digital to analog converters (dac) 3 ©paul godin created march 2008
TRANSCRIPT
Digital to Analog Converters (DAC)
3
©Paul GodinCreated 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
VDD
VEE
Rf
R2R
2R 2R 2R 2R
R R
LSB MSB
DAC 3.3
R/2R Circuit
VDD
VEE
Rf
R2R
2R 2R 2R 2R
R R
LSB MSB
1000
What value of R does the op-amp input sence?
DAC 3.4
Resistor Equivalent
R
2R2R 2R 2R
2R
R R
1
000RRR2||)R)R2||)R)R2||R2(((( EQ
REQ
2R
R
1
DAC 3.5
Resistor Equivalent
REQ
2R
R
1
2R
2R
1
VDD
VEE
Rf
LSB
V5.2RR2V5
VOUT
DAC 3.6
R/2R Circuit – Current Perspective
VDD
VEE
Rf
R
2R
2R 2R 2R 2R
R R
LSB MSB
The resistance seen to the right of any vertical resistor is 2R.
I21
I21
I
I41
I41
I81
I81
I161
I161
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: IdealBlue: Error
Green: Error
Volts
Binary Code
DAC 3.10
Gain Error
Red: IdealBlue: Error
Green: Error
Typical Causes: •wrong VDD/VEE at op amp•wrong RREF value•wrong resistor network values
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: IdealBlue: Error
Green: Error
Volts
Binary Code
DAC 3.12
Offset Error
Red: IdealBlue: Error
Green: Error
Typical Causes: •op amp improperly offset
DAC 3.13
Non-Linearity: Non-Monotonic
◊ A Monotonic Error occurs when the individual voltage steps are non-linear.
Red: IdealBlue: Error
Volts
Binary Code
DAC 3.14
Non-Linearity: Non-Monotonic
Red: IdealBlue: Error
Typical Causes: •incorrect input binary sequence caused by mix-up at binary input or a stuck input
DAC 3.15
Non-Linearity: Differential
◊ A Differential Error occurs when the output steps start to vary in size.
Red: IdealBlue: Error
Volts
Binary Code
DAC 3.16
Non-Linearity: Differential
Red: IdealBlue: Error
Typical Causes: •resistor network unbalanced•binary input voltage values
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
©Paul R. Godinprgodin°@ gmail.com
END DAC 3
DAC 3.19