02-07-2012, 05:38 PM
AD and DA Converter
Introduction
The real time embedded controller is expected to process the real world signals within a specified time. Most of the real world signals are analog in nature. Take the examples of your mobile phone. The overall architecture is shown on Fig.18.1. The Digital Signal Processor (DSP) is fed with the analog data from the microphone. It also receives the digital signals after demodulation from the RF receiver and generates the filtered and noise free analog signal through the speaker. All the processing is done in real time.
The processing of signals in real time is termed as Real Time Signal Processing which has been coined beautifully in the Signal Processing industry.
The DA Converter
A digital word (8-bits or 16-bits) can be converted to its analog equivalent by weighted averaging. Fig. 18.5(a) shows the weighted averaging method for a 3-bit converter.
A switch connects an input either to a common voltage V or to a common ground. Only switches currently connected to the voltage source contribute current to the non-inverting input summing node. The output voltage is given by the expression drawn below the circuit diagram; SX = 1 if switch X connects to V, SX = 0 if it connects to ground. There are eight possible combinations of connections for the three switches, and these are indicated in the columns of the table to the right of the diagram. Each combination is associated with a decimal integer as shown. The inputs are weighted in a 4:2:1 relationship, so that the sequence of values for 4S3 + 2S2 + S1 form a binary-coded decimal number representation. The magnitude of Vo varies in units (steps) of (Rf/4R)V from 0 to 7. This circuit provides a simplified Digital to Analog Converter (DAC). The digital input controls the switches, and the amplifier provides the analog output.
Quantizer
The hold circuit tries to maintain a constant voltage till the next switching. The quantizer is responsible to convert this voltage to a binary number. The number of bits in a binary number decides the approximation and accuracy.
The sample hand hold output can assume any real number in a given range. However because of finite number of bits (say N) the levels possible in the digital domain 0 to 2N-1 which corresponds to a voltage range of 0 to V volts.
Coder
This is an optional device which is used after the conversion is complete. In microprocessor based systems the Coder is responsible for packing several samples and transmitting them onwards either in synchronous or in asynchronous manner. For example in TI DSK kits you will find the AD converters with CODECs are interfaced to McBSP ports (short form of Multi-channel Buffered Serial Ports). Several 16-bit sampled values are packed into a frame and transmitted to the processor or to the memory by Direct Memory Access (DMA). The Coder is responsible for controlling the ADC and transferring the Data quickly for processing. Sometimes the Codec is responsible for compressing several samples together and transmitting them. In your desktop computers you will find audio interfaces which can digitize and record your voice and store them in .wav format. Basically this AD conversion followed by coding. The wav format is the Pulse-Code-Modulated (PCM) format of the original digital voice samples.