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LTC2335-16

233516f

For more information www.linear.com/LTC2335-16

ANALOG INPUTS

The LTC2335-16 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 rejec-

tion 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 LTC2335-16 enables

it to accept a wide variety of signal swings, including

traditionalclassesofanaloginputsignalssuchaspseudo-

differential unipolar, 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 LTC2335-16 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

applicaTions inForMaTion

the sampling switches, each of which has approximately

600Ω (RIN) of on-resistance. This behavior occurs on all

channels, so that the LTC2335-16 may respond instantly

to user-requested changes in multiplexer configuration

with no additional settling time required.

Figure 4. Equivalent Circuit for Differential Analog Inputs,

Single Channel Shown

IN+

RIN

600

RIN

600

CIN

40pF

CIN

40pF

VCC

VEE

BIAS

VOLTAGE

IN–

233516 F04

Figure 3. LTC2335-16 Straight Binary Transfer Function

INPUT VOLTAGE (V)

233516 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

The initial voltage on both capacitors of the just-converted

channel will be approximately the sampled common mode

voltage (VIN+ + VIN–)/2 from the previous conversion.

Other channels’ capacitors will retain approximately the

voltage of their respective IN+/IN– pin at the beginning of

the previous conversion. The external circuitry connected

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.

Bipolar SoftSpan Input Ranges

For conversions configured in SoftSpan ranges 7, 6, 3,

or 2, the LTC2335-16 digitizes the differential analog

input voltage (VIN+ – VIN–) over a bipolar span of

±2.5 • VREFBUF, ±2.5 • VREFBUF/1.024, ±1.25 • VREFBUF, or

±1.25 • VREFBUF/1.024, respectively, as shown in Table

1a. These SoftSpan ranges are useful for digitizing input

signals where IN+ and IN– swing above and below each

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