Apparatus having corresponding methods comprise: a first fixed current source to produce a first fixed current; a second fixed current source to produce a second fixed current; a first adjustable current source to produce a first adjustable current; a first switch to deliver the first fixed current and the first adjustable current to a first output node when the first switch is closed; a second switch to deliver the second fixed current to the first output node when the second switch is closed; and an adjustment circuit to adjust the first adjustable current according to the first fixed current, the second fixed current, and the adjustable current.