1433 The Weighted Resistor DA Converter
This type of converter is very similar to the Resistor Ladder D/A converter. In this case, however, each resistor in the string is given a value proportional to the binary value of the bit it represents. Currents are then summed from each active bit to achieve the output (Figure 14-2).
- Figure 14—2. Binary Weighted D/A Converter
The number of resistors and switches reduced to one per bit, but the range of the resistors is extremely wide for high-resolution converters, making it hard to fabricate all of them on the IC. The resistor used for B3 is the limiting factor for power dissipation from VREF to ground.
This converter architecture is often used to make logarithmic converters. In this case, the R, 2R, 4R, 8R ... resistors are replaced with logarithmically-weighted resistors.
This type of converter, and the R/2R converter described in the next paragraph, use a feedback resistor fabricated on the D/A IC itself. This feedback resistor is not an optional convenience for the designer — it is crucial to the accuracy of the D/A. It is fabricated on the same silicon as the resistor ladder. Therefore, it experiences the same thermal drift as the resistor ladder. The gain of the buffer amplifier is fixed, with a full-scale output voltage limited to VREF. If a different full-scale D/A output voltage is needed, change VREF.
If the full scale Vqut must exceed the maximum rating of the D/A reference voltage, use a gain stage after the buffer op amp (see Paragraph 14.7.2).
The op amp must be selected carefully, because it will be operated in much less than unity gain mode for some combinations of bits. This is probably one of the main reasons why this architecture is not popular, as well as the requirement for a wide range of resistor values for high precision converters.
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