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LTC2335-16

233516f

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

applicaTions inForMaTion

OVERVIEW

The LTC2335-16 is a 16-bit, low noise 8-channel multi-

plexed successive approximation register (SAR) ADC with

differential, wide common mode range inputs. The ADC

operates from a 5V low voltage supply and flexible high

voltage supplies, nominally ±15V. Using the integrated

low-driftreferenceandbuffer(VREFBUF = 4.096Vnominal),

this SoftSpan ADC can be configured on a conversion-by-

conversionbasistoaccept±10.24V,0Vto10.24V,±5.12V,

or 0V to 5.12V signals on any channel. Alternately, the

ADC may be programmed to cycle through a sequence of

channelsandrangeswithoutfurtheruserintervention. The

input signal range may be expanded up to ±12.5V using

an external 5V reference.

The wide input common mode range and high CMRR

(118dB typical, VIN+ = VIN– = 18VP-P 200Hz Sine) of the

LTC2335-16analoginputsallowtheADCtodirectlydigitize

a variety of signals, simplifying signal chain design. The

absolute common mode input range is determined by

the choice of high voltage supplies, which may be biased

asymmetrically around ground and include the ability for

either the positive or negative supply to be tied directly to

ground. This input signal flexibility, combined with ±1LSB

INL, no missing codes at 16-bits, and 94.4dB SNR, makes

the LTC2335-16 an ideal choice for many high voltage

applications requiring wide dynamic range.

The LTC2335-16 supports pin-selectable SPI CMOS (1.8V

to 5V) and LVDS serial interfaces, enabling it to communi-

cate equally well with legacy microcontrollers and modern

FPGAs.TheLTC2335-16typicallydissipates180mWwhen

converting at 1Msps throughput. Optional nap and power

down modes may be employed to further reduce power

consumption during inactive periods.

CONVERTER OPERATION

The LTC2335-16 operates in two phases. During the ac-

quisition phase, the sampling capacitors in each channel

connect to their respective analog input pins and track the

differentialanaloginputvoltage(VIN+–VIN–).Arisingedge

ontheCNVpintransitionstheS/Hcircuitsfromtrackmode

to hold mode, sampling the input signals and initiating a

conversion. During the conversion phase, the selected

channel's sampling capacitors are connected to a 16-bit

chargeredistributioncapacitorD/Aconverter(CDAC).The

CDAC is sequenced through a successive approximation

algorithm,effectivelycomparingthesampledinputvoltage

with binary-weighted fractions of the channel’s SoftSpan

full-scalerange(e.g.,VFSR/2,VFSR/4…VFSR/65536)using

a differential comparator. At the end of this process, the

CDAC output approximates the channel’s sampled analog

input.TheADCcontrollogicthenpreparesthe16-bitdigital

output code for serial transfer.

TRANSFER FUNCTION

The LTC2335-16 digitizes the full-scale voltage range into

216 levels. In conjunction with the ADC master reference

voltage, VREFBUF, the selected SoftSpan configuration

determines its input voltage range, full-scale range, LSB

size, and the binary format of its conversion result, as

shown in Tables 1a and 1b. For example, employing the

internal reference and buffer (VREFBUF = 4.096V nominal),

SoftSpan 7 configures a channel to accept a ±10.24V bi-

polar analog input voltage range, which corresponds to a

20.48Vfull-scalerangewitha312.5μVLSB.OtherSoftSpan

configurationsandreferencevoltagesmaybeemployedto

convert both larger and smaller bipolar and unipolar input

ranges. Conversion results are output in two’s comple-

ment binary format for all bipolar SoftSpan ranges, and

in straight binary format for all unipolar SoftSpan ranges.

The ideal two’s complement transfer function is shown in

Figure 2, while the ideal straight binary transfer function

is shown in Figure 3.

Figure 2. LTC2335-16 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

–1

LSB

233516 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

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