LTC6655 datasheet(37/40 Pages) LINER

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LTC2335-18

233518f

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

applicaTions inForMaTion

ThedataonSDOareformattedasa24-bitpacketconsisting

ofan18-bitconversionresult,3-bitanalogchannelID,and

3-bitSoftSpancode,allpresentedMSBfirst.Assuggested

in Figures 19 and 20, if more than 24 SCKI clocks are

applied, the 24-bit packet is repeated indefinitely on SDO.

The LTC2335-18 guarantees a minimum data transfer

window(tACQ–tQUIET)of400nswhileconvertingat1Msps.

Thus, if an application needs to read the full 24-bit packet

of conversion result plus channel ID and SoftSpan, the

minimum usable SCKI frequency is 30MHz (60Mbps).

Applications needing to read only the conversion result

may send only 18 SCKI edges and thus have a minimum

SCKI frequency of 22.5MHz (45Mbps). The LTC2335-18

supportsLVDSSCKIfrequenciesupto250MHz(500Mbps).

Configuring the Multiplexer and SoftSpan Range in

LVDS I/O Mode

On power-up and after a reset, the LTC2335-18 defaults

to converting channels 0 through 7 sequentially, all in

SoftSpan 7. If this configuration does not need to be

changed, simply hold SDI at an LVDS low level.

The LTC2335-18 multiplexer and SoftSpan range may be

controlled in two ways, depending on the needs of the ap-

plication.IfthedesiredsequenceofchannelsandSoftSpan

rangesareknownaheadoftime,theLTC2335-18’sinternal

sequencer may be programmed with a sequence of up to

16 configurations, and will cycle through those configu-

rations on subsequent conversions without further user

intervention.Alternatelyifultimateflexibilityisdesired,the

LTC2335-18 may be directly controlled by overwriting the

sequencereachconversionwiththechannelandSoftSpan

range for the following conversion. This reconfiguration

has no latency and requires no additional settling time or

digital I/O overhead.

Using the Sequencer

To use the internal sequencer of the LTC2335-18, first

program it as described below with the desired sequence

of up to 16 configurations. Each of these configurations

specifies the desired channel number and SoftSpan range

for one conversion. The LTC2335-18 will then apply the

first configuration to the first conversion, the second

configuration to the second conversion, and so on until

the end of the programmed sequence is reached, at which

point the cycle will start again from the beginning.

Eachdatatransactionwindowisanopportunitytoprogram

thesequencerbyclockinginaseriesof8-bitcontrolwords

on SDI, each specifying a channel number and SoftSpan

range, as shown in Figures 19 and 20. To program the

sequencer with a series of up to 16 conversion configu-

rations, write in the corresponding control words in the

desired conversion order during a single data transaction.

Words beyond the 16th valid word will be ignored.

The control word format is as follows:

C[7]

C[6]

C[5]

C[4]

C[3]

C[2]

C[1]

C[0]

CH[2] CH[1] CH[0] SS[2] SS[1] SS[0]

The V bit (C[7]) controls whether the LTC2335-18 should

consider this a valid word. Any words which have V =

0 are considered invalid and are ignored (though valid

words will still be accepted after an invalid word). Words

which have V = 1 will be added to the sequencer in the

order provided. The C[6] bit is reserved for future use and

should be set to 0. The CH[2:0] (C[5:3]) bits are a binary

value 0 to 7 controlling the channel to be converted. The

SS[2:0] (C[2:0]) bits specify the desired SoftSpan range

for the conversion, as described in Table 1.

Sequencer programming is completed when the next

conversion is started. At this time, any incomplete words

are considered invalid and discarded. If one or more

valid words were provided, the sequencer is completely

overwritten with the new sequence, and the just-initiated

conversion employs the first provided configuration.

If no valid words were provided during the data transac-

tion window, the sequencer program is unchanged, and

the pointer advances to the next entry in the previously

programmed cycle to configure the next conversion.

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