Datasheet

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.
01020
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
30
FREQUENCY (kHz)
(dB)
40 6050
AD7864-1 @ 25°C
5V SUPPLY
SAMPLING AT 131072Hz
INPUT FREQUENCY OF
48,928Hz AND 50,016Hz
4096 SAMPLES TAKEN
0
1341-019
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.
0 500 1000 1500 2000 2500 3000 3500
ADC CODE
3
2
1
0
–1
–2
–3
DNL (LSB)
4000
01341-020
Figure 20. Typical DNL Plot
–0.5
0
0.5
1.0
1.5
2.0
2.5
–2.5
–2.0
–1.5
–1.0
0 500 1000 1500 2000 2500 3000 3500 4000
ADC CODE
INL (LSB)
0
1341-021
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:
()
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×××
×=
σfπ
SNR
IN
JITTER
2
1
log20
10
(3)
where:
SNR
JITTER
is the signal-to-noise due to the rms time jitter.
σ is the rms time jitter.
f
IN
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)
12
11
5
9
8
7
6
10
ENOB
900k 950k 1.00M 1.05M 1.10M
01341-022
Figure 22. ENOB of the AD7864 at 1 MHz