Data Converters DAC or ADC

Data Conversion is the process of changing or converting one form of data in to another form. In processing and communication there are only two types of data forms i.e analog and digital data. The converter which converts the digital data in to analog data is called analog to digital to analog converter (ADC) and in the same the converter which converts digital to analog is called as DAC.

 

Why data conversion?

In the growing digital world processing and transmitting of digital data became easy & secure with the computer world. Most complicated applications or logic can be easily programmable in the digital computer compared to analog circuits. This enabled the use of converting analog form of data in to digital form. Even though the processing has been done in digital form the final element which has to reflect the data is most probably responds to analog signals compared to digital signals. This utilizes the digital to analog conversion techniques. In a summarized way the analog signals at the input of the system converted to digital form and they are converted back to analog before applying final element i.e. at the output of the system.

Digital To Analog Convertor (DAC)

The converter which converters the digital form of data in to analog form is called digital to analog converter. In this the digital data in the form of 1’s and 0’s are used to control the switches which are placed in a analog circuit with  reference voltage, based on this switch condition (ON/OFF) and position (MSB or LSB) the output analog amplitude is calculated.

Types of Digital to Analog Converters

  1. Binary Weighted Resistor DAC or R-2nR DAC
  2. R-2R Ladder DAC

Specifications of a Digital to Analog Converter (DAC)

Resolution:

The resolution of a DAC is the smallest change in the output of the DAC for any change in digital input.i.e. if a input to DAC changes one bit, how much analog output has changed in full scale deflection.

% resolution = [Step size / Full scale output (FSO)] * 100

In other way the resolution is the number of states into which the full scale output is divided. i.e if a 8 bit DAC can resolve the FSO up to 255 levels. Each level of output is called step size and for higher number of bits the resolution will be better.

% resolution = [1/(2N-1) *100

Normally the resolution will be in Milli volts.

Accuracy

The Accuracy of a DAC is the difference between output practical analog output to the ideal expected output for a given digital input. The DAC is contains electronic components where the gain plays a major role which can introduce gain error in the output. Due to the the full scale output may differ compared to ideal one. For an example if a DAC of 10 V is said to have an accuracy of 0.01% there will be 10mv output deviation. The another factor which implicates the accuracy is the zero offset error i.e for a zero input the output of DAC reflects some offset value.

Conversion Speed

The conversion speed of the DAC is output analog value settling time period for a change in the digital input. This is also called settling time period of DAC. Normally it will be micro seconds and in some advanced micro controller DAC it may be nano seconds.

Monotonicity

The Digital to Analog Converter is said to be monotonic if its analog value is either increasing or equal to previous value for an LSB change in input digital signal.

Analog to Digital Converter

The data converter which converts the data from analog values to digital values is called Analog to Digital converter in shot it is called ADC. In ADC the input signal value is sampled at a particular time interval and compared with the analog value produced by the combination of counter and DAC. If the output of the comparator is zero then the value of the counter will be the output digital value. These are very important converters as the environmental analog signals has too be converted in to digital for processing with the digital computer.

Types of  Analog to Digital Converters:

  1. Simultaneous or Flash A/D Converters
  2. Counter-Type A/D Converter
  3. Tracking-Type A/D Converter
  4. Successive Approximation Type A/D Converter
  5. Single-, Dual- and Multislope A/D Converters
  6. Sigma-Delta A/D Converter

Specifications of ADC

Resolution of ADC:

The resolution of an A/D converter is the quantum of the input analogue voltage change required to increment its digital output from one value to the next higher code value. i.e. if an n bit ADC then it needs 1/ 2n-1 of full scaled output to reflect at the output of ADC. For example The resolution of an eight-bit A/D converter can be expressed as one part in 255 or as 0.4% of full scale or simply as eight-bit resolution. If such a converter has a full-scale analogue input range of 10 V, it can resolve a 40 mV change in input.

Aliasing or sampling frequency

It is the rate at which the rate of the analog value is sampled for digitizing. If the sampling frequency is less than the signal frequency then due to aliasing the ADC output will be distorted and cannot reproduce the exact digital value. So to extract the nice digital output the ADC nyquist frequency should be at least 5 or 10 times greater than the signal frequency.

Step recovery of ADC:

It shows that how quickly an ADC changes its output to match a large, sudden change in the analog input.

 

 

 

Recent Posts