(thing) by trondgs Fri Mar 30 2001 at 19:32:10
As the name implies the Analog to Digital Converter (A/D converter, ADC) is a device for converting an analog voltage into a digital format, so it can be used by a microprocessor or microcontroller system. There exists several different A/D converter types; flashconverters, Successive approximation converters, ramping converter and sigma/delta converters.

The flashconverter consists of several resistors connected in serial. One end of this resistor network is connected to ground, the other to a voltage reference. This network works as a voltage divider with many out-voltages. These voltages are compared with the input signal with comparators. When the signal voltage is higher on one of the comparators it will give out a digital '1'. The comparator outputs are sent to a decoder circuit that converts it to a binary number, that can be read by the ยต-processor. 
Simple diagram of a flashconverter.
(A real converter can have tens or houndreds
of resistor/compapator steps)
     Ref
      |
      |
Sin --+--.
     | | |    __
     | | |   |  |
      |  *-\_|  |
      *--+-/ |  |
      |  |   |D |
     | | |   |e |
     | | |   |c |
      |  *-\_|o |
      *--+-/ |d |
      |  |   |e |
     | | |   |r |
     | | |   |  |
      |  *-\_|  |
      *--+-/ |  |
      |  |   |  |
     | | |   |  |
     | | `-\_|  |
      *----/ |__|
      |
     ---
     GND

   |
  | |  Resistor
  | |
   |

  -\_  Comparator
  -/
The flasconverter has a high sampling speed(converters with up to 1 Ghz exist but up to 100Mhz is the most common), but it's accuracy is low. These are used in digital oscilloscopes and spectrum analysers where the high speed is needed.

The successive approximation converter uses a comparator to compare the input signal to the output from a D/A converter. The controller circuit steps up the D/A output bit for bit. When the two voltages match, the comparators output switches to '1'. Since the D/A output voltage is the same as the input signal voltage, and the output from the D/A is known, the voltage of the signal is also known. This type of converter is slow but very accurate, and is used in high accuracy instruments.

The ramp converter charges a capacitor for a fixed time, then discharges it through a known resistance. Since the discharge time, the capacitor's capacitance and the resistor's resistance is known, the input voltage can be computed. This A/D converter type is somewhat less accurate than the Successive approximation type, and is also quite slow. But it is cheap and is the convertor used in most digital multimeters.

The Sigma/Delta or bit stream converter is a 1 bit converter used in CD players, mobile phones and ADSL modems. The output stream is a pulse width encoded representation of the input signal. A Sigma/Delta converter output bit width is increased by oversampling, but for each oversampling the bandwidth decreases. The Sigma/delta converter is cheap to manufacture and has a good linearity.

I like it!