
9
LTC1411
1411f
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:
ENOBS = [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 2.5MHz the LTC1411 maintains good
ENOBs up to the Nyquist input frequency of 1.25MHz.
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
APPLICATIO S I FOR ATIO
WU
UU
INPUT FREQUENCY (kHz)
0
–60
–40
0
1411 G14
–80
–100
–120
–140
–20
AMPLITUDE
(dB)
SINAD = 78.8dB
SFDR = 95dB
fSAMPLE = 2.5MHz
fIN = 100kHz
1000
250
500
750
1250
Figure 2a. LTC1411 Nonaveraged, 4096 Point FFT,
Input Frequency = 100kHz
itself. The out-of-band harmonics alias into the frequency
band between DC and half the sampling frequency. THD is
expressed as:
THD
VV
V
N
=
++
+…
20
2
3
2
4
22
1
log
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 LTC1411 has good distortion
performance up to the Nyquist frequency and beyond.
FREQUENCY (kHz)
0
–40
–20
0
1000
1411 G15
–60
–80
250
500
750
1250
–100
–120
–140
AMPLITUDE
(dB)
SINAD = 75dB
SFDR = 81dB
fSAMPLE = 2.5MHz
fIN = 1MHz
Figure 2b. LTC1411 4096 Point FFT,
Input Frequency = 1MHz
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
INPUT FREQUENCY (kHz)
10
38
S/(N
+
D)
(dB)
EFFECTIVE
BITS
50
62
100
1000
10000
1411 TA02
26
14
86
74
32
44
56
20
80
68
10
14
12
13
11
Figure 4. Distortion vs Input Frequency
INPUT FREQUENCY (kHz)
10
–80
DISTORTION
(dB)
–60
–40
100
1000
10000
1411 G03
–100
–110
0
–20
–90
–70
–50
–10
–30
2ND
3RD
THD