LTC1410
U
W U U
APPLICATIONS INFORMATION
DYNAMIC PERFORMANCE
tofrequencies fromaboveDCandbelowhalfthesampling
frequency. Figures 2a and 2b shows a typical spectral
content with a 1.25MHz sampling rate for 100kHz and
600kHz inputs. The dynamic performance is excellent for
input frequencies up to the Nyquist limit of 625kHz and
beyond.
The LTC1410 has excellent high speed sampling capabil-
ity. Fast Four Transform (FFT) 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.
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:
0
f
= 1.25MHz
SAMPLE
f
= 100.098kHz
IN
–20
–40
N = [S/(N + D) – 1.76]/6.02
SFDR = 90.1dB
SINAD = 72.4dB
where N is the effective number of bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 1.25MHz the LTC1410 maintains very good ENOBs
up to the Nyquist input frequency of 625kHz and beyond.
Refer to Figure 3.
–60
–80
–100
–120
12
10
8
74
68
62
56
50
0
200
300 400 500
600
100
NYQUIST
FREQUENCY (kHz)
1410 F02a
Figure 2a. LTC1410 Nonaveraged 4096 Point FFT, 100kHz Input
6
0
f
f
IN
= 1.25MHz
= 599.975kHz
4
SAMPLE
–20
–40
SFDR = 84.7dB
SINAD = 71.7dB
2
f
= 1.25MHz
SAMPLE
0
1k
10k
100k
1M
10M
–60
INPUT FREQUENCY (Hz)
LTC1410 • TA02
–80
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
–100
–120
0
200
300 400 500
600
100
Total Harmonic Distortion (THD)
FREQUENCY (kHz)
1410 F02b
Total harmonic distortion is the ratio of the RMS sum of all
harmonics oftheinputsignaltothefundamentalitself.The
out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency. THD is
expressed as:
Figure 2b. LTC1410 Nonaveraged 4096 Point FFT, 600kHz Input
Signal-to-Noise Ratio
The Signal-to-Noise plus Distortion ratio [S/(N + D)] is the
ratiobetweentheRMSamplitudeofthefundamentalinput
frequency to the RMS amplitude of all other frequency
components at the ADC output. The output is band limited
2
2
2
V + V + V2 +. . .V
2
3
4
n
THD = 20 log
V
1
8
Linear [ Linear ]
