Very high speed and high resolution D/A converters are required in many applications such as VDSL broadband modems or direct digital frequency synthesizers. Current steering D/A converters are excellent candidates because of their intrinsic high speed characteristics. In order to obtain accuracies in the 14b range, special layout techniques or calibration methods must be adopted.
A solution using background calibration has been chosen in this work, because it offers the advantage of constantly correcting for slow varying errors, such as the ones induced by temperature changes. The calibration of the five most significant bits (MSBs) is done with a fully analog trimming loop that works sequentially on the tail current of each MSB current source, while the MSB head is kept connected to the output through the data switches.
Excellent static linearity is achieved by this technique. At 200MHz update rate a peak spurious-free-dynamic-range (SFDR) of 85dBc has been measured.