LTC2668 datasheet(34/40 Pages) LINER

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LTC2348-16

234816fa

For more information www.linear.com/LTC2348-16

Typically, applications will update the SoftSpan configura-

tion register in the manner shown in Figures 17 and 18.

After the opening of a new data transaction window at the

falling edge of BUSY, the user supplies a 24-bit SoftSpan

configuration word on SDI during the first 24 SCKI cycles.

Thisnewwordoverwritestheinternalconfigurationregister

contentsfollowingthe24thSCKIrisingedge. Theuserthen

holds SDI low for the remainder of the data transaction

windowcausingtheregistertoretainitscontentsregardless

of the number of additional SCKI cycles applied. SoftSpan

settings may be retained across multiple conversions by

holding SDI low for the entire data transaction window,

regardless of the number of SCKI cycles applied.

Serial LVDS I/O Mode

In LVDS I/O mode, information is transmitted using posi-

tive and negative signal pairs (LVDS+/LVDS−) with bits

differentially encoded as (LVDS+ − LVDS−). These signals

are typically routed using differential transmission lines

with 100Ω characteristic impedance. Logical 1’s and 0’s

are nominally represented by differential +350mV and

−350mV,respectively.Forclarity,allLVDStimingdiagrams

and interface discussions adopt the logical rather than

physical convention.

As shown in Figure 20, in LVDS I/O mode the serial data

bus consists of a serial clock differential input, SCKI, serial

data differential input, SDI, serial clock differential output,

SCKO, and serial data differential output, SDO. Communi-

cation with the LTC2348-16 across this bus occurs during

predefined data transaction windows. Within a window,

the device accepts 24-bit SoftSpan configuration words

for the next conversion on SDI and outputs 24-bit packets

containing conversion results and channel configuration

information from the previous conversion on SDO. New

data transaction windows open 10ms after powering up

or resetting the LTC2348-16, and at the end of each con-

version on the falling edge of BUSY. In the recommended

use case, the data transaction should be completed with

a minimum tQUIET time of 20ns prior to the start of the

next conversion, as shown in Figure 20. New SoftSpan

configuration words are only accepted within this recom-

mended data transaction window, but SoftSpan changes

S23

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5

DON’T CARE

SCKI

SDI

SoftSpan CONFIGURATION WORD

CMOS I/O MODE

SoftSpan CONFIGURATION WORD

INTERNAL 24-BIT SoftSpan CONFIGURATION REGISTER

(SAME FOR CMOS AND LVDS)

LVDS I/O MODE

tSCKI

S23

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5

DON’T CARE

SCKI

(LVDS)

SDI

(LVDS)

tSCKI

CHANNEL 7 SoftSpan

CODE SS[2:0]

CHANNEL 6 SoftSpan

CODE SS[2:0]

CHANNEL 5 SoftSpan

CODE SS[2:0]

CHANNEL 4 SoftSpan

CODE SS[2:0]

CHANNEL 3 SoftSpan

CODE SS[2:0]

CHANNEL 2 SoftSpan

CODE SS[2:0]

CHANNEL 1 SoftSpan

CODE SS[2:0]

CHANNEL 0 SoftSpan

CODE SS[2:0]

234816 F19

tSCKIH

tSCKIL

tSSDISCKI

tHSDISCKI

tSCKIH

tSCKIL

tHSDISCKI

tSSDISCKI

Figure 19. Mapping Between Serial SoftSpan Configuration Word, Internal SoftSpan

Configuration Register, and SoftSpan Code for Each Analog Input Channel

applicaTions inForMaTion

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