AD7864 datasheet(23/29 Pages) AD

23 / 29 page

AD7864

Rev. D | Page 22 of 28

terms include (fa + fb) and (fa − fb), whereas the third-order

terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).

Using the CCIF standard where two input frequencies near the

top end of the input bandwidth are used, the second- and third-

order terms are of different significance. The second-order

terms are usually distanced in frequency from the original sine

waves, whereas the third-order terms are usually at a frequency

close to the input frequencies. As a result, the second- and

third-order terms are specified separately. The calculation of the

intermodulation distortion is as per the THD specification

where it is the ratio of the rms sum of the individual distortion

products to the rms amplitude of the fundamental expressed in

decibels. In this case, the input consists of two, equal amplitude,

low distortion sine waves. Figure 19 shows a typical IMD plot

for the AD7864.

010

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

FREQUENCY (kHz)

AD7864-1 @ 25°C

5V SUPPLY

SAMPLING AT 131072Hz

INPUT FREQUENCY OF

48,928Hz AND 50,016Hz

4096 SAMPLES TAKEN

Figure 19. IMD Plot

AC LINEARITY PLOTS

The plots shown in Figure 20 and Figure 21 show typical DNL

and INL plots for the AD7864.

500

1000

1500

2000

2500

3000

3500

ADC CODE

–1

–2

–3

4000

Figure 20. Typical DNL Plot

–0.5

0.5

1.0

1.5

2.0

2.5

–2.5

–2.0

–1.5

–1.0

500

1000

1500

2000

2500

3000

3500

4000

ADC CODE

Figure 21. Typical INL Plot

MEASURING APERTURE JITTER

A convenient way to measure aperture jitter is to use the

relationship it is known to have with SNR (signal-to-noise plus

distortion) given as follows:

(

)⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛

SNR

JITTER

log

(3)

where:

SNRJITTER is the signal-to-noise due to the rms time jitter.

σ is the rms time jitter.

fIN is the sinusoidal input frequency (1 MHz in this case).

Equation 3 demonstrates that the signal-to-noise ratio due to

jitter degrades significantly with frequency. At low input fre-

quencies, the measured SNR performance of the AD7864 is

indicative of noise performance due to quantization noise and

system noise only (72 dB used as a typical figure in this example).

Therefore, by measuring the overall SNR performance

(including noise due to jitter, system, and quantization) of the

AD7864, a good estimation of the jitter performance of the

AD7864 can be calculated.

FREQUENCY (Hz)

900k

950k

1.00M

1.05M

1.10M

Figure 22. ENOB of the AD7864 at 1 MHz

AD7864 Datasheet(PDF) 23 Page - Analog Devices

AD7864 Datasheet(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL