Datasheet

LTC2497
13
2497fa
The LTC2497 has two registers. The output register (24
bits long) contains the last conversion result. The input
register (8 bits long) sets the input channel.
DATA OUTPUT FORMAT
The output register contains the last conversion result.
After each conversion is completed, the device automati-
cally enters the sleep state where the supply current is
reduced to 1µA. When the LTC2497 is addressed for a read
operation, it acknowledges (by pulling SDA low) and acts
as a transmitter. The master/receiver can read up to three
bytes from the LTC2497. After a complete read operation
(3 bytes), a new conversion is initiated. The device will
NAK subsequent read operations while a conversion is
being performed.
The data output stream is 24 bits long and is shifted out
on the falling edges of SCL (see Figure 3a). The first bit
is the conversion result sign bit (SIG) (see Tables 1 and
2). This bit is high if V
IN
≥ 0 and low if V
IN
< 0 (where V
IN
corresponds to the selected input signal IN
+
– IN
–
). The
second bit is the most significant bit (MSB) of the result.
The first two bits (SIG and MSB) can be used to indicate
over and under range conditions (see Table 2). If both bits
Table 1. Output Data Format
Differential Input Voltage
V
IN
*
Bit 23
SIG
Bit 22
MSB
Bit 21 Bit 20 Bit 19 … Bit 6
LSB
Bits 5-0
Always 0
V
IN
* ≥ FS** 1 1 0 0 0 … 0 000000
FS** – 1LSB 1 0 1 1 1 … 1 000000
0.5 • FS** 1 0 1 0 0 … 0 000000
0.5 • FS** – 1LSB 1 0 0 1 1 … 1 000000
0 1 0 0 0 0 … 0 000000
–1LSB 0 1 1 1 1 … 1 000000
–0.5 • FS** 0 1 1 0 0 … 0 000000
–0.5 • FS** – 1LSB 0 1 0 1 1 … 1 000000
–FS** 0 1 0 0 0 … 0 000000
V
IN
* < –FS** 0 0 1 1 1 … 1 000000
*The differential input voltage V
IN
= IN
+
– IN
–
. **The full-scale voltage FS = 0.5 • V
REF
.
are HIGH, the differential input voltage is equal to or above
+FS. If both bits are set low, the input voltage is below –FS.
The function
of these bits is summarized in Table 2. The
16 bits following the MSB bit are the conversion result in
binary two’s complement format. The remaining six bits
are always 0.
As long as the voltage on the selected input channels (IN
+
and IN
–
) remains between –0.3V and V
CC
+ 0.3V (absolute
maximum operating range) a conversion result is gener-
ated for any differential input voltage V
IN
from –FS = –0.5
• V
REF
to +FS = 0.5 • V
REF
. For differential input voltages
greater than +FS, the conversion result is clamped to the
value corresponding to +FS. For differential input voltages
below –FS, the conversion result is clamped to the value
–FS – 1LSB.
Table 2. LTC2497 Status Bits
Input Range
Bit 23
SIG
Bit 22
MSB
V
IN
≥ FS 1 1
0V ≤ V
IN
< FS 1 0
–FS ≤ V
IN
< 0V 0 1
V
IN
< –FS 0 0
applicaTions inForMaTion