TABLE OF CONTENTS Compensating directly the imperfections of the components by tuning them is not always the best solution. Instead of compensating the source of imperfections, it is also possible to act on their consequences. For instance, the offset of an operational amplifier can be tuned by injecting a compensation current into an internal node of the circuit (see chapter 4).
This compensation current injection method is applicable to a large variety of situations. It enables the construction of generic compensation blocks which can be reused in different situations, and a systematic approach for inserting these trimming circuits in the systems to be calibrated.
This chapter presents two different structures of current-mode digital-to-analog converters. In this section, an implementation based on current mirrors is detailed. The rest of the chapter presents another DAC topology based on modified R/2R ladders.
Figure 28 shows the structure of the digital-to-analog converter [23]. The numbers near the transistors represent their respective W/L aspect ratios.
In each stage, the multiple output current mirror (bottom) is designed to divide the current into 2 binary-weighted output currents. The inter-stage current mirrors (top), combined with the input transistor of the next stage, perform a sub-binary current division, with a radix approximately equal to 1.7. Table 4 shows the current division performed by the circuit.
| bit (i) | current (b i) |
|---|---|
| 4 | |
| 3 | |
| 2 | |
| 1 | |
In this example, both stages are designed using the same...
