Spencer,

Both justification methods have advantages and it's up to you what is best.

Left justification requires less housekeeping. Right justification (assuming there is no sign-extend assistance) requires the "stuffing" of the sign bit into the MSB's.

Perhaps some examples example will help.

If you have a 12 bit converter being left justified into a 16 bit word let's design so the unused LSB's are read/written as zero.

The word 800H (midscale 12 bits) if left justified is 8000H (midscale 16 bits).

The word FFFH (max 12 bits) when left justified is FFF0 (nearly full scale 16 bits)

The word 000H (min 12 bits) is 0000H (min 16 bits) when left justified.

Now right justification just asks that you "sign extend" the data so;

800H sign extended to 16 bits is F800H.

FFFH sign extended to 16 bits is FFFFH

000H sign extends to 0000H

Notice that right justification maintains symetry while left justification does not. Also note that this error is very small.

Some folks "lsb extend" to stuff lsb's when left justifying, i do not.

regards,
p