Datasheet

LTC2486
16
2486fd
APPLICATIONS INFORMATION
+FS. If both Bit 21 and Bit 20 are LOW, the differential
input voltage is below –FS. The function of these bits is
summarized in Table 1.
Table 1. LTC2486 Status Bits
Input Range Bit 23
EOC
Bit 22
DMY
Bit 21
SIG
Bit 20
MSB
V
IN
≥ 0.5 • V
REF
/Gain 0011
0V ≤ V
IN
< 0.5 • V
REF
/Gain 0 0 1/0 0
–0.5 • V
REF
/Gain ≤ V
IN
< 0V0001
V
IN
< –0.5 • V
REF
/Gain 0000
Bits 20 to 4 are the 16-bit plus sign conversion result
MSB fi rst.
Bit 4 is the least signifi cant bit (LSB
16
).
Bits 3 to 0 are always LOW.
Data is shifted out of the SDO pin under control of the
serial clock (SCK) (see Figure 3). Whenever CS is HIGH,
SDO remains high impedance and SCK is ignored.
In order to shift the conversion result out of the device,
CS must fi rst be driven LOW. EOC is seen at the SDO pin
of the device once CS is pulled LOW. EOC changes in real
time as a function of the internal oscillator or the clock
applied to the f
O
pin from HIGH to LOW at the completion
of a conversion. This signal may be used as an interrupt for
an external microcontroller. Bit 23 (EOC) can be captured
on the fi rst rising edge of SCK. Bit 22 is shifted out of the
device on the fi rst falling edge of SCK. The fi nal data bit
(Bit 0) is shifted out on the on the falling edge of the 23rd
SCK and may be latched on the rising edge of the 24th SCK
pulse. On the falling edge of the 24th SCK pulse, SDO goes
HIGH indicating the initiation of a new conversion cycle.
This bit serves as EOC (Bit 23) for the next conversion
cycle. Table 2 summarizes the output data format.
As long as the voltage on the IN
+
and IN
–
pins remains
between –0.3V and V
CC
+ 0.3V (absolute maximum op-
erating range) a conversion result is generated for any
differential input voltage V
IN
from –FS = –0.5 • V
REF
/Gain
to +FS = 0.5 • V
REF
/Gain. For differential input voltages
greater than +FS, the conversion result is clamped to the
value corresponding to +FS + 1LSB. For differential input
voltages below –FS, the conversion result is clamped to
the value –FS – 1LSB.
INPUT DATA FORMAT
The LTC2486 serial input word is 16 bits long and contains
two distinct sets of data. The fi rst set (SGL, ODD, A2,
A1, A0) is used to select the input channel. The second
set of data (IM, FA, FB, SPD, GS2, GS1, GS0) is used
to select the frequency rejection, speed mode (1x, 2x),
temperature measurement, and gain.
After power up, the device initiates an internal reset cycle
which sets the input channel to CH0-CH1 (IN
+
= CH0, IN
–
=
CH1), the frequency rejection to simultaneous 50Hz/60Hz,
1x output rate (auto-calibration enabled), and gain = 1. The
fi rst conversion automatically begins at power up using this
default confi guration. Once the conversion is complete, a
new word may be written into the device.
Figure 3. Channel Selection, Confi guration Selection and Data Output Timing
EOC
CS
SCK
(EXTERNAL)
SDI
SDO
2486 F03
CONVERSION
SLEEP
DATA INPUT/OUTPUT
MSB
BIT 20 BIT 19 BIT 18 BIT 17 BIT 16 BIT 15 BIT 14 BIT 13 BIT 12 BIT 11
SIG
BIT 21
“0”
BIT 22BIT 23
1 0 EN SGL A2 A1 A0 EN2 IM FA FB SPD GS2 GS1 GS0ODD
BIT 10 BIT 9 BIT 0
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