Datasheet

LTC2486
17
2486fd
APPLICATIONS INFORMATION
The fi rst three bits shifted into the device consist of two
preamble bits and an enable bit. These bits are used to
enable the device confi guration and input channel selec-
tion. Valid settings for these three bits are 000, 100 and
101. Other combinations should be avoided. If the fi rst
three bits are 000 or 100, the following data is ignored
(don’t care) and the previously selected input channel and
confi guration remain valid for the next conversion.
If the fi rst three bits shifted into the device are 101, then
the next fi ve bits select the input channel for the next
conversion cycle (see Table 3).
The fi rst input bit (SGL) following the 101 sequence
determines if the input selection is differential (SGL =
0) or single-ended (SGL = 1). For SGL = 0, two adjacent
channels can be selected to form a differential input. For
SGL = 1, one of four channels is selected as the positive
input. The negative input is COM for all single ended
operations. The remaining four bits (ODD, A2, A1, A0)
determine which channel(s) is/are selected and the polarity
(for a differential input).
The next serial input bit immediately following the input
channel selection is the enable bit for the conversion
confi guration (EN2). If this bit is set to 0, then the next
conversion is performed using the previously selected
converter confi guration.
The second set of confi guration data can be loaded into
the device by setting EN2 = 1 (see Table 4). The fi rst bit
(IM) is used to select the internal temperature sensor. If
IM = 1, the following conversion will be performed on
the internal temperature sensor rather than the selected
input channel. The next two bits (FA and FB) are used to
set the rejection frequency. The next bit (SPD) is used to
select either the 1x output rate if SPD = 0 (auto-calibration
is enabled and the offset is continuously calibrated and
removed from the fi nal conversion result) or the 2x output
rate if SPD = 1 (offset calibration disabled, multiplexing
output rates up to 15Hz with no latency). When IM = 1
(temperature measurement) SPD, GS2, GS1 and GS0 will
be ignored and the device will operate in 1x mode. The
fi nal 3 bits (GS2, GS1, GS0) are used to set the gain. The
confi guration remains valid until a new input word with
EN = 1 (the fi rst three bits are 101) and EN2 = 1 is shifted
into the device.
Table 3 Channel Selection
MUX ADDRESS CHANNEL SELECTION
SGL
ODD/
SIGNA2A1A0 0 1 2 3COM
*0 0 0 0 0 IN
+
IN
–
00001 IN
+
IN
–
01000IN
–
IN
+
01001 IN
–
IN
+
10000IN
+
IN
–
10001 IN
+
IN
–
11000 IN
+
IN
–
11001 IN
+
IN
–
*Default at power up
Table 2. LTC2486 Output Data Format
DIFFERENTIAL INPUT VOLTAGE
V
IN
*
BIT 23
EOC
BIT 22
DMY
BIT 21
SIG
BIT 20
MSB
BIT 19 BIT 18 BIT 17 … BIT 4 BITS 3 TO 0
V
IN
* ≥ FS** 0011000…0 0000
FS** – 1LSB 0010111…1 0000
0.5 • FS** 0010100…0 0000
0.5 • FS** – 1LSB 0010011…1 0000
0 0 0 1/0*** 0 0 0 0 … 0 0000
–1LSB 0001111…1 0000
–0.5 • FS** 0001100…0 0000
–0.5 • FS** – 1LSB 0001011…1 0000
–FS** 0001000…0 0000
V
IN
* < –FS** 0000111…1 0000
*The differential input voltage V
IN
= IN
+
– IN
–
. **The full-scale voltage FS = 0.5 • V
REF
/Gain.
***The sign bit changes state during the 0 output code when the device is operating in the 2x speed mode.