LTC1400 LINER [Linear Technology], LTC1400 Datasheet - Page 8

• Manufacturer Part Number: LTC1400
• Description: Complete SO-8, 12-Bit, 400ksps ADC with Shutdown
• Manufacturer: LINER [Linear Technology]

LTC1400
APPLICATIONS
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).
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 of the sampling frequency. THD
is expressed as:
8
Figure 2b. LTC1400 Nonaveraged, 4096 Point FFT
Plot with 200kHz Input Frequency in Bipolar Mode
Figure 3. Effective Bits and Signal-to-Noise +
Distortion vs Input Frequency in Bipolar Mode
–100
–110
–120
12
11
10
– 60
–10
–20
–30
–40
–50
–70
–80
–90
9
8
7
6
5
4
3
2
1
0
10k
0
0
f
SAMPLE
f
f
SINAD = 72.1dB
THD = – 80dB
SAMPLE
IN
20
= 199.121kHz
U
40
= 400kHz
INPUT FREQUENCY (Hz)
FREQUENCY
= 400kHz
NYQUIST
60
FREQUENCY (kHz)
INFORMATION
80 100
U
100k
120
140
W
160
LTC1400 • F03
LTC1400 • F02b
180
1M
200
74
68
62
56
50
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
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 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.
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 vs input frequency is
shown in Figure 4. The LTC1400 has good distortion
performance up to the Nyquist frequency and beyond.
IMD fa
THD
Figure 4. Distortion vs Input Frequency in
Bipolar Mode
–100
20
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
log
fb
10k
f
SAMPLE
3RD HARMONIC
V
20log
2
= 400kHz
2
INPUT FREQUENCY (Hz)
V
Amplitude at (
3
2ND HARMONIC
100k
2
Amplitude at fa
V
1
V
4
THD
2
LTC1400 • F04
fa
Vn
1M
2
fb
)