Datasheet
10
LTC1864/LTC1865
18645f
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
finished. 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 indefinitely. See Figure 4.
Analog Inputs
The two bits of the input word (SDI) assign the MUX
configuration 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
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
1864 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
Figure 4. LTC1865 Operating Sequence
APPLICATIO S I FOR ATIO
WUUU
MUX ADDRESS
Table 1. Multiplexer Channel Selection
SGL/DIFF
1
1
0
0
ODD/SIGN
0
1
0
1
CHANNEL #
0
+
+
–
1
+
–
+
GND
–
–
1864 TBL1
SINGLE-ENDED
MUX MODE
DIFFERENTIAL
MUX MODE
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 occurs 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 fixed 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 defines the span of the
A/D converter. The LTC1865 MSOP package can operate
with reference voltages from 1V to V
CC
.