n our daily life, anything we deal like sound, pressure, 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 converters come to play. Usually we call them ADC (Analog to digital converter). 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 conversion methods:

  • Digital-Ramp ADC
  • Successive Approximation ADC
  • Flash ADC

Digital-Ramp ADC


Digital Ramp ADC Block

Conversion 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 ADC


Illustration 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.


Flow chart for Successive Approximation ADC

Flash ADC

pull-right clearfix

Flash ADC Block Diagram

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.

Commonly Used ADCs

Now 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)
ADS1115 16-Bit ADC - 4 Channel with Programmable Gain Amplifier
MAX1204 5V, 8-Channel, Serial, 10-Bit ADC with 3V Digital Interface
AD571 10-Bit, A/D Converter, Complete with Reference and Clock
MAX1202 5V, 8-Channel, Serial, 12-Bit ADCs with 3V Digital Interface
MAX195 16-Bit, Self-Calibrating, 10us Sampling ADC
Note: List may be old :)

See Also

Help & Queries

If you have any queries, doubts or feedback on this tutorial please share in our discussion forum. If you want us to write tutorial for more devices please let us know in the forum.

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This page was last modified on 3 March 2015, at 14:54.
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