LTC2756 datasheet(20/40 Pages) LINER

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LTC2358-16

Rev A

For more information www.analog.com

The ideal two’s complement transfer function is shown in

Figure 2, while the ideal straight binary transfer function

is shown in Figure 3.

INPUT VOLTAGE (V)

–1

LSB

235816 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. LTC2358-16 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

235816 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. LTC2358-16 Straight Binary Transfer Function

BUFFERED ANALOG INPUTS

Each channel of the LTC2358-16 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 (VEE + 4V) and (VCC – 4V). This feature of the

APPLICATIONS INFORMATION

LTC2358-16 enables it to accept a wide variety of signal

swings,includingtraditionalclassesofanaloginputsignals

such as pseudo-differential unipolar, pseudo-differential

true bipolar, and fully differential, simplifying signal chain

design. For conversion of signals extending to VEE, the

unbuffered LTC2348-16 ADC is recommended.

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 individually allowed operating

ranges, including the ability for VEE to be tied directly to

ground. This feature enables the common mode input

range of the LTC2358-16 to be tailored to specific ap-

plication 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 sampling capacitors (CSAMP)

connect to the integrated buffers Buffer+/Buffer– through

the sampling switches. The sampled voltage is reset dur-

ing the conversion process and is therefore re-acquired

for each new conversion.

The diodes between the inputs and the VCC and VEE sup-

pliesprovideinputESDprotection.Whilewithinthesupply

voltages, the analog inputs of the LTC2358-16 draw only

5pA typical DC leakage current and the ESD protection

diodes don’t turn on. This offers a significant advantage

over external op amp buffers, which often have diode

protection that turns on during transients and corrupts

the voltage on any filter capacitors at their inputs.

IN+

RSAMP

750

RSAMP

750

CSAMP

30pF

CSAMP

30pF

VCC

VEE

BIAS

VOLTAGE

IN–

235816 F04

BUFFER+

BUFFER–

Figure 4. Equivalent Circuit for Differential Analog

Inputs, Single Channel Shown

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