Digital to Analog Conversion DAC

1
Digital to Analog Conversion - DAC

• Input - Digital number
• Output Analog/continuous voltage

2
R/2R Ladder Network

• A 3 bit example
• Switches represent binary state
• S2 S1 S0
• S0 0 when open
• S0 1 when closed
• S0 is lsb least significant bit
• S2 is msb most significant bit

3
R/2R ladder operation

• Current injected - switches active
• Currents split at T nodes
• Operational amplifier
• OpAmp
• Input current 3 components
• Weighting 1/2 1/4 1/8
• Switch S2 S1 S0

4
DAC positive output

• Refer to diagram of DAC
• Uses unsigned numbers
• 000 represents 0 volts output
• 111 represents max output
• 111 -- full-scale
• Number of bits n
• Number of steps 2n

5
DAC equation

• The input output relationship is
• Vout -Vr ( 4/8S2 2/8S1 1/8S0 )
• Vout -Vr ( 4/81 2/81 1/81 )
• Vout max is Vr ( 1/21/41/8)
• 7/8 Vr
• Vout -Vr ( 1/20 1/40 1/80 )
• Vout min is Vr ( 000) 0

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3 bit DAC Ranges

• 4/812/811/81 7/8 Max
• 4/812/811/80 6/8
• 4/812/801/81 5/8
• 4/812/801/80 4/8
• 4/802/811/81 3/8
• 4/802/811/80 2/8
• 4/802/801/81 1/8
• 4/802/801/80 0/8 Min

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DACs

• Unipolar DAC
• Typical example 8 bits
• Range 0 to 10Volts
• Step size 10 / 256 volts
• Or 10 1000/256 40 mV

8
16bit DAC

• Range typically 2 volts
• Steps 216 64K
• Output steps size given by
• 2 / (216) 106
• Answer students calculate

9
Biploar DAC

• 4/802/811/81 3/8 POS MAX
• 4/802/811/80 2/8 POS
• 4/802/801/81 1/8 POS
• 4/802/801/80 0/8 ZERO
• 4/812/811/81 -1/8 NEG
• 4/812/811/80 -2/8 NEG
• 4/812/801/81 -3/8 NEG
• 4/812/801/80 -4/8 NEG MIN

10
Bipolar DAC offset

• Bipolar analogue (audio)
• Normal to use - zero offset of 1/2 lsb
• Example 8 bit
• 001
• Threshold 20mv
• 000 noise /- 10mV
• Threshold -20mV
• 111