LTC2484CDD datasheet(13/40 Pages) LINER

13 / 40 page

LTC2484

2484fa

APPLICATIO S I FOR ATIO

While in this sleep state, power consumption is reduced by

two orders of magnitude. 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 exits the low power

mode and enters the data output state. If CS is pulled HIGH

before the first rising edge of SCK, the device returns to the

low power sleep mode and the conversion result is still

held in the internal static shift register. If CS remains LOW

after the first rising edge of SCK, the device begins

outputting the conversion result. Taking CS high at this

point will terminate the data input and output state and

start a new conversion. The conversion result is shifted

out of the device through the serial data output pin (SDO)

on the falling edge of the serial clock (SCK) (see Figure 2).

The LTC2484 includes a serial data input pin (SDI) in

which data is latched by the device on the rising edge of

SCK (Figure 2). The bit stream applied to this pin can be

used to select various features of the LTC2484, including

an on-chip temperature sensor, line frequency rejection

and output data rate. Alternatively, this pin may be tied to

ground and the part will perform conversions in a default

state. In the default state (SDI grounded) the device simply

performs conversions on the user applied input with

simultaneous rejection of 50Hz and 60Hz line frequencies.

Through timing control of the CS and SCK pins, the

LTC2484 offers several flexible modes of operation

(internal or external SCK and free-running conversion

modes). These various modes do not require program-

ming configuration registers; moreover, they do not dis-

turb the cyclic operation described above. These modes of

operation are described in detail in the Serial Interface

Timing Modes section.

Easy Drive Input Current Cancellation

The LTC2484 combines a high precision delta-sigma ADC

with an automatic differential input current cancellation

front end. A proprietary front-end passive sampling

network transparently removes the differential input cur-

rent. This enables external RC networks and high imped-

ance sensors to directly interface to the LTC2484 without

external amplifiers. The remaining common mode input

current is eliminated by either balancing the differential

input impedances or setting the common mode input

equal to the common mode reference (see Automatic

Input Current Cancellation section). This unique architec-

ture does not require on-chip buffers enabling input sig-

nals to swing all the way to ground and up to VCC.

Furthermore, the cancellation does not interfere with the

transparent offset and full-scale autocalibration and the

absolute accuracy (full scale + offset + linearity) is main-

tained with external RC networks.

Accessing the Special Features of the LTC2484

The LTC2484 combines a high resolution, low noise

∆Σ

analog-to-digital converter with an on-chip selectable tem-

perature sensor, programmable digital filter and output

rate control. These special features are selected through a

single 8-bit serial input word during the data input/output

cycle (see Figure 2).

The LTC2484 powers up in a default mode commonly

used for most measurements. The device will remain in

this mode as long as the serial data input (SDI) is low. In

this default mode, the measured input is external, the

digital filter simultaneously rejects 50Hz and 60Hz line

frequency noise, and the speed mode is 1x (offset auto-

matically, continuously calibrated).

A simple serial interface grants access to any or all special

functions contained within the LTC2484. In order to

change the mode of operation, an enable bit (EN) followed

by up to 7 bits of data are shifted into the device

(see Table 1). The first 3 bits, in order to remain pin

compatible with the LTC2480, are DON’T CARE and can

be either HIGH or LOW. The 4th bit (IM) is used to select

the internal temperature sensor as the conversion input,

while the 5th and 6th bits (FA, FB) combine to determine

the line frequency rejection mode. The 7th bit (SPD) is

used to double the output rate by disabling the offset auto

calibration.

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