Download Digital to Analog Converters (DAC)

Digital to Analog Converters
(DAC)
2
Technician Series
©Paul Godin
Created March 2015
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 2.2
R/2R Circuit
LSB
MSB
Rf
2R
2R
2R
2R
R
R
2R
VDD
R
VEE
The resistance seen to the right of any vertical resistor is 2R.
DAC 2.3
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 2.4
Typical DAC chip (DAC 08)
B1-B8: Binary inputs
Io & Io’: Analog Output
V- & V+: Supply Voltages
VLC & Comp: Special Purpose
Most DACs are designed to output current, not voltage. An
external resistor is needed to convert current to voltage. The
value of this resistor can aid in scaling the output.
DAC 2.5
ERRORS IN DAC
DAC 2.6
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 2.7
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 2.8
Gain Error
Typical Causes:
•wrong VDD/VEE at op amp
•wrong RREF value
•wrong resistor network
values
Red: Ideal
Blue: Error
Green: Error
DAC 2.9
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 2.10
Offset Error
Typical Causes:
•op amp improperly offset
Red: Ideal
Blue: Error
Green: Error
DAC 2.11
Non-Linearity: Non-Monotonic
◊ A Monotonic Error occurs when the individual
voltage steps are non-linear.
Red: Ideal
Blue: Error
Volts
Binary Code
DAC 2.12
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 2.13
Non-Linearity: Differential
◊ A Differential Error occurs when the output steps
start to vary in size.
Red: Ideal
Blue: Error
Volts
Binary Code
DAC 2.14
Non-Linearity: Differential
Typical Causes:
•resistor network unbalanced
•binary input voltage values
Red: Ideal
Blue: Error
DAC 2.15
END DAC 2
©Paul R. Godin
prgodin°@ gmail.com
DAC 2.16