Datasheet

LTC2440
19
2440fd
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 fi rst 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 fi ltering. Combined
with a large oversampling ratio, the LTC2440 signifi cantly
simplifi es antialiasing fi lter requirements.
The LTC2440’s speed/resolution is determined by the
over sample ratio (OSR) of the on-chip digital fi lter. 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 fi lter characteristics of the device.
The fi rst NULL of the digital fi lter 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 ±5% (over supply and temperature variations). At
an OSR of 32,768, the fi rst NULL is at f
N
= 55Hz and the
no latency output rate is f
N
/8 = 6.9Hz. At the maximum
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)