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 V DD V EE RfRf R 2R R R LSB MSB DAC 3.3

R/2R Circuit V DD V EE RfRf R 2R R R LSB MSB What value of R does the op-amp input sence? DAC 3.4

Resistor Equivalent R 2R R R R EQ 2R R 1 DAC 3.5

Resistor Equivalent R EQ 2R R 1 1 V DD V EE RfRf LSB DAC 3.6

R/2R Circuit – Current Perspective V DD V EE RfRf R 2R R R LSB MSB 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 Red: Ideal Blue: Error Green: Error Typical Causes: wrong V DD /V EE at op amp wrong R REF 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: Ideal Blue: Error Green: Error Volts Binary Code DAC 3.12

Offset Error Red: Ideal Blue: 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: Ideal Blue: Error Volts Binary Code DAC 3.14

Non-Linearity: Non-Monotonic Red: Ideal Blue: 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: Ideal Blue: Error Volts Binary Code DAC 3.16

Non-Linearity: Differential Red: Ideal Blue: 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. Godin prgodin gmail.com END DAC 3 DAC 3.19