Bipolar DAC; weighted resistor DAC drawbacks

Bipolar DAC

A bipolar DAC gives an output of both polarities (−FS/2 to +FS/2) instead of 0 to +FS. It is needed for signals such as audio and for systems that use offset-binary or 2's-complement codes.

Method (offset binary): a unipolar DAC is used and a fixed offset current equal to half of full scale is subtracted at the summing node, so the output is shifted down by FS/2.

 -Vref -> R-2R ladder (b1..bn) --Iout--+
                                       |
 +Vref ----2R---- Ioff ----------------+
                                       |     Rf
                                       +---/\/\/--+
                                       |          |
                                       +--(-)     |
                                 GND -----(+) >---+-- Vo

Ladder current (reference -Vref): Iout = -(Vref/R)·D/2ⁿ
Offset current: Ioff = Vref/(2R)
Vo = -Rf (Iout + Ioff)
   = (Rf/R)·Vref·(D/2ⁿ - 1/2)

With Rf = R, Vref = 8 V and 3 bits: Vo = 8(D/8 − 1/2) = (D − 4) V.

So the MSB acts as a sign bit (1 = positive, 0 = negative). Inverting the MSB converts 2's-complement input to offset binary. The output range is −FS/2 to +FS/2 − 1 LSB.

Disadvantages of the weighted-resistor DAC

• Wide range of resistor values: R, 2R, 4R, …, 2ⁿ⁻¹R. For 12 bits the largest is 2048R, e.g. 10 kΩ to 20.48 MΩ.
• Hard to make accurately in ICs: very large and very small resistors, each needing a different precision, are hard to fabricate and match.
• MSB resistor needs the highest accuracy (tolerance better than 1/2ⁿ), which is difficult and costly.
• Switch resistance error: the switch on-resistance is significant compared with the small MSB resistor.
• Poor temperature tracking of resistors of very different values.
• Current drawn from Vref changes with the input code, loading the reference.
• So it is practical only up to about 6 to 8 bits; the R-2R ladder is preferred.