LTC2487
applicaTions inForMaTion
TheLTC2487hastworegisters.Theoutputregister(24bits
long)containsthelastconversionresult.Theinputregister
(16bitslong)setstheinputchannel,selectsthetemperature
sensor, rejection mode, gain and speed mode.
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
DATA OUTPUT FORMAT
–
and IN ) remains between –0.3V and V + 0.3V (absolute
CC
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
reducedto1µA.WhentheLTC2487isaddressedforaread
operation, it acknowledges (by pulling SDA low) and acts
as a transmitter. The master/receiver can read up to three
bytes from the LTC2487. 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.
maximum operating range) a conversion result is gener-
ated for any differential input voltage V from –FS = –0.5
IN
• V /Gain to +FS = 0.5 • V /Gain. For differential input
REF
REF
voltagesgreaterthan+FS,theconversionresultisclamped
to the value corresponding to +FS. For differential input
voltages below –FS, the conversion result is clamped to
the value –FS – 1LSB.
Table 2. LTC2487 Status Bits
Bit 23
SIG
Bit 22
MSB
Input Range
≥ FS
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 ≥ 0 and low if V < 0 (where V
V
IN
1
1
0
0
1
0
1
0
0V ≤ V < FS
IN
–FS ≤ V < 0V
IN
V
IN
< –FS
IN
IN
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
are HIGH, the differential input voltage is equal to or above
+FS. Ifbothbitsaresetlow, theinputvoltageisbelow–FS.
INPUT DATA FORMAT
The serial input word to the LTC2487 is 16 bits long and
is written into the device input register in two 8-bit words.
The first word (SGL, ODD, A2, A1, A0) is used to select
Table 1. Output Data Format
Differential Input Voltage
V *
Bit 23
SIG
Bit 22
MSB
Bit 21
Bit 20
Bit 19
…
Bit 6
LSB
Bits 5-0
Always 0
IN
V * ≥ FS**
1
1
1
1
1
0
0
0
0
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
…
…
…
…
…
…
…
…
…
…
0
1
0
1
0
1
0
1
0
1
000000
000000
000000
000000
000000
000000
000000
000000
000000
000000
IN
FS** – 1LSB
0.5 • FS**
0.5 • FS** – 1LSB
0
–1LSB
–0.5 • FS**
–0.5 • FS** – 1LSB
–FS**
V * < –FS**
IN
0
+
–
*The differential input voltage V = IN – IN . **The full-scale voltage FS = 0.5 • V /Gain.
IN
REF
2487fa
ꢀꢄ
Linear [ Linear ]
