Datasheet

LTC2440
20
2440fd
APPLICATIONS INFORMATION
OSR, the noise performance of the device is 200nV
RMS
with
better than 80dB rejection of 50Hz ±2% and 60Hz ±2%.
Since the OSR is large (32,768) the wide band rejection
is extremely large and the antialiasing requirements are
simple. The fi rst multiple of f
S
occurs at 55Hz • 32,768 =
1.8MHz, see Figure 13.
The fi rst NULL becomes f
N
= 7.04kHz with an OSR of 256
(an output rate of 880Hz) and f
O
grounded. While the NULL
has shifted, the sample rate remains constant. As a result
of constant modulator sampling rate, the linearity, offset
and full-scale performance remains unchanged as does
the fi rst multiple of f
S
.
Figure 12. LTC2440 Normal Mode Rejection (Internal Oscillator) Figure 13. LTC2440 Normal Mode Rejection (Internal Oscillator)
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
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
1440 F13
–80
–100
1000000 2000000
–120
1.8MHz
–140
–20
NORMAL MODE REJECTION (dB)
REJECTION > 120dB
Table 5. 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 50/60 rejection