LTC1418 LINER [Linear Technology], LTC1418 Datasheet - Page 10

• Manufacturer Part Number: LTC1418
• Description: Low Power, 14-Bit, 200ksps ADC with Serial and Parallel I/O
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1418
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:
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
10
THD
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
20
14
13
12
11
10
–100
–120
9
8
7
6
5
4
3
2
Figure 4. Distortion vs Input Frequency
–20
–40
–60
–80
Log
1k
0
1k
V
U
INPUT FREQUENCY (Hz)
2
2
10k
INPUT FREQUENCY (Hz)
10k
INFORMATION
V
U
3
2
V
THD
1
100k
V
3RD
4
100k
2
W
1418 F03
2ND
Vn
1418 G03
1M
2
1M
U
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:
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.
IMD fa fb
–100
–120
Figure 5. Intermodulation Distortion Plot
– 40
– 60
– 20
– 80
0
0
f
f
f
10
SAMPLE
IN1
IN2
20
= 97.65625kHz
= 104.248046kHz
20
Log
= 200kHz
30 40
FREQUENCY (kHz)
Amplitude
Amplitude at fa
50
60
70
at fa + fb
80 90
1418 G05
100