AD7868

–10–

REV. B

AD7868

DAC

LOW-PASS

FILTER

16-BIT

DIGITIZER

MICRO-

CONTROLLER

Figure 11. AD7868 DAC Dynamic Performance Test Circuit

The digitizer sampling is synchronized with the DAC update

rate to ease FFT calculations. The digitizer samples the DAC

output after the output has settled to its new value. Therefore, if

the digitizer were to sample the output directly it would effec-

tively be sampling a dc value each time. As a result, the dynamic

performance of the DAC would not be measured correctly. Us-

ing the digitizer directly on the DAC output would give better

results than the actual performance of the DAC. Using a filter

between the DAC and the digitizer means that the digitizer

samples a continuously moving signal and the true dynamic per-

formance of the AD7868 DAC output is measured.

Figure 12 shows a typical 2048 point Fast Fourier Transform

plot for the AD7868 DAC with an update rate of 83 kHz and an

output frequency of 1 kHz. The SNR obtained from the graph is

73 dBs.

Figure 12. AD7868 DAC FFT Plot

Some applications will require improved performance versus fre-

quency from the AD7868 DAC. In these applications, a simple

sample-and-hold circuit such as that outlined in Figure 13 will

extend the very good performance of the DAC to 20 kHz. Other

applications will already have an inherent sample-and-hold

function following the AD7868 DAC output. An example of

this type of application is driving a switched-capacitor filter

where the updating of the DAC is synchronized with the

switched-capacitor filter. This inherent sample-and-hold

function also extends the frequency range performance.

Performance versus Frequency

The typical performance plots of Figures 14 and 15 show the

AD7868’s DAC performance over a wide range of input fre-

quencies at an update rate of 83 kHz. The plot of Figure 14 is

without a sample-and-hold on the DAC output while the plot of

Figure 15 is generated with a sample-and-hold on the output.

AD7868*

LDAC

Q

ADG201HS

S1

D1

IN1

AD711

*ADDITIONAL PINS OMITTED FOR CLARITY

R2

2k2

C9

330pF

1µs

ONE

SHOT

DELAY

R1

2k2

LDAC

Figure 13. DAC Sample-and-Hold Circuit

FREQUENCY – kHz

80

70

0

05

1

234

40

30

20

10

60

50

Figure 14. DAC Performance vs. Frequency (No Sample-

and-Hold)

FREQUENCY – kHz

80

70

0

0

20

5

10

15

40

30

20

10

60

50

Figure 15. DAC Performance vs. Frequency (Sample-and-

Hold)