TOTAL HARMONIC DISTORTION + NOISE

The THD figure of an audio DAC represents the amount of un-

desirable signal produced during reconstruction and playback of

an audio waveform. The THD specification, therefore, provides

a direct method to classify and choose an audio DAC for a de-

sired level of performance.

By combining noise measurement with the THD measurement,

a THD+N specification is realized. This specification indicates

all of the undesirable signal produced by the DAC, including

harmonic products of the test tone as well as noise.

Analog Devices tests all AD1862s on the basis of THD+N per-

formance. In this test procedure, a digital data stream represent-

ing a 0 dB, –20 dB or –60 dB sine wave is sent to the device

under test. The frequency of the waveform is 990.5 Hz. Input

data is sent to the AD1862 at an 8

× F

AD1862 under test produces an output current which is con-

verted to an output voltage by an external amplifier. Figure 10

illustrates the recommended test circuit. Deglitchers and trims

are not used during this test procedure. The automatic test

equipment digitizes 4096 samples of the output test waveform,

incorporating 23 complete cycles of the sine wave. A 4096 point

FFT is performed on the test data.

V

OUT

AD1862

TOP VIEW

(Not to Scale)

1

2

3

4

5

6

7

89

10

11

12

13

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15

16

Figure 9. External Amplifier Connections

Based upon the harmonics of the fundamental 990.5 Hz test

tone, and the noise components in the audio band, the total har-

monic distortion + noise of the device is calculated. The

AD1862 is available in two performance grades. The AD1862N

produces a maximum of 0.0025% THD+N at 0 dB signal lev-

els. The higher performance AD1862N-J produces a maximum

of 0.0016% THD+N at 0 dB signal levels.

SIGNAL-TO-NOISE RATIO

The Signal-to-Noise Ratio (SNR) of the AD1862 is tested in the

following manner. The amplitude of a 0 dB signal is measured.

The device under test is then set to midscale output voltage (0

volts). The amplitude of all noise present to 30 kHz is mea-

sured. The SNR is the ratio of these two measurements. The

SNR figure for the AD1862 includes the output noise contrib-

uted by the NE5534 op amp used in the test fixture but does

not include the noise contributed by the low-pass filter used in

the test fixture.

The AD1862N has a minimum SNR of 110 dB. The higher

performance AD1862N-J has a minimum SNR of 113 dB.

Testing the AD1862

REV. A

–7–

AD1862

TOP VIEW

(Not to Scale)

1

2

3

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5

6

7

89

10

11

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0.1

µF

0.1

µF

0.1

µF

0.1

µF

10

µF

1

µF

3-POLE

LOW PASS

FILTER

12V

–

12V

12V

–

17MHz

352.8kHz

DIGITAL

COMMON

SE5534A

OUTPUT

VOLTAGE

ANALOG

COMMON

12V

360pF

+

+

Figure 10. Recommended Test Circuit