Data Acquisition

Like the weighted-current source network, this DAC produces an output current I T,proportional to the input code and the voltage reference source. The principle of operation ofthe ladder network relies on the binary divisions of the current as it flows through the ladderresistance network. A simple resistance calculation at point A shows that the resistance to theright adds up to 2R, and the resistance to the left is 2R. Hence the current flowing in theresistive leg of the MSB is I 0 = V REF /3R. At node A, this current splits, half flowing to the leftof node A and half flowing into node B. At node B the current splits in half again, halfflowing into node C and half flowing to ground through the resistance 2R in the leg of this,the next most significant bit. This continues, with the current from the LSB being divided by2 n when it reaches the summing junction of the operational amplifier. The same analysis canbe applied for each switch that connects the voltage reference source to the ladder network,with the current contributed by each finally being added at the operational amplifier’ssumming junction.The main advantages, which make this type of DAC popular, are the easy matching ofresistances (R or 2R), the constant input resistance for the output amplifier, and the fact thatlow resistor values can be used, thus ensuring high-speed operations.5.8.2 Parameters of D/A convertersMost of the performance parameters and errors associated with A/D converters are applicableto D/A converters. In addition, several specifications for D/A converters determine thequality of the output signal produced. These are settling time, slew rate, and resolution.ResolutionThis is a measure of the size of the output step associated with a change of 1 LSB at the input.A greater number of bits, in the digital input code generating the analog output, reduce themagnitude of each output voltage increment. This allows the D/A converter to generate amore smoothly changing output signal for applications, where there is a wide dynamic outputrange.Output rangeOutput from a D/A converter can be in two forms, current, and voltage. If a DAC produces acurrent output where the application requires an output voltage, an external operationalamplifier is required.The feedback resistors that would be used to set the offset, gain, and therefore range of theoutput, are usually provided within the D/A converter. Internal resistors are provided whichtrack the temperature characteristics of the internal resistors of the ladder network. Thiseliminates the need to use an external resistance, which may introduce tracking errors. Ifmore than one feedback resistor is provided, a choice of analog output ranges is available.Bipolar output voltage ranges are usually obtained by simply on-setting the unipolar offsetvoltage, with an internal bipolar offset resistor.The selection and range of a unipolar or bipolar output range is commonly made withjumper connections.Input data codesThere are a number of ways in which the digital data can be presented to D/A converters. Thetype of coding (i.e. binary, binary offset, two’s complement, BCD, arbitrary etc), and itssense (positive true and negative true) must be applicable to the D/A converter used.