Download Digital to Analog Converters (DAC)

Digital to Analog Converters
(DAC)
3
©Paul Godin
Created March 2008
R/2R DAC
◊ The Binary-Weighted DAC limitations include the
need for progressively larger resistors.
◊ The R/2R ladder method eliminates the need for
the variety of resistors required. It operates on a
ratio between resistors.
◊ The Op Amp is still an important component in the
circuit.
DAC 3.2
R/2R Circuit
LSB
MSB
Rf
2R
2R
2R
2R
R
R
2R
VDD
R
VEE
DAC 3.3
R/2R Circuit
LSB
MSB
1
0
0
0
Rf
2R
2R
2R
2R
R
R
2R
VDD
R
VEE
What value of R does the op-amp input sence?
DAC 3.4
Resistor Equivalent
1
2R
2R
2R
0
R
R
2R
2R
0
R
1
0
2R
R
((((2R || 2R)  R) || 2R)  R) || 2R  R E Q  R
REQ
DAC 3.5
Resistor Equivalent
1
2R
1
2R
R
REQ
Rf
VDD
2R
LSB
VEE
 5V 
VO U T  
R  2.5V
 2R 
DAC 3.6
R/2R Circuit – Current Perspective
LSB
MSB
I
2R
R
1
I
2
2R
1
I
2
2R
1
I
4
R
1
I
4
2R
1
I
8
R
1
I
8
Rf
2R
1
I
16
VDD
1
I
16
VEE
The resistance seen to the right of any vertical resistor is 2R.
DAC 3.7
ERRORS IN DAC
DAC 3.8
Errors
◊ Digital to Analog systems have several possible
sources of error.
◊ The errors can be the result of:
◊ Binary Input
◊ Non-linear input sequences or incorrect values
◊ Voltage issues
◊ Resistor network
◊ Resistors out of specification
◊ Noise
◊ Op Amp output errors
◊ Improper input voltage
◊ Improper Offset
◊ Delay
DAC 3.9
Gain Error
◊ A Gain Error occurs when the Op Amp produces an
output at a different voltage scale than desired.
The output is linear but the steps are either larger
or smaller than they should be. Output may
appear clipped.
Red: Ideal
Blue: Error
Green: Error
Volts
Binary Code
DAC 3.10
Gain Error
Typical Causes:
•wrong VDD/VEE at op amp
•wrong RREF value
•wrong resistor network
values
Red: Ideal
Blue: Error
Green: Error
DAC 3.11
Offset Error
◊ An Offset Error occurs when the Op Amp output
has the same voltage per step but the starting
voltage is different.
Red: Ideal
Blue: Error
Green: Error
Volts
Binary Code
DAC 3.12
Offset Error
Typical Causes:
•op amp improperly offset
Red: Ideal
Blue: Error
Green: Error
DAC 3.13
Non-Linearity: Non-Monotonic
◊ A Monotonic Error occurs when the individual
voltage steps are non-linear.
Red: Ideal
Blue: Error
Volts
Binary Code
DAC 3.14
Non-Linearity: Non-Monotonic
Typical Causes:
•incorrect input binary
sequence caused by mix-up
at binary input or a stuck
input
Red: Ideal
Blue: Error
DAC 3.15
Non-Linearity: Differential
◊ A Differential Error occurs when the output steps
start to vary in size.
Red: Ideal
Blue: Error
Volts
Binary Code
DAC 3.16
Non-Linearity: Differential
Typical Causes:
•resistor network unbalanced
•binary input voltage values
Red: Ideal
Blue: Error
DAC 3.17
Other Types of DACs
◊ MOSFET Current Scaling that use a configuration
of MOSFETs in a similar manner to R/2R.
◊ Voltage Scaled DACs employ a resistor string as a
large voltage divider for reference voltage values,
and use enable/disable circuitry for the output
voltage.
◊ Charge Scaling DACs utilize capacitors instead of
resistors. Can be more accurate and potentially
faster but are more difficult to implement.
DAC 3.18
END DAC 3
©Paul R. Godin
prgodin°@ gmail.com
DAC 3.19