REV. B

AD7865

–16–

ADC CODE

7000

0

6000

5000

4000

3000

2000

1000

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 AD7865 with an input signal of 100 kHz

and a sampling frequency of 350 kHz. The SNR obtained from

this graph is 80.5 dB. It should be noted that the harmonics are

taken into account when calculating the SNR.

FREQUENCY – Hz

–140

0

35000

70000

105000

140000

175000

fs = 350kHz

SNR = 80.5dB

–120

–100

–80

–60

–40

–20

0

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

obtain a measure of performance expressed in effective number

of bits (N).

N

SNR

=

−176

602

.

.

(2)

The effective number of bits for a device can be calculated

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

of effective number of bits versus frequency for an AD7865-2.

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 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 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 AD7865 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 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

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

distortion sine waves. Figure 17 shows a typical IMD plot for

the AD7865.

FREQUENCY – Hz

0

–140

0

25000

50000

100000

125000

175000

75000

150000

–120

–100

–80

–60

–40

–20

fa = 49.113kHz

fb = 50.183kHz

fs = 350kHz

Figure 17. IMD Plot