Digital to Analog Converters (DAC) - ia Institute ….ppt

Digital to Analog Converters (DAC)15 March 2006Doug HinckleyLee HuynhDooroo KimWhat is a DAC??A digital to analog converter (DAC) converts a digital signal to an analog voltage or current …What is a DAC?101110011010011110000110010101000011001000010000Digital Input SignalAnalog Output SignalTypes of DACs?Many types of DACs available.?Usually switches, resistors, and op-amps used to implement conversion?Two Types:?Binary Weighted Resistor?R-2R LadderBinary Weighted Resistor?Utilizes a summing op-amp circuit?Weighted resistors are used to distinguish each bit from the most significant to the least significant?Transistors are used to switch between Vref and ground (bit high or low)Binary Weighted Resistor?Assume Ideal Op-amp?No current into op-amp?Virtual ground at inverting input?Vout= -IRf-+R2R4R2nRRfVoutIVrefBinary Weighted Resistor?????????????RVRVRVRVRIRV1-nn321ffout242?MSBLSBVoltages V1 through Vn are either Vref if corresponding bit is high or ground if corresponding bit is lowV1 is most significant bitVn is least significant bitI-+R2R4R2n-1RRfVoutVrefV1V2V3VnBinary Weighted ResistorIf Rf=R/2?????????????nn321fout2842VVVVIRV?For example, a 4-Bit converter yields???????????16**********refoutbbbbVVWhere b3 corresponds to Bit-3, b2 to Bit-2, Weighted Resistor?Advantages?Simple Construction/Analysis?Fast Conversion?Disadvantages?Requires large range of resistors (2000:1 for 12-bit DAC) with necessary high precision for low resistors?Requires low switch resistances in transistors ?Can be expensive. Therefore, usually limited to 8-bit -2R Ladder Bit: 0 0 0 0 Each bit correspondsto a switch:If the bit is high, the correspondingswitch is connected tothe inverting input of the op- the bit is low, the corresponding switchis connected to ground. 4-Bit ConverterVoutVref