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LTC2335-18

233518f

For more information www.linear.com/LTC2335-18

applicaTions inForMaTion

OVERVIEW

The LTC2335-18 is an 18-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

LTC2335-18analoginputsallowtheADCtodirectlydigitize

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 ±3LSB

INL, no missing codes at 18-bits, and 96.7dB SNR, makes

the LTC2335-18 an ideal choice for many high voltage

applications requiring wide dynamic range.

The LTC2335-18 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.TheLTC2335-18typicallydissipates180mWwhen

converting at 1Msps throughput. Optional nap and power

down modes may be employed to further reduce power

consumption during inactive periods.

CONVERTER OPERATION

The LTC2335-18 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 differential analog input voltage (VIN+ – VIN–). A ris-

ing edge on the CNV pin transitions the S/H circuits from

track mode to hold mode, sampling the input signals and

initiating a conversion. During the conversion phase, the

selected channel's sampling capacitors are connected to

an 18-bit charge redistribution capacitor D/A converter

(CDAC). The CDAC is sequenced through a successive ap-

proximationalgorithm,effectivelycomparingthesampled

input voltage with binary-weighted fractions of the chan-

nel’s SoftSpan full-scale range (e.g., VFSR/2, VFSR/4 …

VFSR/262144) using a differential comparator. At the end

of this process, the CDAC output approximates the chan-

nel’s sampled analog input. The ADC control logic then

prepares the 18-bit digital output code for serial transfer.

TRANSFER FUNCTION

The LTC2335-18 digitizes the full-scale voltage range into

218 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

bipolar analog input voltage range, which corresponds

to a 20.48V full-scale range with a 78.125μV LSB. Other

SoftSpan configurations and reference voltages may be

employed to convert both larger and smaller bipolar and

unipolar input ranges. Conversion results are output in

two’s complement 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. LTC2335-18 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

–1

LSB

233518 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/262144

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