
18
LTC1854/LTC1855/LTC1856
185456fa
SGL/
DIFF
SELECT
1
SELECT
0
DON'T
CARE
DON'T
CARE
MUX ADDRESS
18545 AI03
ODD
SIGN
NAP
POWER DOWN
SELECTION
SLEEP
Figure 8. Examples of Multiplexer Options on the LTC1854/LTC1855/LTC1856
0
1
2
3
4
5
6
7
CHANNEL
COM (
–)
8 Single-Ended
+
0,1
CHANNEL
4 Differential
2,3
4,5
6,7
+ (–)
+
+ (–)
– (+)
4
5
6
7
CHANNEL
COM (
–)
Combinations of
Differential and Single-Ended
+
0,1
2,3
–
COM (UNUSED)
Changing the
MUX Assignment “On the Fly”
COM (
–)
4,5
6,7
4,5
1ST CONVERSION
2ND CONVERSION
+
–
+
–
+
–
+
7
6
{
18545 F08
APPLICATIO S I FOR ATIO
WU
UU
SERIAL DATA INPUT (SDI) INTERFACE
The LTC1854/LTC1855/LTC1856 communicate with mi-
croprocessors and other external circuitry via a synchro-
nous, full duplex, 3-wire serial interface (see Figure 7).
The shift clock (SCK) synchronizes the data transfer with
each bit being transmitted on the falling SCK edge and
captured on the rising SCK edge in both transmitting and
receiving systems. The data is transmitted and received
simultaneously (full duplex).
An 8-bit input word is shifted into the SDI input which
configures the LTC1854/LTC1855/LTC1856 for the next
conversion. Simultaneously, the result of the previous
conversion is output on the SDO line. At the end of the data
exchange the requested conversion begins by applying a
rising edge on CONVST. After tCONV, the conversion is
complete and the results will be available on the next data
transfer cycle. As shown below, the result of a conversion
is delayed by one conversion from the input word re-
questing it.
SDI
SDO
SDO WORD 0
SDI WORD 1
DATA
TRANSFER
SDO WORD 2
SDI WORD 3
SDO WORD 1
SDI WORD 2
DATA
TRANSFER
tCONV
A/D
CONVERSION
tCONV
A/D
CONVERSION
18545 AI02
MUX ADDRESS
DIFFERENTIAL CHANNEL SELECTION
Table 1. Multiplexer Channel Selection
MUX ADDRESS
SGL/
DIFF
SELECT
1
0
ODD
SIGN
10
0
+
–
10
0
1
+
–
10
1
0
+
–
10
1
+
–
11
0
+
–
11
0
1
+
–
11
1
0
+
–
11
1
+
–
SGL/
DIFF
ODD
SIGN
SELECT
1
0
00
0
+
–
00
0
1
+
–
00
1
0
+
–
00
1
+
–
01
0
–
+
01
0
1
–
+
01
1
0
–
+
01
1
–
+
0
1
23
4
56
7
0
1
2
3
4
5
6
7
COM
SINGLE-ENDED CHANNEL SELECTION
INPUT DATA WORD
The LTC1854/LTC1855/LTC1856 8-bit data word is clocked
into the SDI input on the first eight rising SCK edges.
Further inputs on the SDI pin are then ignored until the next
conversion. The eight bits of the input word are defined as
follows: