LTC2435-1IGN datasheet(33/40 Pages) LINER

33 / 40 page

LTC2435/LTC2435-1

24351fa

Figure 35. Input Normal Mode Rejection

Figure 36. Input Normal Mode Rejection

Figure 37. LTC2435-1 Input Normal Mode Rejection

As a result of these remarkable normal mode specifica-

tions, minimal (if any) antialias filtering is required in front

of the LTC2435/LTC2435-1. If passive RC components

are placed in front of the LTC2435/LTC2435-1, the input

dynamic current should be considered (see Input Current

section). In cases where large effective RC time constants

are used, an external buffer amplifier may be required to

minimize the effects of dynamic input current.

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 LTC2435/LTC2435-1

third order modulator resolves this problem and guaran-

tees 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 over volt level perturbations and

LTC2435/LTC2435-1 are 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 LTC2435/

LTC2435-1 have a full-scale differential input range of 5V

peak-to-peak. Figure 40 shows measurement results for

the LTC2435-1 normal mode rejection ratio with a 7.5V

peak-to-peak (150% of full scale) input signal superim-

posed over the more traditional normal mode rejection

ratio results obtained with a 5V peak-to-peak (full scale)

input signal. The same performance is obtained for the

LTC2435 with the frequency scaled to have the notch fre-

quency at 60Hz (FO = GND) or 50Hz (FO = VCC). It is clear

that the LTC2435/LTC2435-1 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.

INPUT SIGNAL FREQUENCY (fN)

2435 F35

–20

–40

–60

–80

–100

–120

2fN 3fN 4fN 5fN

6fN 7fN

8fN

INPUT SIGNAL FREQUENCY (fN)

2435 F36

–20

–40

–60

–80

–100

–120

250

248

252 254 256 258 260 262 264

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–70

–80

–90

–100

–110

–120

–130

–140

2435 F37

NO AVERAGE

WITH

RUNNING

AVERAGE

APPLICATIO S I FOR ATIO

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