Q6Analog Circuits
Question
How D/A converter operates in bipolar range? Explain.
Answer
Bipolar operation in a DAC is achieved using Offset Binary coding and adding a fixed negative reference current to shift the output range.
Standard D/A Converters naturally operate in a unipolar range (e.g., to ). To operate in a bipolar range (e.g., to ), the DAC must be able to output negative voltages for certain digital inputs.
How it operates:
This is typically achieved using Offset Binary or Two's Complement digital encoding, combined with analog hardware shifting.
In hardware, an external fixed reference resistor is used to inject a constant negative offset current (equal to half the full-scale current) into the op-amp's summing junction.
- When the digital input is exactly at half-scale (e.g., 1000 0000 in an 8-bit DAC), the DAC's positive current exactly cancels the negative offset current, producing output.
- Inputs below half-scale produce a net negative current, resulting in negative output voltages.
- Inputs above half-scale overcome the offset, producing positive output voltages.