Datasheet

LTC2481
30
2481fc
APPLICATIONS INFORMATION
In 1x speed mode, the regions of low rejection occurring
at integer multiples of f
S
have a very narrow bandwidth.
Magnifi ed details of the normal mode rejection curves
are shown in Figure 33 (rejection near DC) and Figure 34
(rejection at f
S
= 256f
N
) where f
N
represents the notch
frequency. These curves have been derived for the exter-
nal oscillator mode but they can be used in all operating
modes by appropriately selecting the f
N
value.
The user can expect to achieve this level of performance
using the internal oscillator as it is demonstrated by
Figures 35, 36 and 37. Typical measured values of the
normal mode rejection of the LTC2481 operating with an
internal oscillator and a 60Hz notch setting are shown in
Figure 35 superimposed over the theoretical calculated
curve. Similarly, the measured normal mode rejection of
the LTC2481 for the 50Hz rejection mode and 50Hz/60Hz
rejection mode are shown in Figures 36 and 37.
As a result of these remarkable normal mode specifi ca-
tions, minimal (if any) antialias fi ltering is required in front
of the LTC2481. If passive RC components are placed in
front of the LTC2481, the input dynamic current should
be considered (see Input Current section). In this case,
the differential input current cancellation feature of the
LTC2481 allows external RC networks without signifi cant
degradation in DC performance.
Traditional high order delta-sigma modulators, while
providing very good linearity and resolution, suffer
from potential instabilities at large input signal levels.
The proprietary architecture used for the LTC2481 third
order modulator resolves this problem and guarantees a
predictable stable behavior at input signal levels of up to
150% of full-scale. In many industrial applications, it is
not uncommon to have to measure microvolt level signals
superimposed on volt level perturbations and the LTC2481
is eminently suited for such tasks. When the perturbation
is differential, the specifi cation of interest is the normal
mode rejection for large input signal levels. With a refer-
ence voltage V
REF
= 5V, the LTC2481 has a full-scale dif-
ferential input range of 5V peak-to-peak. Figures 38 and
39 show measurement results for the LTC2481 normal
mode rejection ratio with a 7.5V peak-to-peak (150% of
full scale) input signal superimposed over the more tradi-
tional normal mode rejection ratio results obtained with a
5V peak-to-peak (full-scale) input signal. In Figure 38, the
LTC2481 uses the internal oscillator with the notch set at
60Hz and in Figure 39 it uses the internal oscillator with
the notch set at 50Hz. It is clear that the LTC2481 rejection
performance is maintained with no compromises in this
extreme situation. When operating with large input signal
levels, the user must observe that such signals do not
violate the device absolute maximum ratings.
Figure 33. Input Normal Mode Rejection at DC Figure 34. Input Normal Mode Rejection at f
s
= 256f
N
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2481 F33
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
INPUT SIGNAL FREQUENCY (Hz)
250f
N
252f
N
254f
N
256f
N
258f
N
260f
N
262f
N
INPUT NORMAL MODE REJECTION (dB)
2481 F34
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120