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EE597G Presentation:
Design of Digital-to-Analog Converter
Qin Chen
Yong Wang
Dept. of Electrical Engineering
Feb. 6th, 2006
What’s DAC?
Smpl
Object
A/D
Digital
System
D/A
Ctrl
Object
• A device that converts digital signals to analog
signals
• It’s usually used to control analog devices, such
as actuator, audio and video output.
Weighted-Resistor DAC
– VOUT is a weighted sum of
V3, …, V0 with weights
proportional to the
conductances G3, .., G0.
– If X3:0 is a binary number
we want conductances in
the ratio 8:4:2:1.
–Fast: gate slew rate ≈ 3 V/ns.
– We can scale the resistors
to give the output
impedance we want.
– Not good for many bits DAC
R-2R Ladder
• resistances of the two
branches at V1 both equal 2R
so the current into this node
will split evenly.
• using only two resistor values,
can generate a whole series of
currents where In=2nI0.
• From the voltage drop across
the
• horizontal resistors, we see
that
• Vn = 2RIn = 2n+1RI0 . For an Nbit ladder the input voltage is
therefore I0=2–N Vin/R.
Current-Switched DAC
• Total current into summing junction is X3:0 ×I0,
Vout = X3:0 × Vin /16R × (–Rf)
• Use CMOS transmission gates as switches: adjust ladder resistors
to account for switch resistance.
• All the switch output terminals are at 0 V. Ladder outputs are always
connected either to ground or to a virtual earth.
Advantages:
• Very fast, No need to charge/discharge node capacitances
• Only two value resistors can satisfy many bits DAC
Glitches in DAC output voltages
• Switches in DAC operate at different speeds ⇒ output
glitches occur when several input bits change together:
0111->1000
• Glitches are very noticeable on a video display:
Deglitching with sample/hold circuit
• To minimize the effect
of glitches:
– Use a register to
make inputs change
as simultaneously as
possible
– Use a sample/hold
circuit to disconnect
the DAC output while
it is changing
Chip Specifications
•
•
•
•
•
•
Resolution: 8 bits
No. of channels: 2
Interface: Parallel
Output type: Voltage
Reference: External
Supply voltage: Single, 5V
Chip Specifications (cont.)
•
•
•
•
•
•
Power consumption (Max. 200mW)
Update rate (100MHz)
Settling time (10μs)
Area (1mm2)
Integral Nonlinearity ( 1 LSB)
Differential Nonlinearity (1 LSB)
Chip Diagram
-
DAC A
+
Input(8 bit)
Input Latch
A
Out A
Input Latch
B
DAC B
+
Address
Control
Reference
Address
Out B
Project Schedule
Week 1
Systematic design of chip
Week 2
Systematic design of chip (cont.)
Week 3
Circuit design
Week 4
Circuit design (cont.)
Week 5
Layout design
Week 6
Layout design (cont.)
Week 7
Final adjustments and verification
Week 8
Final adjustments and verification
(cont.)