 channel 100us ADC |
| ADC0809 | 8-Bit 8 channel ADC (=~ADC0808) |
| AD571 | 10-Bit, A/D Converter, Complete with Reference and Clock |
| MAX1204 | 5V, 8-Channel, Serial, 10-Bit ADC with 3V Digital Interface |
| MAX1202 | 5V, 8-Channel, Serial, 12-Bit ADCs with 3V Digital Interface |
| MAX195 | 16-Bit, Self-Calibrating, 10us Sampling ADC |
More information on how to interface the above listed ADC can be obtained from the datasheets of respective ICs. In the next part of tutorial we will look into the interfacing and programming of a simple 8-bit ADC (ADC0804).
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