LTC1860/LTC1861
12
18601fb
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applicaTions inForMaTion
LTC1861 OPERATION
Operating Sequence
The LTC1861 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 LTC1861
goes into sleep mode. The LTC1861’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 trans
-
mitted 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
single-ended mode, all input channels are measured with
respect to GND (or AGND). 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.
Figure 4. LTC1861 Operating Sequence
Figure 5. LTC1861 Transfer Curve
CONV
SDI
SCK
121110987654321
SDO
B11
B10 B8 B6 B4 B2 B0*
Hi-Z
B9
B7 B5 B3 B1
S/D O/S
DON’T CAREDON’T CARE
t
CONV
1860 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
0V
1LSB
V
CC
– 2LSB
V
CC
– 1LSB
V
CC
V
IN
*
*V
IN
= (SELECTED “+” CHANNEL) –
(SELECTED “–” CHANNEL)
REFER TO TABLE 1
0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 0
•
•
•
1860 F05
MUX ADDRESS
Table 1. Multiplexer Channel Selection
SGL/DIFF
1
1
0
0
ODD/SIGN
0
1
0
1
CHANNEL #
0
+
+
–
1
+
–
+
GND
–
–
SINGLE-ENDED
MUX MODE
DIFFERENTIAL
MUX MODE
