Datasheet

10
LTC1418
APPLICATIONS INFORMATION
WUU
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Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
shown in Figure 4. The LTC1418 has good distortion
performance up to the Nyquist frequency and beyond.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
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 frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3,
etc. For example, the 2nd order IMD terms include
(fa + fb). If the two input sine waves are equal in magni-
tude, the value (in decibels) of the 2nd order IMD products
can be expressed by the following formula:
IMD fa fb Log
Amplitude
+
()
=
()
20
at fa+ fb
Amplitude at fa
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamental
itself. The out-of-band harmonics alias into the frequency
band between DC and half the sampling frequency. THD is
expressed as:
THD Log
VVV Vn
V
=
+++
20
234
1
222 2
...
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through Vn are the amplitudes of the
second through nth harmonics. THD vs Input Frequency is
Figure 4. Distortion vs Input Frequency
INPUT FREQUENCY (Hz)
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
0
–20
–40
–60
–80
–100
–120
1418 G03
1k 100k 1M10k
THD
2ND
3RD
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral component excluding the input signal and DC. This
value is expressed in decibels relative to the RMS value of
a full-scale input signal.
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
20 50
70
100
1418 G05
–20
0
10 30 40
60
80 90
f
SAMPLE
= 200kHz
f
IN1
= 97.65625kHz
f
IN2
= 104.248046kHz
Figure 5. Intermodulation Distortion Plot
INPUT FREQUENCY (Hz)
1k
EFECTIVE BITS
14
13
12
11
10
9
8
7
6
5
4
3
2
10k 100k 1M
1418 F03