LTC2485CDD datasheet(30/40 Pages) LINER

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LTC2485

2485fa

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

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 VREF = 5V, the LTC2485 has a full-scale differential

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.

APPLICATIO S I FOR ATIO

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