10
LTC1417
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 3b shows a typical spectral content with
a 400kHz sampling rate and a 200kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist limit of 200kHz.
Figure 4. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
APPLICATIONS INFORMATION
WUU
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Figure 3a. LTC1417 Nonaveraged, 4096 Point FFT,
Input Frequency = 10kHz
Figure 3b. LTC1417 Nonaveraged, 4096 Point FFT,
Input Frequency = 200kHz
FREQUENCY (kHz)
0 10050
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
150
1417 G07
200
f
SAMPLE
= 400kHz
f
IN
= 10.05859375kHz
SFDR = –97.44dB
SINAD = 81.71dB
FREQUENCY (kHz)
0 50 100
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
150
1417 G08
200
f
SAMPLE
= 400kHz
f
IN
= 197.949188kHz
SFDR = –98dB
SINAD = 81.1dB
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:
ENOB (N) = [S/(N + D) – 1.76]/6.02
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 LTC1417 maintains near ideal ENOBs
up to the Nyquist input frequency of 200kHz (refer to
Figure 4).
INPUT FREQUENCY (Hz)
4
EFFECTIVE BITS
S/(N + D) (dB)
6
8
10
12
10k 100k 1M
1417 TA02
2
1k
14 86
80
74
68
62
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:
THD Log
VVV Vn
V
=
+++
20
234
1
222 2
...
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 5. The LTC1417 has good distortion
performance up to the Nyquist frequency and beyond.
