LTC2414IGN datasheet(34/48 Pages) LINER

34 / 48 page

LTC2414/LTC2418

241418fa

The combined normal mode rejection performance is

shown in Figure 33 for the internal oscillator with 50Hz

notch setting (FO = HIGH) and in Figure 34 for the internal

oscillator with 60Hz notch setting (FO = LOW) and for the

external oscillator mode. The regions of low rejection

occurring at integer multiples of fS have a very narrow

bandwidth. Magnified details of the normal mode rejection

curves are shown in Figure 35 (rejection near DC) and

Figure 36 (rejection at fS = 256fN) where fN represents the

notch frequency. These curves have been derived for the

external oscillator mode but they can be used in all

operating modes by appropriately selecting the fN value.

The user can expect to achieve in practice this level of

performance using the internal oscillator as it is demon-

strated by Figures 37 and 38. Typical measured values of

the normal mode rejection of the LTC2414/LTC2418

operating with an internal oscillator and a 60Hz notch

setting are shown in Figure 37 superimposed over the

theoretical calculated curve. Similarly, typical measured

values of the normal mode rejection of the LTC2414/

LTC2418 operating with an internal oscillator and a 50Hz

notch setting are shown in Figure 38 superimposed over

the theoretical calculated curve.

As a result of these remarkable normal mode specifica-

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

of the LTC2414/LTC2418. If passive RC components are

placed in front of the LTC2414/LTC2418, the input dy-

namic 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.

Figure 35. Input Normal Mode Rejection

Figure 36. Input Normal Mode Rejection

Figure 37. Input Normal Mode Rejection vs Input Frequency

with Input Perturbation of 100% Full Scale (60Hz Notch)

Figure 38. Input Normal Mode Rejection vs Input Frequency

with Input Perturbation of 100% Full Scale (50Hz Notch)

APPLICATIO S I FOR ATIO

INPUT SIGNAL FREQUENCY (Hz)

2414/18 F35

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fN 3fN 4fN 5fN 6fN 7fN 8fN

INPUT SIGNAL FREQUENCY (Hz)

250fN 252fN 254fN 256fN 258fN 260fN 262fN

2414/18 F36

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

INPUT FREQUENCY (Hz)

105 120 135 150 165 180 195 210 225 240

2414/18 F37

–20

–40

–60

–80

–100

–120

VCC = 5V

REF+ = 5V

REF – = GND

VINCM = 2.5V

VIN(P-P) = 5V

SDI = GND

FO = GND

TA = 25°C

MEASURED DATA

CALCULATED DATA

INPUT FREQUENCY (Hz)

12.5

37.5

62.5

87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

2414/18 F38

–20

–40

–60

–80

–100

–120

VCC = 5V

REF+ = 5V

REF – = GND

VINCM = 2.5V

VIN(P-P) = 5V

SDI = GND

FO = 5V

TA = 25°C

MEASURED DATA

CALCULATED DATA

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