Datasheet

LTC2444/LTC2445/
LTC2448/LTC2449
22
2444589fb
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
47
–140
NORMAL MODE REJECTION (dB)
–130
–120
–110
–100
51 55
59
63
2440 F12
–90
–80
49 53
57
61
Figure 11. LTC2444/LTC2445/LTC2448/LTC2449
Normal Mode Rejection (Internal Oscillator)
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
180
2440 F11
–80
–100
60 120 240
–120
–140
–20
NORMAL MODE REJECTION (dB)
SINC
4
ENVELOPE
Figure 10. LTC2444/LTC2445/LTC2448/LTC2449
Normal Mode Rejection (Internal Oscillator)
APPLICATIO S I FOR ATIO
WUU
U
Table 7. OSR vs Notch Frequency (f
N
) (with Internal Oscillator
Running at 9MHz)
OSR NOTCH (f
N
)
64 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 f
S
determine the filter characteristics of the device. The first
NULL of the digital filter is at f
N
and multiples of f
N
where
f
N
= f
S
/OSR, see Figure 10 and Table 7. The rejection at the
frequency f
N
±14% is better than 80dB, see Figure 11.