Datasheet

LTC2446/LTC2447
21
24467fa
Figure 11. LTC2446/LTC2447
Normal Mode Rejection (Internal Oscillator)
Figure 10. LTC2446/LTC2447
Normal Mode Rejection (Internal Oscillator)
APPLICATIO S I FOR ATIO
WUU
U
Table 6. 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 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 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
6. The rejection at the frequency f
N
±14% is better than
80dB, see Figure 11.
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
180
24467 F10
–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
24467 F11
–90
–80
49 53
57
61
If F
O
is grounded, f
S
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 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
(LTC2446) and 200nV
RMS
(LTC2447) 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 first
multiple of f
S
occurs at 55Hz • 32,768 = 1.8MHz, see
Figure 12.
The first 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,
Figure 12. LTC2446/LTC2447
Normal Mode Rejection (Internal Oscillator)
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
24467 F12
–80
–100
1000000 2000000
–120
1.8MHz
–140
–20
NORMAL MODE REJECTION (dB)
REJECTION > 120dB