LTC1416 Linear Technology, LTC1416 Datasheet - Page 8

• Manufacturer Part Number: LTC1416
• Description: Low Power 14-Bit/ 400ksps Sampling ADC
• Manufacturer: Linear Technology

LTC1416
APPLICATIONS
DYNAMIC PERFORMANCE
The LTC1416 has excellent high speed sampling capabil-
ity. FFT (Fast Fourier Transform) test techniques are used
to test the ADC’s frequency response, distortion and noise
at the rated throughput. By applying a low distortion sine
wave and analyzing the digital output using an FFT algo-
rithm, the ADC’s spectral content can be examined for
frequencies outside the fundamental. Figure 2 shows a
typical LTC1416 FFT plot.
8
Figure 2a. LTC1416 Nonaveraged, 4096 Point FFT,
Input Frequency = 100kHz
Figure 2b. LTC1416 Nonaveraged, 4096 Point FFT,
Input Frequency = 190kHz
–100
–120
–140
–100
–120
–140
–20
–20
–40
–60
–80
–40
–60
–80
0
0
0
0
f
f
SFDR = 94.8dB
SINAD = 80.2dB
f
f
SFDR = 95.2dB
SINAD = 80.5dB
SAMPLE
IN
SAMPLE
IN
25
= 189.9414kHz
25
= 101.5625kHz
U
= 400kHz
= 400kHz
50
50
FREQUENCY (kHz)
FREQUENCY (kHz)
75
75
INFORMATION
U
100
100
125
125
W
150
150
175
175
1416 F02b
1416 F02a
200
200
U
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 2a shows a typical spectral content with
a 400kHz sampling rate and a 100kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist limit of 200kHz, Figure 2b.
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 ENOB is the Effective Number of Bits of resolution
and S/(N + D) is expressed in dB. At the maximum
sampling rate of 400kHz the LTC1416 maintains near ideal
ENOBs up to the Nyquist input frequency of 200kHz (refer
to Figure 3).
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
ENOB = [S/(N + D) – 1.76]/6.02
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1k
f
SAMPLE
= 400kHz
INPUT FREQUENCY (Hz)
10k
FREQUENCY
NYQUIST
100k
1416 TA02
1M 2M
86
80
74
68
62