LTC1416 Datasheet, PDF (9/20 Pages) Linear Technology – Low Power 14-Bit, 400ksps Sampling ADC

LTC1416
APPLICATIONS INFORMATION
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 = 20 log ^{V22 + V32 + V42 + ...Vn2}
V1
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.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
1k
3RD
THD
2ND
10k
100k
INPUT FREQUENCY (Hz)
1M 2M
1416 G03
Figure 4. Distortion vs Input Frequency
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 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 = 20 log
Amplitude at fa
0
f_SAMPLE = 400kHz
–20
f_a=87.01171876kHz
f_b=113.1835938kHz
–40
–60
–80
–100
–120
–140
0 20 40 60 80 100 120 140 160 180 200
FREQUENCY (Hz)
1416 G05
Figure 5. Intermodulation Distortion Plot
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.
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