LTC2440
21
2440fe
For more information www.linear.com/LTC2440
APPLICATIONS INFORMATION
Figure 14. LTC2440 Normal Mode Rejection
(External Oscillator at 90kHz)
The sample rate f
S
and NULL f
N
, my 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
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 22. By floating pin 4 (DIV) of the
LTC1799, the output oscillator frequency is:
f MHz
k
R
OSC
=
10
10
10
•
•
The normal mode rejection characteristic shown in
Figure 14 is achieved by applying the output of the LTC1799
(with R
SET
= 100k) to the f
O
pin on the LTC2440 with SDI
tied HIGH (OSR = 32768).
Reduced Power Operation
In addition to adjusting the speed/resolution of the
LTC2440, the speed/resolution/power dissipation may
also be adjusted using the automatic sleep mode. During
the conversion cycle, the LTC2440 draws 8mA supply
current independent of the programmed speed. Once the
conversion cycle is completed, the device automatically
enters a low power sleep state drawing 8µA. The device
remains in this state as long as CS is HIGH and data is not
shifted out. By adjusting the duration of the sleep state
(hold CS HIGH longer) and the duration of the conversion
cycle (programming OSR) the DC power dissipation can
be reduced, see Figure 16.
For example, if the OSR is programmed at the fastest rate
(OSR = 64, t
CONV
= 0.285ms) and the sleep state is 10ms,
the effective output rate is approximately 100Hz while the
average supply current is reduced to 240µA. By further
extending the sleep state to 100ms, the effective output
rate of 10Hz draws on average 30µA. Noise, power, and
speed can be optimized by adjusting the OSR (Noise/
Speed) and sleep mode duration (Power).
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–40
–20
0
8
2440 F14
–60
–80
2 4 6 10
–100
–120
–140
NORMAL MODE REJECTION (dB)
