LTC1286 Linear Technology, LTC1286 Datasheet - Page 19

• Manufacturer Part Number: LTC1286
• Description: Micropower Sampling 12-Bit A/D Converters In S0-8 Packages
• Manufacturer: Linear Technology

APPLICATION INFORMATION
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 of the sampling frequency. THD
is defined as:
where V
quency and V
second through the N
fication in the Dynamic Accuracy table includes the 2nd
through 5th harmonics. With a 7kHz input signal, the
LTC1286/LTC1298 have typical THD of 80dB with V
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.
THD 20log
Figure 12. Effective Bits and S/(N + D) vs Input Frequency
1
is the RMS amplitude of the fundamental fre-
12
11
10
9
8
7
6
5
4
3
2
1
0
1
2
T
V
f
f
CLK
SMPL
A
CC
= 25°C
through V
= 5V
= 200kHz
V
= 12.5kHz
2
2
U
INPUT FREQUENCY (kHz)
th
10
harmonics. The typical THD speci-
V
3
2
U
N
V
V
1
are the amplitudes of the
4
2
100
W
LTC 1286/98 G20
V
N
2
1000
74
68
62
56
50
44
38
U
CC
= 5V.
If two pure sine waves of frequencies f
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at sum and difference
frequencies of mf
For example, the 2nd order IMD terms include (f
(f
(2f
waves are equal in magnitudes, the value (in dB) of the 2nd
order IMD products can be expressed by the following
formula:
For input frequencies of 5kHz and 6kHz, the IMD of the
LTC1286/LTC1298 is 73dB with a 5V supply.
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral component excluding the input signal and DC. This
value is expressed in dBs relative to the RMS value of a full-
scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is re-
duced by 3dB for a full-scale input.
The full-linear bandwidth is the input frequency at which
the effective bits rating of the ADC falls to 11 bits. Beyond
this frequency, distortion of the sampled input signal
increases. The LTC1286/LTC1298 have been designed to
optimize input bandwidth, allowing the ADCs to
undersample input signals with frequencies above the
converters’ Nyquist Frequency.
a
a
IMD f
– f
– f
b
b
) while 3rd order IMD terms include (2f
), (f
a
a
f
b
+ 2f
20log
a
b
), and (f
nf
b
, where m and n = 0, 1, 2, 3, etc.
a
LTC1286/LTC1298
mplitude f
amplitude at f
a
– 2f
b
). If the two input sine
a
a
and f
f
a
b
b
are applied
a
+ f
a
19
b
+ f
) and
b
),