DAC729

11

FIGURE 12. Differential Linearity Adjustment Circuit for the

4MSBs.

FIGURE 13. 0 to 10V FSR.

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2

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4

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8

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40

39

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18-Bit

DAC

V

OUT

The DAC729 offers superb dynamic range. Dynamic range

is a measure of the ratio of the smallest signals the converter

can produce to the full-scale range, usually expressed in

decibels (dB). The theoretical dynamic range of a converter

is approximately 6dB per bit. For the DAC729 the theoreti-

cal range is 108dB! The actual dynamic range is limited by

noise (signal-to-noise) and linearity errors. The DAC729’s

6

µV typical noise floor, fast settling op amp, and adjustable

18-bit linearity minimize the limitation.

Total harmonic distortion (THD) is the measure of the

magnitude and distribution of the linearity error, differential

linearity error, noise, and quantization error. The THD is

defined as the ratio of the square root of the sum of the

squares of the harmonics to the values of the input funda-

mental frequency. The rms value of a DAC error can be

shown to be

where n is the number of samples in one cycle of any given

sine wave, E

sampling point, and E

sampling point. The THD can then be expressed as

where E rms is the rms signal-voltage level.

This expression indicates that, in general, there is a correla-

tion between the THD and the square root of the sum of the

squares of the linearity errors at each digital word of interest.

However, this expression does not mean that the worst-case

linearity error of the D/A is directly correlated to the THD.

The DAC729 has demonstrated THD of 0.0009% at full

scale (at 1kHz). This is the level of distortion that is desired

to test other professional audio products, making the DAC729

ideal for professional audio test equipment.

The ability to adjust the linearity of the 4MSBs, the 18-bit

resolution, fast settling and low noise give the DAC729

unmatched performance.

AUTOMATIC TEST EQUIPMENT

The pin functions of the DAC729 are convenient for use in

automatic test equipment systems. The ability to use internal

or external reference and internal or external op amp means

versatility for the system designer. For example, in auto-

matic test systems with several DACs and ADCs, it is

desirable to operate all of the high accuracy converters from

the same reference, improving the tracking characteristics of

those components to one another. The reference in the

DAC729 is a very stable precision reference, and is suitable

for use as the system reference.

Test systems, and other large systems are the ideal applica-

tion for a DAC of this accuracy, because the DAC will be

calibrated in the environment in which it will be used. Since

the environment is very stable, the manual calibration (Fig-

ure 12) may be adequate. However, highly automated sys-

tems will go to an automatic calibration routine. Replacing

n

1

ε

∑ [E

2

i =1

n

1.0M

Ω

100k

Ω

0.01µF

(1)

36

–V

CC

1.0M

Ω

100k

Ω

0.01µF

(1)

37

–V

CC

1.0M

Ω

100k

Ω

0.01µF

(1)

38

–V

CC

1.0M

Ω

Bit 1

Adjust

100k

Ω

0.01µF

(1)

39

V

POT

40

–V

CC

21

–V

CC

150k

Ω

150k

Ω

150k

Ω

150k

Ω

NOTE: (1) Low leakage film type.

Bit 2

Adjust

Bit 3

Adjust

Bit 4

Adjust

THD =

=

X x 100%

E

RMS

ε

RMS

E

RMS

i =1

n

1

n ∑

[E

2

(2)