LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 30

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2485
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 LTC2485 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 LTC2485 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 specifica-
tions, minimal (if any) antialias filtering is required in front
of the LTC2485. If passive RC components are placed in
front of the LTC2485, the input dynamic current should be
considered (see Input Current section). In this case, the
differential input current cancellation feature of the LTC2485
allows external RC networks without significant degrada-
tion in DC performance.
Traditional high order delta-sigma modulators, while pro-
viding very good linearity and resolution, suffer from
potential instabilities at large input signal levels. The pro-
prietary architecture used for the LTC2485 third order
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modulator resolves this problem and guarantees a predict-
able stable behavior at input signal levels of up to 150% of
full scale. In many industrial applications, it is not uncom-
mon to have to measure microvolt level signals superim-
posed on volt level perturbations and the LTC2485 is
eminently suited for such tasks. When the perturbation is
differential, the specification of interest is the normal mode
rejection for large input signal levels. With a reference
voltage V
input range of 5V peak-to-peak. Figures 38 and 39 show
measurement results for the LTC2485 normal mode rejec-
tion ratio with a 7.5V peak-to-peak (150% of full scale)
input signal superimposed over the more traditional nor-
mal mode rejection ratio results obtained with a 5V peak-
to-peak (full scale) input signal. In Figure 38, the LTC2485
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 LTC2485 rejection performance
is maintained with no compromises in this extreme situa-
tion. When operating with large input signal levels, the user
must observe that such signals do not violate the device
absolute maximum ratings.
REF
= 5V, the LTC2485 has a full-scale differential
2485fa