Datasheet

LTC2440
19
2440fe
For more information www.linear.com/LTC2440
APPLICATIONS INFORMATION
Figure 10. Internal Serial Clock, Continuous Operation
falling edge of SCK. The internally generated serial clock
is output to the SCK pin. This signal may be used to shift
the conversion result into external circuitry. EOC can be
latched on the first rising edge of SCK and the last bit of
the conversion result can be latched on the 32nd rising
edge of SCK. After the 32nd rising edge, SDO goes HIGH
(EOC = 1) indicating a new conversion is in progress. SCK
remains HIGH during the conversion.
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 LTC2440 significantly
simplifies antialiasing filter requirements.
The LTC2440’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 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 5. 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,
Figure 11. LTC2440 Normal Mode Rejection (Internal Oscillator)
SDO
BUSY
SCK
(INTERNAL)
CS
LSB
24
MSBSIG
BIT 5 BIT 0BIT 27 BIT 26BIT 28BIT 29BIT 30
EOC
BIT 31
SLEEP
DATA OUTPUT CONVERSIONCONVERSION
2410 F10
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
V
CC
f
O
REF
+
REF
–
SCK
BUSY
IN
+
IN
–
SDO
GND
CS
EXT
2
14
3
4
13
5
6
12
1, 8, 9, 16
11
10
15
SDI
V
CC
7
REFERENCE VOLTAGE
0.1V TO V
CC
ANALOG INPUT RANGE
–0.5V
REF
TO 0.5V
REF
1µF
4.5V TO 5.5V
LTC2440
2-WIRE
SPI INTERFACE
200nV NOISE, 50/60Hz REJECTION
10-SPEED/RESOLUTION PROGRAMMABLE
2µV NOISE, 880Hz OUTPUT RATE
V
CC
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
180
2440 F11
–80
–100
60 120 240
–120
–140
–20
NORMAL MODE REJECTION (dB)
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