Datasheet

LTC2444/LTC2445/
LTC2448/LTC2449
23
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional 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/
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 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 11 and Table 7. The rejection at
the frequency f
N
±14% is better than 80dB, see Figure12.
If F
O
is grounded, f
S
is set by the on-chip oscillator at
1.8MHz (over supply and temperature variations). At an
OSR of 32,768, the first NULL is at f
N
= 55Hz and the
no latency output rate is f
N
/8 = 6.9Hz. At the maximum
OSR, the noise performance of the device is 280nV
RMS
(LTC2444/LTC2448) and 200nV
RMS
(LTC2445/LTC2449)
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
Figure 11. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator)
Figure 12. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator)
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
180
2444589 F11
–80
–100
60 120
240
–120
–140
–20
NORMAL MODE REJECTION (dB)
SINC
4
ENVELOPE
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