AD1380

Rev. D | Page 11 of 12

APPLICATIONS

High performance sampling analog-to-digital converters like

the AD1380 require dynamic characterization to ensure that

they meet or exceed their desired performance parameters for

signal processing applications. Key dynamic parameters include

signal-to-noise ratio (SNR) and total harmonic distortion

(THD), which are characterized using Fast Fourier Transform

(FFT) analysis techniques.

Increasing the input signal amplitude to –0.4 dB of full scale

causes THD to increase to –80.6 dB as shown in Figure 12.

At lower input frequencies, however, THD performance is

improved. Figure 13 shows a full-scale (−0.3 dB) input signal at

1.41 kHz. THD is now −96.0 dB.

0

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

1

44

86

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FREQUENCY (

20V SPAN

2f (dB) = –97.8

3f (dB) = –102.8

4f (dB) = –106.9

FUNDAMENTAL = 1416

SAMPLE RATE = 50000

SIGNAL

(dB) = –0.3

NOISE

(dB) = –91.9

THD

(dB) = –96.0

The results of that characterization are shown in Figure 11. In

at a level of 10 dB below full scale; the AD1380 is operated at a

word rate of 50 kHz (its maximum sampling frequency). The

results of a 1024-point FFT demonstrate the exceptional

performance of the converter, particularly in terms of low noise

and harmonic distortion.

In Figure 11, the vertical scale is based on a full-scale input

referenced as 0 dB. In this way, all (frequency) energy cells can be

calculated with respect to full-scale rms inputs. The resulting

signal-to-noise ratio is 83.2 dB, which corresponds to a noise floor

of −93.2 dB. Total harmonic distortion is calculated by adding the

rms energy of the first four harmonics and equals –97.5 dB.

Figure 13. FFT of 1.4 kHz Input Signal at −0.3 dB with a 50 kHz Sample Rate

0

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

1

44

86

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FREQUENCY (×48.8281Hz)

2f (dB) = –100.9

3f (dB) = –101.8

4f (dB) = –111.9

FUNDAMENTAL = 13232

SAMPLE RATE = 50000

SIGNAL

(dB) = –10.0

NOISE

(dB) = –93.2

THD

(dB) = –97.5

The ultimate noise floor can be seen with low level input signals

of any frequency. In Figure 14, the noise floor is at −94 dB, as

demonstrated with an input signal of 24 kHz at −39.8 dB.

0

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

1

44

86

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FREQUENCY (

20V SPAN

2f (dB) = –116.0

3f (dB) = –113.6

4f (dB) = –112.4

FUNDAMENTAL = 23975

SAMPLE RATE = 50000

SIGNAL

(dB) = –39.8

NOISE

(dB) = –94.3

THD

(dB) = –107.9

Figure 11. FFT of 13.2 kHz Input Signal at −10 dB with a 50 kHz Sample Rate

0

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

1

44

86

129 171 214 257 299 342 384 427 469 512

FREQUENCY (×48.8281Hz)

2f (dB) = –80.7

3f (dB) = –99.9

4f (dB) = –102.9

FUNDAMENTAL = 13232

SAMPLE RATE = 50000

SIGNAL

(dB) = –0.4

NOISE

(dB) = –91.0

THD

(dB) = –80.6

Figure 14. FFT of 24 kHz Input Signal at −39.8 dB with a 50 kHz Sample Rate

Figure 12. FFT of 13.2 kHz Input Signal at −0.4 dB with a 50 kHz Sample Rate