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AD7863

Rev. B | Page 15 of 24

TOTAL HARMONIC DISTORTION (THD)

Total harmonic distortion (THD) is the ratio of the rms sum of

harmonics to the rms value of the fundamental. For the

AD7863, THD is defined as

()

THD

log

(3)

where:

V1 is the rms amplitude of the fundamental.

V2, V3, V4, and V5 are the rms amplitudes of the second through

the fifth harmonic.

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 creates distortion

products at sum and difference frequencies of mfa ± nfb where

m, n = 0, 1, 2, 3 . . . Intermodulation terms are those for which

neither m nor n is equal to zero. For example, the second order

terms include (fa + fb) and (fa − fb) and the third order terms

include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).

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 15 shows a typical IMD plot for the AD7863.

1020

405060

708090

FREQUENCY (kHz)

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

–140

–150

INPUT FREQUENCIES

F1 = 50.13kHz

F2 = 49.13kHz

fSAMPLE = 175kHz

IMD

2ND ORDER TERM

–98.21dB

3RD ORDER TERM

–93.91dB

Figure 15. IMD Plot

PEAK HARMONIC OR SPURIOUS NOISE

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 is

determined by the largest harmonic in the spectrum, but for

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

peak is a noise peak.

DC LINEARITY PLOT

Figure 16 and Figure 17 show typical DNL and INL plots for

the AD7863.

2048

4096

6144

8192

10240 12288 14336 16383

ADC CODE

1.0

0.5

–0.5

–1.0

Figure 16. DC DNL Plot

2048

4096

6144

8192

10240 12288 14336 16383

ADC CODE

1.0

0.5

–0.5

–1.0

Figure 17. DC INL Plot

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