REV. A

AD7899

–13–

FREQUENCY – Hz

–140

0

50000

100000

150000

200000

fs = 400kHz

fIN = 100kHz

SNR = 80.5dB

–120

–100

–80

–60

–40

–20

0

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).

N

=

SNR

−1.76

6.02

(2)

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.

INPUT FREQUENCY – kHz

0

0

100

1000

10000

–55 C

+25 C

1

2

3

4

5

6

7

8

9

10

11

12

13

14

+125 C

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).

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

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.

FREQUENCY – Hz

–140

0

50000

100000

150000

200000

–120

–100

–80

–60

–40

–20

0

Figure 12. IMD Plot

AC Linearity Plots

The plots in Figure 13 show typical DNL and INL for the

AD7899.

0

2000

4000

16000

1.00

ADC – Code

–1.00

6000

8000

10000 12000

14000

–0.50

0

0.50

0

2000

4000

16000

1.00

ADC – Code

–1.00

6000

8000

10000 12000

14000

–0.50

0

0.50

Figure 13. Typical DNL and INL Plots