LTC2420_15 Datasheet, PDF (19/28 Pages) Linear Technology – 20-Bit mPower No Latency ADC in SO-8

LTC2440
APPLICATIONS INFORMATION
4.5V TO 5.5V
1μF
2 _VCC
15
BUSY
REFERENCE VOLTAGE
0.1V TO V_CC
ANALOG INPUT RANGE
–0.5V_REF TO 0.5V_REF
3
LTC2440
REF⁺
f_O
14
4 _REF–
SCK ¹³
5 _IN+
SDO ¹²
6 _IN–
CS ¹¹
7
SDI
1, 8, 9, 16 _GND
EXT ¹⁰
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
2-WIRE
SPI INTERFACE
V_CC
200nV NOISE, 50/60Hz REJECTION
10-SPEED/RESOLUTION PROGRAMMABLE
2μV NOISE, 880Hz OUTPUT RATE
V_CC
CS
BIT 31 BIT 30
BIT 29
BIT 28
BIT 27
BIT 26
BIT 5
BIT 0
SDO
EOC
SIG
MSB
LSB₂₄
SCK
(INTERNAL)
BUSY
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2410 F10
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 ±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
0
–20
–40
–60
–80
–100
–120
–140
0
60
120
180
240
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2440 F11
Figure 11. LTC2440 Normal Mode Rejection (Internal Oscillator)
2440fd
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