Datasheet

8
LTC1414
APPLICATIONS INFORMATION
WUU
U
FREQUENCY (kHz)
0 400 800200 600 1000
AMPLITUDE (dB)
1414 F02b
0
–20
–40
–60
–80
–100
–120
SINAD = 78dB
SFDR = 84dB
f
SAMPLE
= 2.2MHz
f
IN
= 997.949kHz
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
the 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 f
a
and f
b
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 mf
a
± nf
b
, where m and n = 0, 1, 2, 3
etc. For example, the 2nd order IMD terms include (f
a
± f
b
).
If the two input sine waves are equal in magnitude, the
value (in dB) of the 2nd order IMD products can be
expressed by the following formula:
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
INPUT FREQUENCY (Hz)
EFFECTIVE BITS
S/(N + D) (dB)
10k 100k 1M 10M
1414 TA02
1k
14
13
12
11
10
9
8
7
6
5
4
3
2
86
80
74
68
f
SAMPLE
= 2.2MHz
Figure 4. Distortion vs Input Frequency
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
ENOB
S
= [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 2.2MHz the LTC1414 maintains near ideal
ENOBs up to the Nyquist input frequency of 1.1MHz. Refer
to Figure␣ 3.
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
VVV V
V
N
=
+++…
20
2
2
3
2
4
22
1
log
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD vs input frequency is
shown in Figure 4. The LTC1414 has good distortion
performance up to the Nyquist frequency and beyond.
Figure 2b. LTC1414 2048 Point FFT,
Input Frequency = 1MHz
INPUT FREQUENCY (Hz)
DISTORTION (dB)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
1 100k 1M 10M
1414 F04
10k
3rd
THD
2nd