Digital to Analog Converters (DAC) 3 ©Paul Godin Created March 2008.

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


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.


Green: Error

Volts

Binary Code

DAC 3.12

Offset 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.


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

DAC 3.15

Non-Linearity: Differential

◊ A Differential Error occurs when the output steps start to vary in size.


Volts

Binary Code

DAC 3.16

Non-Linearity: Differential


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

Digital to Analog Converters (DAC) 3 ©Paul Godin Created March 2008.

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Transcript of Digital to Analog Converters (DAC) 3 ©Paul Godin Created March 2008.