AD7899
second and third order terms are specified separately. The cal-
culation of the intermodulation distortion is as per the THD
specification where it is the ratio of the rms sum of the indi-
vidual distortion products to the rms amplitude of the fundamental
expressed in dBs. In this case, the input consists of two, equal
amplitude, low distortion sine waves. Figure 12 shows a typical
IMD plot for the AD7899.
0
–20
–40
–60
–80
f
f
= 400kHz
= 100kHz
s
IN
SNR = 80.5dB
0
–20
–40
–60
–80
–100
–120
–140
0
50000
100000
FREQUENCY – Hz
150000
200000
Figure 10. FFT Plot
Effective Number of Bits
The formula given in Equation 1 relates the SNR to the number
of bits. Rewriting the formula, as in Equation 2, it is possible to
obtain a measure of performance expressed in effective number
of bits (N).
–100
–120
–140
0
50000
100000
150000
200000
FREQUENCY – Hz
SNR −1. 76
Figure 12. IMD Plot
N =
(2)
6.02
AC Linearity Plots
The effective number of bits for a device can be calculated directly
from its measured SNR. Figure 11 shows a typical plot of effec-
tive number of bits versus frequency for an AD7899.
The plots in Figure 13 show typical DNL and INL for the
AD7899.
1.00
0.50
14
–55؇C
13
12
11
10
9
+25؇C
+125؇C
8
0
–0.50
–1.00
7
6
5
4
3
2
1
0
0
2000
4000
6000 8000 10000 12000 14000 16000
0
100
INPUT FREQUENCY – kHz
1000
10000
ADC – Code
1.00
0.50
Figure 11. Effective Numbers of Bits vs. Frequency
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa nfb where
m, n = 0, 1, 2, 3 . . ., etc. Intermodulation terms are those for
which neither m nor n are equal to zero. For example, the sec-
ond order terms include (fa + fb) and (fa – fb) while the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
0
–0.50
–1.00
The AD7899 is tested using two input frequencies. In this case
the second and third order terms are of different significance.
The second order terms are usually distanced in frequency from
the original sine waves while the third order terms are usually at
a frequency close to the input frequencies. As a result, the
0
2000
4000
6000 8000 10000 12000 14000 16000
ADC – Code
Figure 13. Typical DNL and INL Plots
–13–
REV. A
ADI [ ADI ]
