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LTC2358-16

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

Bipolar SoftSpan Input Ranges

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

or 2, the LTC2358-16 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.

TheseSoftSpanrangesareusefulfordigitizinginputsignals

where IN+ and IN– swing above and below each other.

Traditionalexamplesincludefullydifferentialinputsignals,

where IN+ and IN– are driven 180 degrees out-of-phase

with respect to each other centered around a common

mode voltage (VIN+ + VIN–)/2, and pseudo-differential

true bipolar input signals, where IN+ swings above and

below a ground reference level, driven on IN–. Regardless

of the chosen SoftSpan range, the wide common mode

input range and high CMRR of the IN+/IN– analog inputs

allow them to swing with an arbitrary relationship to each

other, provided each pin remains between (VCC – 4V) and

(VEE + 4V). The output data format for all bipolar SoftSpan

ranges is two’s complement.

Unipolar SoftSpan Input Ranges

For channels configured in SoftSpan ranges 5, 4, or 1, the

LTC2358-16 digitizes the differential analog input voltage

(VIN+ – VIN–) over a unipolar span of 0V to 2.5 • VREFBUF,

0V to 2.5 • VREFBUF/1.024, or 0V to 1.25 • VREFBUF, respec-

tively, as shown in Table 1a. These SoftSpan ranges are

useful for digitizing input signals where IN+ remains above

IN–. A traditional example includes pseudo-differential

unipolar input signals, where IN+ swings above a ground

reference level, driven on IN–. Regardless of the chosen

SoftSpan range, the wide common mode input range and

high CMRR of the IN+/IN– analog inputs allow them to

swingwithanarbitraryrelationshiptoeachother,provided

each pin remains between (VCC – 4V) and (VEE + 4V).

The output data format for all unipolar SoftSpan ranges

is straight binary.

INPUT DRIVE CIRCUITS

The CMOS buffer input stage offers a very high degree of

transient isolation from the sampling process. Most sen-

sors,signalconditioningamplifiersandfilternetworkswith

less than 10kΩ of impedance can drive the passive 3pF

analog input capacitance directly. For higher impedances

and slow-settling circuits, add a 680pF capacitor at the

pins to maintain the full DC accuracy of the LTC2358-16.

The very high input impedance of the unity gain buffers in

the LTC2358-16 greatly reduces the drive requirements of

the differential amplifier and make it possible to include

optional RC filters with kΩ impedance and arbitrarily slow

time constants for anti-aliasing or other purposes. Micro-

power op amps with limited drive capability are also well

suited to drive the high impedance analog inputs directly.

The LTC2358-16 features proprietary circuitry to achieve

exceptional internal crosstalk isolation between channels

(109dB typical). The PC board wiring to the analog inputs

shouldbeshortandshieldedtopreventexternalcapacitive

crosstalkbetweenchannels.Thecapacitancebetweenadja-

cent package pins is 0.16pF.Low source resistance and/or

high source capacitance help reduce external capacitively

coupled crosstalk. Single ended input drive also enjoys

additional external crosstalk isolation because every other

input pin is grounded, or at a low impedance DC source,

and serves as a shield between channels.

INPUT OVERDRIVE TOLERANCE

DrivingananaloginputaboveVCConanychannelupto10mA

willnotaffectconversionresultsonotherchannels.Approx-

imately70%ofthisoverdrivecurrentwillflowoutoftheVCC

pinandtheremaining30%willflowoutofVEE.Thiscurrent

flowing out of VEE will produce heat across the VCC – VEE

voltage drop and must be taken into account for the total

Absolute Maximum power dissipation of 500mW. Driving

an analog input below VEE may corrupt conversion results

on other channels. This product can handle input currents

of up to 100mA below VEE or above VCC without latchup.

Keep in mind that driving the inputs above VCC or below

VEE may reverse the normal current flow from the external

power supplies driving these pins.

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