AD668

REV. A

–13–

Figure 18. Small Signal Bandwidth vs. DC Reference Level

Noise Spectrum

Figure 19 shows the noise spectrum of the DAC with all bits on.

The noise floor of –78 dB is just above the noise floor of the in-

strument being used, in part due to the relatively small (1 V)

output signal of the DAC in voltage output mode.

Figure 19. Noise Spectrum

Analog Feedthrough vs. Frequency

Analog feedthrough is a measure of the effective signal at the

DAC output when all bits are off and a full-scale signal is placed

at the analog input. At dc, the feedthrough is a result of analog

input dependent ground drops, predominantly through the lad-

der ground. Good grounding practices will minimize this effect.

At high frequencies, the signal may propagate to the output

through a variety of capacitive paths. Proper shielding and rout-

ing should be implemented to eliminate external coupling be-

tween the analog input and the DAC output node.

Figure 20. Analog Feedthrough vs. Frequency

Reference Channel THD

THD, or total harmonic distortion, is the ratio of the

rootmean-square (rms) sum of the harmonics to the fundamen-

tal and is expressed in dBs. Figure 21 shows the typical THD of

the AD668 reference channel for both large and small signals.

Figure 21. Reference Channel THD vs. Frequency

TRANSIENT PERFORMANCE

High accuracy settling time measurements of less than one hun-

dred nanoseconds are extremely diliicult to make. The conven-

tional analog amplifiers used in oscilloscope front ends,

typically, cannot recover from the overdrive resulting from a

full-scale step in sufficient time. Sampling scopes can track

much quicker rise times but often provide insufficient accuracy

for 12-bit characterization. Data Precision’s new 640 sampling

scope provides a good combination of speed and resolution that

provides just enough performance to measure the AD668’s

performance.

Digital Settling Time

Figure 22 illustrates the typical settling characteristic of the

AD668 to a full-scale change in digital inputs with the analog

input fixed at 100%. The digital driving circuity is shown in

Figure 23. This circuit allows the DAC to be toggled between

any two codes, and so provides an excellent means of character-

izing both settling and glitch performance.

Figure 22. Typical Digital Settling Characteristics