DAC312

–12–

REV. C

One of the characteristics of an R-2R DAC in standard form is

that any transition which causes a zero LSB change (i.e., the

same output for two different codes) will exhibit the same out-

put each time that transition occurs. The same holds true for

transitions causing a 2 LSB change. These two problem transi-

tions are allowable for the standard definition of monotonicity

and also allow the device to be specified very tightly for INL.

The major problem arising from this error type is in A/D con-

verter implementations. Inputs producing the same output are

now represented by ambiguous output codes for an identical in-

put. Also, 2 LSB gaps can cause large errors at those input lev-

els (assuming 1/2 LSB quantizing levels). It can be seen from

the two figures that the DNL specified D/A converter will yield

much finer grained data than the INL specified part, thus im-

proving the ability of the A/D to resolve changes in the analog

input.

DIFFERENTIAL LINEARITY COMPARISON

D/A Converter with

±1/2 LSB INL, ±1 LSB DNL

Video Deflection by DACs

ENLARGED “POSITIONAL” OUTPUTS

D/A Converter with

±2 LSB INL, ±1/2 LSB DNL

Video Deflection by DACs

ENLARGED “POSITIONAL” OUTPUTS

DIFFERENTIAL VS. INTEGRAL NONLINEARITY

Integral nonlinearity, for the purposes of the discussion, refers

to the “straightness”of the line drawn through the individual re-

sponse points of a data converter. Differential nonlinearity, on

the other hand, refers to the deviation of the spacing of the adja-

cent points from a 1 LSB ideal spacing. Both may be expressed

as either a percentage of full-scale output or as fractional LSBs

or both. The following figures define the manner in which these

parameters are specified. The left figure shows a portion of the

transfer curve of a DAC with 1/2 LSB INL and the (implied)

DNL spec of 1 LSB. Below this is a graphic representation of

the way this would appear on a CRT, for example, if the D/A

converter output were to be applied to the Y input of a CRT as

shown in the application schematic titled “CRT Display Drive.”

On the right is a portion of the transfer curve of a DAC speci-

fied for 2 LSB INL with 1/2 LSB DNL specified and the

graphic display below it.