LTC2446 datasheet(21/28 Pages) LINER

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LTC2446/LTC2447

24467fa

Figure 11. LTC2446/LTC2447

Normal Mode Rejection (Internal Oscillator)

Figure 10. LTC2446/LTC2447

Normal Mode Rejection (Internal Oscillator)

APPLICATIO S I FOR ATIO

Table 6. 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 LTC2446/LTC2447 signifi-

cantly simplify antialiasing filter requirements.

The LTC2446/LTC2447’s speed/resolution 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 1x 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

6. The rejection at the frequency fN ±14% is better than

80dB, see Figure 11.

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–60

–40

180

24467 F10

–80

–100

120

240

–120

–140

–20

SINC4 ENVELOPE

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–140

–130

–120

–110

–100

24467 F11

–90

–80

If FO is grounded, fS is set by the on-chip oscillator at

1.8MHz

±5% (over supply and temperature variations). At

an OSR of 32,768, the first NULL is at fN = 55Hz and the no

latency output rate is fN/8 = 6.9Hz. At the maximum OSR,

the noise performance of the device is 280nVRMS

(LTC2446) and 200nVRMS (LTC2447) with better than

80dB rejection of 50Hz

±2%and60Hz±2%.SincetheOSR

is large (32,768) the wide band rejection is extremely large

and the antialiasing requirements are simple. The first

multiple of fS occurs at 55Hz • 32,768 = 1.8MHz, see

Figure 12.

The first NULL becomes fN = 7.04kHz with an OSR of 256

(an output rate of 880Hz) and FO grounded. While the

NULL has shifted, the sample rate remains constant. As a

result of constant modulator sampling rate, the linearity,

Figure 12. LTC2446/LTC2447

Normal Mode Rejection (Internal Oscillator)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–60

–40

24467 F12

–80

–100

1000000

2000000

–120

1.8MHz

–140

–20

REJECTION > 120dB

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