
9
LTC1415
APPLICATIONS INFORMATION
WU
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:
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 1.25MHz the LTC1415 maintains very good ENOBs
up to the Nyquist input frequency of 625kHz (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:
THD
Log
VV
V
Vn
V
=
+++…
20
23
4
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 4. The LTC1415 has good distortion
performance up to the Nyquist frequency and beyond.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition to
THD. IMD is the change in one sinusoidal input caused by
INPUT FREQUENCY (Hz)
EFFECTIVE
BITS
SIGNAL/(NOISE
+
DISTORTION)
(dB)
12
11
10
9
8
7
6
5
4
3
2
1
0
74
68
62
56
1k
100k
1M 2M
LT1415 F03
10k
Figure 3. Effective Bits and Signal/(Noise +
Distortion) vs Input Frequency
Figure 4. Distortion vs Input Frequency
FREQUENCY (Hz)
0
AMPLITUDE
(dB)
0
–20
–40
–60
–80
–100
–120
200k
300k
600k
LTC1415 F05
100k
400k
500k
fb – fa
2fb – fa
2fa – fb
2fa
2fb
3fb
fa + 2fb
3fa
2fa + fb
fa + fb
fSAMPLE = 1.25MHz
fIN1 = 86.97509766kHz
fIN2 = 113.2202148kHz
Figure 5. Intermodulation Distortion Plot
INPUT FREQUENCY (Hz)
AMPLITUDE
(dB
BELOW
THE
FUNDAMENTAL)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
1k
100k
1M
2M
LTC1415 F04
10k
THD
2ND
3RD