LTC2668 datasheet(20/40 Pages) LINER

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LTC2348-16

234816fa

For more information www.linear.com/LTC2348-16

INPUT VOLTAGE (V)

–1

LSB

234816 F02

011...111

011...110

000...001

000...000

100...000

100...001

111...110

LSB

BIPOLAR

ZERO

111...111

FSR/2 – 1LSB

–FSR/2

FSR = +FS – –FS

1LSB = FSR/65536

Figure 2. LTC2348-16 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

234816 F03

111...111

111...110

100...001

100...000

000...000

000...001

011...110

UNIPOLAR

ZERO

011...111

FSR – 1LSB

FSR = +FS

1LSB = FSR/65536

Figure 3. LTC2348-16 Straight Binary Transfer Function

ANALOG INPUTS

Each channel of the LTC2348-16 simultaneously samples

the voltage difference (VIN+ – VIN–) between its analog

input pins over a wide common mode input range while

attenuating unwanted signals common to both input

pins by the common-mode rejection ratio (CMRR) of the

ADC. Wide common mode input range coupled with high

CMRR allows the IN+/IN– analog inputs to swing with

an arbitrary relationship to each other, provided each

pin remains between (VCC – 4V) and VEE. This unique

feature of the LTC2348-16 enables it to accept a wide

variety of signal swings, including traditional classes of

analog input signals such as pseudo-differential unipolar,

applicaTions inForMaTion

pseudo-differential true bipolar, and fully differential,

simplifying signal chain design.

The wide operating range of the high voltage supplies

offers further input common mode flexibility. As long as

the voltage difference limits of 10V ≤ VCC – VEE ≤ 38V

are observed, VCC and VEE may be independently biased

anywhere within their own individual allowed operating

ranges, including the ability for either of the supplies to be

tied directly to ground. This feature enables the common

mode input range of the LTC2348-16 to be tailored to the

specific application’s requirements.

In all SoftSpan ranges, each channel’s analog inputs can

be modeled by the equivalent circuit shown in Figure 4.

At the start of acquisition, the 40pF sampling capacitors

(CIN) connect to the analog input pins IN+/IN– through the

samplingswitches,eachofwhichhasapproximately600Ω

(RIN)ofon-resistance.Theinitialvoltageonbothsampling

capacitorsatthestartofacquisitionisapproximatelyequal

to the sampled common-mode voltage (VIN+ + VIN–)/2

fromthepriorconversion.Theexternalcircuitryconnected

to IN+ and IN– must source or sink the charge that flows

through RIN as the sampling capacitors settle from their

initial voltages to the new input pin voltages over the

course of the acquisition interval. During conversion, nap,

and power down modes, the analog inputs draw only a

small leakage current. The diodes at the inputs provide

ESD protection.

IN+

RIN

600

RIN

600

CIN

40pF

CIN

40pF

VCC

VEE

BIAS

VOLTAGE

IN–

234816 F04

Figure 4. Equivalent Circuit for Differential Analog

Inputs, Single Channel Shown

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