LTC1864/LTC1865
14
18645fb
APPLICATIONS INFORMATION
LTC1865 OPERATION
Operating Sequence
The LTC1865 conversion cycle begins with the rising edge
of CONV. After a period equal to t
CONV
, the conversion is
fi nished. If CONV is left high after this time, the LTC1865
goes into sleep mode drawing only leakage current. The
LTC1865’s 2-bit data word is clocked into the SDI input
on the rising edge of SCK after CONV goes low. Additional
inputs on the SDI pin are then ignored until the next CONV
cycle. 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). After completing the data
transfer, if further SCK clocks are applied with CONV low,
SDO will output zeros indefi nitely. See Figure 4.
Analog Inputs
The two bits of the input word (SDI) assign the MUX
confi guration for the next requested conversion. For a
given channel selection, the converter will measure the
voltage between the two channels indicated by the “+”
and “–” signs in the selected row of the following table. In
single-ended mode, all input channels are measured
with respect to GND. A zero code will occur when the
“+” input minus the “–” input equals zero. Full scale oc-
curs when the “+” input minus the “–” input equals V
REF
minus 1LSB. See Figure 5. Both the “+” and “–” inputs
are sampled at the same time so common mode noise
is rejected. The input span in the SO-8 package is fi xed
at V
REF
= V
CC
. If the “–” input in differential mode is
grounded, a rail-to-rail input span will result on the “+”
input.
Reference Input
The reference input of the LTC1865 SO-8 package is
internally tied to V
CC
. The span of the A/D converter is
therefore equal to V
CC
. The voltage on the reference
input of the LTC1865 MSOP package defi nes the span
of the A/D converter. The LTC1865 MSOP package can
operate with reference voltages from 1V to V
CC
.
Figure 4. LTC1865 Operating Sequence
MUX ADDRESS
Table 1. Multiplexer Channel Selection
SGL/DIFF
1
1
0
0
ODD/SIGN
0
1
0
1
CHANNEL #
0
+
+
–
1
+
–
+
GND
–
–
18645 TBL1
SINGLE-ENDED
MUX MODE
DIFFERENTIAL
MUX MODE
CONV
SDI
SCK
16151413121110987654321
SDO
B15
B14 B12 B10 B8 B6 B4 B2
B0*
Hi-Z
B13
B11 B9 B7 B5 B3 B1
S/D O/S
DON’T CAREDON’T CARE
t
CONV
18645 F04
SLEEP MODE
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER SCK CLOCKS ARE
APPLIED WITH CONV LOW, THE ADC WILL OUTPUT ZEROS INDEFINITELY
Hi-Z
t
SMPL