LTC1414 Linear Technology, LTC1414 Datasheet - Page 8

• Manufacturer Part Number: LTC1414
• Description: 14-Bit/ 2.2Msps/ Sampling A/D Converter
• Manufacturer: Linear Technology

LTC1414
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:
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:
where V
quency and V
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.
8
APPLICATIONS
ENOB
THD
1
S
is the RMS amplitude of the fundamental fre-
20
= [S/(N + D) – 1.76]/6.02
–100
–120
Figure 2b. LTC1414 2048 Point FFT,
Input Frequency = 1MHz
–20
–40
–60
–80
log
0
2
0
through V
SINAD = 78dB
SFDR = 84dB
f
f
SAMPLE
IN
V
U
= 997.949kHz
2
200
2
= 2.2MHz
V
INFORMATION
FREQUENCY (kHz)
400
U
3
2
N
V
are the amplitudes of the
1
V
600
4
2
W
800
V
N
1000
1414 F02b
2
U
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
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence frequencies of mf
etc. For example, the 2nd order IMD terms include (f
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
Figure 4. Distortion vs Input Frequency
14
13
12
11
10
–100
9
8
7
6
5
4
3
2
–10
–20
–30
–40
–50
–60
–70
–80
–90
1k
0
f
SAMPLE
1
= 2.2MHz
10k
INPUT FREQUENCY (Hz)
10k
a
INPUT FREQUENCY (Hz)
nf
100k
2nd
b
, where m and n = 0, 1, 2, 3
100k
1M
1M
a
THD
and f
1414 TA02
3rd
1414 F04
10M
b
10M
86
80
74
68
are applied
a
f
b
).