AD7864AS-2 datasheet(15/19 Pages) AD

15 / 19 page

AD7864

–15–

REV. A

ADC CODE

1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064

9000

8000

4000

3000

2000

1000

6000

5000

7000

Figure 14. Histogram of 8192 Conversions of a DC Input

The output spectrum from the ADC is evaluated by applying a

sine wave signal of very low distortion to the analog input. A

Fast Fourier Transform (FFT) plot is generated from which the

SNR data can be obtained. Figure 15 shows a typical 4096 point

FFT plot of the AD7864 with an input signal of 99.9 kHz and a

sampling frequency of 500 kHz. The SNR obtained from this

graph is 72.6 dB. It should be noted that the harmonics are

taken into account when calculating the SNR.

FREQUENCY – Hz

50000

100000

150000

200000

250000

–10

–90

–50

–60

–70

–80

–30

–40

–20

–110

–100

AD7864-1 @ +25 C

5V SUPPLY

SAMPLING AT 499712Hz

INPUT FREQUENCY OF 99857Hz

8192 SAMPLES TAKEN

Figure 15. 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

get a measure of performance expressed in effective number of

bits (N).

SNR –1.76

6.02

(2)

The effective number of bits for a device can be calculated di-

rectly from its measured SNR. Figure 16 shows a typical plot

of effective number of bits versus frequency for an AD7864-2.

FREQUENCY – kHz

3000

500

1000

1500

2000

2500

–40 C

+25 C

+105 C

Figure 16. 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 or n are equal to zero. For example, the second

order terms include (fa + fb) and (fa – fb) while 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

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 calculation of the intermodu-

lation 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 dBs. In this

case, the input consists of two, equal amplitude, low distortion

sine waves. Figure 17 shows a typical IMD plot for the AD7864.

FREQUENCY – Hz

10000

20000

30000

40000

50000

–10

–90

–50

–60

–70

–80

–30

–40

–20

–100

60000

AD7864-1 @ +25 C

5V SUPPLY

SAMPLING AT 131072Hz

INPUT FREQUENCIES OF

48928Hz AND 50016Hz

4096 SAMPLES TAKEN

Figure 17. IMD Plot

AD7864AS-2 Datasheet(PDF) 15 Page - Analog Devices

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