LTC2445 datasheet(22/28 Pages) LINER

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LTC2444/LTC2445/

LTC2448/LTC2449

sn2444589 2444589fs

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–140

–130

–120

–110

–100

2440 F12

–90

–80

Figure 11. LTC2444/LTC2445/LTC2448/LTC2449

Normal Mode Rejection (Internal Oscillator)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–60

–40

180

2440 F11

–80

–100

120

240

–120

–140

–20

SINC4 ENVELOPE

Figure 10. LTC2444/LTC2445/LTC2448/LTC2449

Normal Mode Rejection (Internal Oscillator)

APPLICATIO S I FOR ATIO

Table 7. OSR vs Notch Frequency (fN) (with Internal Oscillator

Running at 9MHz)

OSR

NOTCH (fN)

28.16kHz

128

14.08kHz

256

7.04kHz

512

3.52kHz

1024

1.76kHz

2048

880Hz

4096

440Hz

8192

220Hz

16384

110Hz

32768*

55Hz

*Simultaneous 50/60Hz rejection

Normal Mode Rejection and Antialiasing

One of the advantages delta-sigma ADCs offer over con-

ventional ADCs is on-chip digital filtering. Combined with

a large oversampling ratio, the LTC2444/LTC2445/

LTC2448/LTC2449 significantly simplify antialiasing filter

requirements.

The LTC2444/LTC2445/LTC2448/LTC2449’s speed/reso-

lution is determined by the over sample ratio (OSR) of the

on-chip digital filter. The OSR ranges from 64 for 3.5kHz

output rate to 32,768 for 6.9Hz (in No Latency mode)

output rate. The value of OSR and the sample rate fS

determine the filter characteristics of the device. The first

NULL of the digital filter is at fN and multiples of fN where

fN = fS/OSR, see Figure 10 and Table 7. The rejection at the

frequency fN ±14% is better than 80dB, see Figure 11.

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