LTC1419 LINER [Linear Technology], LTC1419 Datasheet - Page 8

• Manufacturer Part Number: LTC1419
• Description: 14-Bit, 800ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1419
frequencies outside the fundamental. Figure 2 shows a
typical LTC1419 FFT plot.
Signal-to-Noise Ratio
The signal-to-noise plus distortion ratio [S/(N + D)] is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies from above DC and below half the sampling
frequency. Figure 2 shows a typical spectral content with
a 800kHz sampling rate and a 100kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist limit of 400kHz.
8
Figure 2a. LTC1419 Nonaveraged, 4096 Point FFT,
Input Frequency = 100kHz
Figure 2b. LTC1419 Nonaveraged, 4096 Point FFT,
Input Frequency = 375kHz
–100
–140
–120
–140
–100
–120
– 20
– 40
– 60
– 80
– 20
– 60
– 40
– 80
0
0
0
0
f
f
SFDR = 88.3dB
SINAD = 80.1
SAMPLE
IN
50
50
= 375kHz
U
100
100
= 800kHz
FREQUENCY (kHz)
FREQUENCY (kHz)
150
150
INFORMATION
U
200
200
f
f
SFDR = 98dB
THD = – 93.3dB
SAMPLE
IN
250
250
= 99.804687kHz
300
= 800kHz
300
W
350
350
1419 F02a
1419 F02b
400
400
U
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 N is the effective number of bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 800kHz the LTC1419 maintains near ideal ENOBs
up to the Nyquist input frequency of 400kHz (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 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 LTC1419 has good distortion
performance up to the Nyquist frequency and beyond.
N = [S/(N + D) – 1.76]/6.02
THD
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
20
14
13
12
11
10
9
8
7
6
5
4
3
2
1k
Log
f
SAMPLE
V
= 800kHz
INPUT FREQUENCY (Hz)
2
10k
2
V
3
2
100k
V
1
V
4
2
1419 TA02
1M 2M
Vn
86
80
74
68
62
2