LTC1416 Linear Technology, LTC1416 Datasheet - Page 9

• Manufacturer Part Number: LTC1416
• Description: Low Power 14-Bit/ 400ksps Sampling ADC
• Manufacturer: Linear Technology

APPLICATIONS
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 versus input fre-
quency is shown in Figure 4. The LTC1416 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-
THD
20 log
–100
–110
Figure 4. Distortion vs Input Frequency
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
1k
U
THD
V2
2
INPUT FREQUENCY (Hz)
10k
INFORMATION
U
V
3
2
3RD
V
100k
1
V
4
2
W
2ND
Vn
1M 2M
1416 G03
2
U
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 magnitude, 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.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is
reduced by 3dB for a full-scale input signal. The full-linear
bandwidth is the input frequency at which the S/(N + D)
has dropped to 77dB (12.5 effective bits). The LTC1416
has been designed to optimize input bandwidth, allowing
the ADC to undersample input signals with frequencies
above the converter’s Nyquist frequency. The noise floor
stays very low at high frequencies; S/(N + D) becomes
dominated by distortion at frequencies far beyond Nyquist.
IMD fa fb
–100
–120
–140
–20
–40
–60
–80
0
Figure 5. Intermodulation Distortion Plot
0
20
40 60
20 log
FREQUENCY (Hz)
80
Amplitude at fa + fb
100
Amplitude at fa
120
f
f
f
SAMPLE
a
b
=87.01171876kHz
=113.1835938kHz
140
= 400kHz
160
180 200
1416 G05
LTC1416
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