LTC2440IGN datasheet(10/28 Pages) LINER

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LTC2440

sn2440, 2440fas

operation cycle and data out format is compatible with the

LTC2410.

Initially, the LTC2440 performs a conversion. Once the

conversion is complete, the device enters the sleep state.

While in this sleep state, power consumption is reduced

below 10

µA. The part remains in the sleep state as long as

CS is HIGH. The conversion result is held indefinitely in a

static shift register while the converter is in the sleep state.

Once CS is pulled LOW, the device begins outputting the

conversion result. There is no latency in the conversion

result. The data output corresponds to the conversion just

performed. This result is shifted out on the serial data out

pin (SDO) under the control of the serial clock (SCK). Data

is updated on the falling edge of SCK allowing the user to

reliably latch data on the rising edge of SCK (see Figure 3).

The data output state is concluded once 32-bits are read

out of the ADC or when CS is brought HIGH. The device au-

tomatically initiates a new conversion and the cycle repeats.

Through timing control of the CS, SCK and EXT pins, the

LTC2440 offers several flexible modes of operation

(internal or external SCK). These various modes do not

require programming configuration registers; moreover,

they do not disturb the cyclic operation described above.

These modes of operation are described in detail in the

Serial Interface Timing Modes section.

Ease of Use

The LTC2440 data output has no latency, filter settling

delay or redundant data associated with the conversion

cycle. There is a one-to-one correspondence between the

conversion and the output data. Therefore, multiplexing

multiple analog voltages is easy. Speed/resolution ad-

justments may be made seamlessly between two conver-

sions without settling errors.

The LTC2440 performs offset and full-scale calibrations

every conversion cycle. This calibration is transparent to

the user and has no effect on the cyclic operation de-

scribed above. The advantage of continuous calibration is

extreme stability of offset and full-scale readings with re-

spect to time, supply voltage change and temperature drift.

APPLICATIO S I FOR ATIO

Power-Up Sequence

The LTC2440 automatically enters an internal reset state

when the power supply voltage VCC drops below

approximately 2.2V. This feature guarantees the integrity

of the conversion result and of the serial interface mode

selection.

When the VCC voltage rises above this critical threshold,

the converter creates an internal power-on-reset (POR)

signal with a duration of approximately 0.5ms. The POR

signal clears all internal registers. Following the POR

signal, the LTC2440 starts a normal conversion cycle and

follows the succession of states described above. The first

conversion result following POR is accurate within the

specifications of the device if the power supply voltage is

restored within the operating range (4.5V to 5.5V) before

the end of the POR time interval.

Reference Voltage Range

This converter accepts a truly differential external refer-

ence voltage. The absolute/common mode voltage speci-

fication for the REF+ and REF– pins covers the entire range

from GND to VCC. For correct converter operation, the

REF+ pin must always be more positive than the REF– pin.

The LTC2440 can accept a differential reference voltage

from 0.1V to VCC. The converter output noise is deter-

mined by the thermal noise of the front-end circuits, and

as such, its value in microvolts is nearly constant with

reference voltage. A decrease in reference voltage will not

significantly improve the converter’s effective resolution.

On the other hand, a reduced reference voltage will im-

prove the converter’s overall INL performance.

Input Voltage Range

The analog input is truly differential with an absolute/

common mode range for the IN+ and IN– input pins

extending from GND – 0.3V to VCC + 0.3V. Outside

these limits, the ESD protection devices begin to turn on

and the errors due to input leakage current increase

rapidly. Within these limits, the LTC2440 converts the

bipolar differential input signal, VIN = IN+ – IN–, from

– FS = – 0.5 • VREF to +FS = 0.5 • VREF where VREF =

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