LTC2480 datasheet(14/42 Pages) LINER

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LTC2480

2480fc

APPLICATIONS INFORMATION

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 LTC2480 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 LTC2480, including

an on-chip temperature sensor, programmable GAIN, 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 a GAIN of 1 and simultaneous

rejection of 50Hz and 60Hz line frequencies.

Through timing control of the CS and SCK pins, the LTC2480

offers several ﬂexible modes of operation (internal or

external SCK and free-running conversion modes). 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.

Easy Drive Input Current Cancellation

The LTC2480 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

current. This enables external RC networks and high

impedance sensors to directly interface to the LTC2480

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

the Automatic Input Current Cancellation section). This

unique architecture does not require on-chip buffers

enabling input signals to swing all the way to ground

and up to VCC. Furthermore, the cancellation does

not interfere with the transparent offset and full-scale

auto-calibration and the absolute accuracy (full-scale

+ offset + linearity) is maintained even with external

RC networks.

Accessing the Special Features of the LTC2480

The LTC2480 combines a high resolution, low noise

ΔΣ

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

temperature sensor, programmable gain, 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 LTC2480 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

GAIN is 1, the digital ﬁlter simultaneously rejects 50Hz

and 60Hz line frequency noise, and the speed mode is 1x

(offset automatically, continuously calibrated).

A simple serial interface grants access to any or all special

functions contained within the LTC2480. 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 (GS2, GS1, GS0) control the GAIN of the

converter from 1 to 256. 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.

LTC2480 Datasheet(PDF) 14 Page - Linear Technology

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