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AD766

16-Bit DSP DACPORT

Analog Devices 

AD766

system where only a single positive and a single negative supply

are available. In this case, the positive logic and positive analog

supplies may both be connected to the single positive supply.

The negative logic and negative analog supplies may both be

connected to the single negative supply. Performance would

benefit from a measure of isolation between the supplies intro-

duced by using simple low-pass filters in the individual power

supply leads.

Figure 6. Alternate Recommended Schematic

in signal processing applications, the DAC is tested at two

output frequencies and at three signal levels over the full oper-

ating temperature ranges.

A block diagram of the test setup is shown in Figure 8. In this

test setup, a digital data stream, representing a 0 dB, –20 dB or

–60 dB sine wave is sent to the device under test. The frequen-

cies used are 1037 Hz and 49.07 kHz. Input data is latched into

the AD766 at 500 kSPS. The AD766 under test produces an

analog output signal using the on-board op amp for 1 kHz and

an external op amp for 50 kHz.

The automatic test equipment digitizes the output test wave-

form, and then an FFT to 250 kHz is performed on the results

of the test. Based on the first 9 harmonics of the fundamental

1037 Hz and the first 3 harmonics of the 49.07 kHz output

waves, the total harmonic distortion of the device is calculated.

Neither a deglitcher nor an MSB trim is used during the THD

test.

The circuit design, layout and manufacturing techniques em-

ployed in the production of the AD766 result in excellent THD

performance. Figure 9 shows the typical unadjusted THD per-

formance of the AD766 for various amplitudes of 1 kHz and

50 kHz sine waves. As can be seen, the AD766 offers excellent

performance even at amplitudes as low as 60 dB. Figure 10

illustrates the typical THD versus frequency performance from

the internal amplifier for a filtered AD766 output. At frequen-

cies greater than approximately 30 kHz, depending on the low-

pass filter used, an improvement in THD of 3–4 dB over the

performance shown in the figure can be achieved. Figure 11

illustrates the consistent THD performance of the AD766

over temperature.

Figure 7. Power Dissipation vs. Clock Frequency

As with most linear circuits, changes in the power supplies will

affect the output of the DAC. Analog Devices recommends that

well regulated power supplies with less than 1% ripple be incor-

porated into the design of any system using these device.

MEASUREMENT OF TOTAL HARMONIC DISTORTION

The THD specification of a DSP DAC represents the amount

of undesirable signal produced during reconstruction of a digital

waveform. To account for the variety of operating conditions

Figure 9. Typical Unadjusted THD

Figure 8. Distortion Test Circuit

–6–

REV. A

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