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Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing. Tim Mewes 5. Computer Interfacing – DAQ cards. V Supl. +. Inverting input. Output. Non- inverting input. V Supl. -. 5.4 Digital to Analog conversion. 5.4.1 Operational Amplifiers Short: “Op-Amp”

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Digital Electronics and Computer Interfacing

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  1. Digital Electronics and Computer Interfacing Tim Mewes 5. Computer Interfacing – DAQ cards

  2. VSupl. + Inverting input Output Non- inverting input VSupl. - 5.4 Digital to Analog conversion 5.4.1 Operational Amplifiers • Short: “Op-Amp” • DC coupled amplifier withtwo inputs and one output • Feedback network determines gain Golden rules (Horowitz & Hill): Voltage Rule: The output attempts to do whatever is necessary to make the voltage difference between the inputs zero. Current Rule:The inputs draw no current. Digital Electronics and Computer Interfacing

  3. R3 R2 A V2 VSupl.+ R1 V1 VOut VSupl. - 5.4.1 Operational Amplifiers Summing amplifier Current Rule  non-inverting input ‘+’ draws no current thus it is at ground potential Voltage Rule  inverting input ‘-’ (and thus A) is at virtual ground potential Current Rule Digital Electronics and Computer Interfacing

  4. VSupl.+ VSupl. - 5.4.2 DAC using scaled resistors 4-Bit Digital to Analog converter R MSB 3 For n bits the largest resistor needs to be 2n-1 times as large as the smallest one! 2R 2 R 4R Resistors need to be veryprecise! 1 8R 0 LSB VOut V Impractical for large numberof bits n Digital Electronics and Computer Interfacing

  5. VSupl.+ VSupl. - 5.4.2 DAC using scaled resistors 4-Bit Digital to Analog converter R 1 MSB 3 How does it work?! 2R Voltage rule: Inverting input ‘-’ is at ground 0 2 R 4R Current rule: 0 1 8R 1 0 LSB VOut V=-8 V 10012=18+04+02+11=910 Digital Electronics and Computer Interfacing

  6. VSupl.+ VSupl. - 5.4.2 DAC using scaled resistors 4-Bit Digital to Analog converter R 0 MSB 3 How does it work?! 2R Voltage rule: Inverting input ‘-’ is at ground 0 2 R 4R Current rule: 1 1 8R 1 0 LSB VOut V=-8 V 00112=08+04+12+11=310 Digital Electronics and Computer Interfacing

  7. 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter 2R 2R Only requires resistance values R and 2R LSB 0 R 2R 1 Resistors still need to be precise! R 2R R 2 R VSupl.+ 2R 3 MSB VOut V VSupl. - Can be used for arbitrarynumber of bits Digital Electronics and Computer Interfacing

  8. 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter 2R 2R How does it work?! 0 LSB 0 R Voltage rule: Inverting input ‘-’ is at ground 2R 0 1 R Current rule: Why?! 2R R 1 2 R VSupl.+ 2R 1 3 MSB VOut VSupl. - V=-16 Volts 11002=18+14+02+01=1210 Digital Electronics and Computer Interfacing

  9. 2R 2R R 2R 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter How does it work?! 0 LSB 0 Why?! R 2R 0 1 R 2R 2R R 1 2 R VSupl.+ 2R 1 3 MSB VOut VSupl. - V=-16 Volts Digital Electronics and Computer Interfacing

  10. 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter How does it work?! 0 LSB 0 Why?! 0 1 2R 2R R 1 2 R VSupl.+ 2R 1 3 MSB VOut VSupl. - V=-16 Volts Digital Electronics and Computer Interfacing

  11. IGND I IVGND 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter How does it work?! Why?! 2R 2R V=-16 Volts R Digital Electronics and Computer Interfacing

  12. 5.4.2 R-2R ladder DAC 4-Bit Digital to Analog converter 2R 2R Summary 0 LSB 0 R 2R 1 1 R 2R R 0 2 R VSupl.+ 2R 1 3 MSB VOut VSupl. - V=-16 Volts 10102=18+04+12+01=1010 Digital Electronics and Computer Interfacing

  13. t • Sampling rate fS 5.4.3 Definitions • Voltage resolution V:smallest voltage difference possible with DACdepends on the number n of bits used: Digital Electronics and Computer Interfacing

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