AD7883

REV. 0

–8–

12

11.5

11

10.5

10

INPUT FREQUENCY – kHz

15

25

20

10

5

SAMPLE FREQUENCY = 50kHz

Figure 11. Effective Number of Bits vs. Frequency

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of harmonics to the rms value

of the fundamental. For the AD7883, THD is defined as:

THD

= 20 log

V

2

2

+V

3

2

+V

4

2

+V

5

2

+V

6

2

(3)

sixth harmonic. The THD is also derived from the FFT plot of

the ADC output spectrum.

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 second or-

der 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 in-

termodulation distortion is as per the THD specification where

it is the ratio of the rms sum of the individual distortion prod-

ucts to the rms amplitude of the fundamental expressed in dBs.

In this case, the input consists of two, equal amplitude, low dis-

tortion, sine waves. Figure 12 shows a typical IMD plot for the

AD7883.

0

–30

–60

–90

–120

0

2.5

25

FREQUENCY – kHz

INPUT FREQUENCY

F1 = 0.983kHz

F2 = 1.05kHz

SAMPLE FREQUENCY = 50kHz

TA = +25

IMD

ALL TERMS = 81.5dB

2ND ORDER TERMS = 83.6dB

3RD ORDER TERMS = 85.4dB

Figure 12. IMD Plot

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the

rms value of the next largest component in the ADC output

spectrum (up to FS/2 and excluding dc) to the rms value of the

fundamental. Normally, the value of this specification will be

determined by the largest harmonic in the spectrum, but for

parts where the harmonics are buried in the noise floor the peak

will be a noise peak.