Datasheet

Total Harmonic Distortion
If a pure sine wave is sampled by an ADC at greater than
the Nyquist frequency, the nonlinearities in the ADC’s
transfer function create harmonics of the input frequency
in the sampled output data.
Total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics (in the frequency band above DC
and below one-half the sample rate, but not including the
DC component) to the RMS amplitude of the fundamental
frequency. This is expressed as follows:
222 2
234 N
1
V V V ... V
THD 20log
V
++ +
=
where V
1
is the fundamental RMS amplitude, and V
2
to
V
N
are the amplitudes of the 2nd through Nth harmonics.
The THD specification in the Electrical Characteristics
table includes the 2nd through 5th harmonics.
lntermodulation Distortion
If the ADC input signal consists of more than one spec-
tral component, the ADC transfer function nonlinearities
produce intermodulation distortion (IMD) in addition to
THD. IMD is the change in one sinusoidal input caused
by the presence of another sinusoidal input at a different
frequency
If two pure sine waves of frequency fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion create distortion products at sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
THD includes those distortion products with m or n equal
to zero. lntermodulation distortion consists of all distor-
tion products for which neither m nor n equal zero. For
example, the 2nd-order IMD terms include (fa + fb) and
(fa - fb) while the 3rd-order IMD terms include (2fa + fb),
(2fa - fb) , (fa + 2fb), and (fa - 2fb).
If the two input sine waves are equal in magnitude, the
value (in decibels) of the 2nd-order IMD products can be
expressed by the following formula:
amplitude at (fa fb)
IMD (fa fb) 20log
amplitude at fa

±
±=


Figure 21. Inverting Bipolar Transfer Function
Figure 22. MAX120 FFT Plot
Figure 23. Effective Bits vs. Input Frequency
MAX120/MAX122 500ksps, 12-Bit ADCs with Track/Hold
and Reference
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