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
present in the sampled output data.
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all the 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:
2222
2
34N
1
V +V +V
THD =
+V
20 log
V
where V
1
is the fundamental RMS amplitude, and
V
2
through V
N
are the amplitudes of the 2nd through
Nth harmonics. The THD specification in the Electrical
Characteristics includes the 2nd through 5th harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range is the ratio of the fundamen-
tal RMS amplitude to the amplitude of the next largest
spectral component (in the frequency band above DC and
below one-half the sample rate). Usually the next largest
spectral component occurs at some harmonic of the input
frequency. However, if the ADC is exceptionally linear, it
may occur only at a random peak in the ADC’s noise floor.
Transfer Function
The plot in Figure 31 graphs the bipolar input/output
transfer function for the MAX121. Code transitions occur
halfway between successive integer LSB values. Output
coding is two’s-complement binary, with 1 LSB = 610µV
(10V/16384).
Figure 30. Effective Bits vs. Input Frequency Figure 31. Bipolar Transfer Function
INPUT FREQUENCY (Hz)
EFFECTIVE BITS
1M100k
8
9
10
11
12
13
14
7
10k 10M
f
S
= 300kHz
T
A
= +25°C
0V-5V
100...001
111...101
111...110
111...111
000...000
000...001
000...010
011...110
011...111
100...000
4.99939V
MAX121 308ksps ADC with DSP Interface and 78dB SINAD
www.maximintegrated.com
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