Datasheet

LTC1860/LTC1861
11
18601fa
APPLICATIONS INFORMATION
LTC1860 OPERATION
Operating Sequence
The LTC1860 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 LTC1860
goes into sleep mode drawing only leakage current. On the
falling edge of CONV, the LTC1860 goes into sample mode
and SDO is enabled. SCK synchronizes the data transfer
with each bit being transmitted from SDO on the falling
SCK edge. The receiving system should capture the data
from SDO on the rising edge of SCK. After completing the
data transfer, if further SCK clocks are applied with CONV
low, SDO will output zeros indefi nitely. See Figure 1.
Analog Inputs
The LTC1860 has a unipolar differential analog input. The
converter will measure the voltage between the “IN
+
”
and “IN
–
” inputs. A zero code will occur when IN
+
minus
IN
–
equals zero. Full scale occurs when IN
+
minus IN
–
equals V
REF
minus 1LSB. See Figure 2. Both the “IN
+
” and
“IN
–
” inputs are sampled at the same time, so common
mode noise on the inputs is rejected by the ADC. If “IN
–
”
is grounded and V
REF
is tied to V
CC
, a rail-to-rail input
span will result on “IN
+
” as shown in Figure 3.
Reference Input
The voltage on the reference input of the LTC1860 (and the
LTC1861 MSOP package) defi nes the full-scale range of
the A/D converter. These ADCs can operate with reference
voltages from V
CC
to 1V.
Figure 1. LTC1860 Operating Sequence
Figure 3. LTC1860 with Rail-to-Rail Input SpanFigure 2. LTC1860 Transfer Curve
CONV
t
CONV
SCK
SDO
121110987654321
B11
B10 B8 B6 B4 B2 B0*
Hi-Z
Hi-Z
B9
B7 B5 B3 B1
SLEEP MODE
t
SMPL
t
suCONV
*AFTER COMPLETING THE DATA TRANSFER
,
IF FURTHER
0V 1LSB
V
REF
– 2LSB
V
REF
– 1LSB
V
REF
V
IN
*
*V
IN
= IN
+
– IN
–
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 F02
1
2
3
4
8
7
6
5
V
REF
IN
+
IN
–
GND
V
CC
SCK
SDO
CONV
LTC1860
1860 F03
V
IN
= 0V TO V
CC
V
CC
1MF
SERIAL DATA LINK TO
ASIC, PLD, MPU, DSP
OR SHIFT REGISTERS