LTC2440IGN datasheet(19/28 Pages) LINER

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LTC2440

sn2440, 2440fas

Figure 10. Internal Serial Clock, Continuous Operation

SDO

BUSY

SCK

(INTERNAL)

LSB24

MSB

SIG

BIT 5

BIT 0

BIT 27

BIT 26

BIT 28

BIT 29

BIT 30

EOC

BIT 31

SLEEP

DATA OUTPUT

CONVERSION

2410 F10

= EXTERNAL OSCILLATOR

= INTERNAL OSCILLATOR

VCC

REF+

REF–

SCK

BUSY

IN+

IN–

SDO

GND

EXT

1, 8, 9, 16

SDI

VCC

REFERENCE VOLTAGE

0.1V TO VCC

ANALOG INPUT RANGE

–0.5VREF TO 0.5VREF

µF

4.5V TO 5.5V

LTC2440

2-WIRE

SPI INTERFACE

200nV NOISE, 50/60Hz REJECTION

10-SPEED/RESOLUTION PROGRAMMABLE

µV NOISE, 880Hz OUTPUT RATE

VCC

APPLICATIO S I FOR ATIO

ends after the 32nd rising edge. Data is shifted out the SDO

pin on each 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 con-

ventional 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 fS

determine the filter characteristics of the device. The first

NULL of the digital filter is at fN and multiples of fN where

fN = fS/OSR, see Figure 11 and Table 5. The rejection at the

frequency fN ±14% is better than 80dB, see Figure 12.

If FO is grounded, fS 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 fN = 55Hz and the no

latency output rate is fN/8 = 6.9Hz. At the maximum OSR,

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–60

–40

180

2440 F11

–80

–100

120

240

–120

–140

–20

Figure 11. LTC2440 Normal Mode Rejection (Internal Oscillator)

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