LTC1400
U U
W U
APPLICATIO S I FOR ATIO
0
f
f
= 400kHz
= 199.121kHz
SAMPLE
IN
V22 + V32 + V42 +…Vn2
–10
–20
THD= 20log
SINAD = 72.1dB
THD = –80dB
V1
–30
–40
where V1 is the RMS amplitude of the fundamental fre-
quencyandV2throughVnaretheamplitudesofthesecond
through nth harmonics. THD vs input frequency is shown
inFigure4.TheLTC1400hasgooddistortionperformance
up to the Nyquist frequency and beyond.
–50
–60
–70
–80
–90
–100
–110
–120
0
0
20 40 60 80 100 120 140 160 180 200
FREQUENCY (kHz)
f
= 400kHz
SAMPLE
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
1400 F02b
Figure 2b. LTC1400 Nonaveraged, 4096 Point FFT
Plot with 200kHz Input Frequency in Bipolar Mode
where N is the effective number of bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 400kHz, the LTC1400 maintains very good ENOBs
up to the Nyquist input frequency of 200kHz (refer to
Figure 3).
3RD HARMONIC
THD
2ND HARMONIC
10k
100k
1M
INPUT FREQUENCY (Hz)
1400 F04
12
11
10
9
74
68
62
56
50
Figure 4. Distortion vs Input Frequency in Bipolar Mode
NYQUIST
FREQUENCY
8
Intermodulation Distortion
7
6
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.
5
4
3
2
1
f
= 400kHz
SAMPLE
0
10k
100k
INPUT FREQUENCY (Hz)
1M
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 sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb)
and (fa – fb) while the 3rd order IMD terms includes (2fa
+ fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). 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.
1400 F03
Figure 3. Effective Bits and Signal-to-Noise +
Distortion vs Input Frequency in Bipolar Mode
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sumofallharmonicsoftheinputsignaltothefundamental
itself. The out-of-band harmonics alias into the frequency
band between DC and half of the sampling frequency. THD
is expressed as:
Amplitude at (fa ± fb)
IMD fa ± fb = 20log
(
)
Amplitude at fa
1400fa
8
Linear [ Linear ]
