LTC2444/LTC2445/
LTC2448/LTC2449
24
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
clock applied to F
O
results in a NULL at 0.6Hz plus all
harmonics up to 20kHz, see Figure 14. This is useful in
applications requiring digitalization of the DC component
of a noisy input signal and eliminates the need of placing
a 0.6Hz filter in front of the ADC.
An external oscillator operating from 100kHz to 12MHz can
be implemented using the LTC1799 (resistor set SOT-23
oscillator), see Figure 17. By floating pin 4 (DIV) of the
LTC1799, the output oscillator frequency is:
MHz
R
OSC
=
10
10
•
•
The normal mode rejection characteristic shown in
Figure13 is achieved by applying the output of the LTC1799
(with R
SET
= 100k) to the F
O
pin on the LTC2444/LTC2445/
LTC2448/LTC2449 with SDI tied HIGH (OSR = 32768).
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
–80
–100
1000000
–120
1.8MHz
–140
–20
NORMAL MODE REJECTION (dB)
REJECTION > 120dB
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–40
–20
8
–60
–80
2 4 6
–100
–120
–140
NORMAL MODE REJECTION (dB)
Figure 13. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator)
Figure 14. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (External Oscillator at 90kHz)
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 13.
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, offset
and full-scale performance remains unchanged as does
the first multiple of f
S
.
The sample rate f
S
and NULL f
N
, may also be adjusted by
driving the F
O
pin with an external oscillator. The sample
rate is f
S
= f
EOSC
/5, where f
EOSC
is the frequency of the
clock applied to F
O
. Combining a large OSR with a reduced
sample rate leads to notch frequencies f
N
near DC while
maintaining simple antialiasing requirements. A 100kHz