Datasheet

11
LTC1273
LTC1275/LTC1276
127356fa
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S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
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 300kHz the LTC1273/LTC1275/LTC1276 maintain
very good ENOBs up to the Nyquist input frequency of
150kHz. Refer to Figure 3.
INPUT FREQUENCY (Hz)
10k
0
EFFECTIVE BITS
3
5
7
10
100k 2M
LTC1273/75/76 • F03
1
4
6
9
12
11
8
2
1M
62
56
74
68
50
S/(N + D) (dB)
f
SAMPLE
= 300kHz
Figure 3. Effective Bits and Signal to (Noise + Distortion)
vs Input Frequency
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 = 20log
√V
2
2
+ V
3
2
+ V
4
2
... + V
N
2
V
1
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD versus input fre-
quency is shown in Figure 4. The LTC1273/LTC1275/
LTC1276 have good distortion performance up to Nyquist
and beyond.
Figure 4. Distortion vs Input Frequency
INPUT FREQUENCY (Hz)
–80
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
–60
–40
–20
0
1k 100k 1M 10M
LTC1273/75/76 • F04
–100
10k
–90
–70
–50
–30
–10
f
SAMPLE
= 300kHz
THD
2nd HARMONIC
3rd HARMONIC
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
the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb) and
(fa – fb) while the 3rd order IMD terms include (2fa + fb),
(2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine
waves are equal in magnitude, the value (in decibels) of the
2nd order IMD products can be expressed by the following
formula:
IMD (fa ± fb) = 20log
Amplitude at (fa ± fb)
Amplitude at fa