LTC2487 Linear Technology, LTC2487 Datasheet - Page 15

• Manufacturer Part Number: LTC2487
• Description: 16-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

www.DataSheet4U.com
applicaTions inForMaTion
The LTC2487 has two registers. The output register (24 bits
long) contains the last conversion result. The input register
(16 bits long) sets the input channel, selects the temperature
sensor, rejection mode, gain and speed mode.
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 LTC2487 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 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.
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
corresponds to the selected input signal IN
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. If both bits are set low, the input voltage is below –FS.
Table 1. Output Data Format
Differential Input Voltage
V
V
FS** – 1LSB
0.5 • FS**
0.5 • FS** – 1LSB
0
–1LSB
–0.5 • FS**
–0.5 • FS** – 1LSB
–FS**
V
*The differential input voltage V
IN
IN
IN
*
* ≥ FS**
* < –FS**
IN
≥ 0 and low if V
IN
= IN
+
– IN
IN
Bit 23
–
SIG
. **The full-scale voltage FS = 0.5 • V
< 0 (where V
1
1
1
1
1
0
0
0
0
0
+
– IN
Bit 22
MSB
–
1
0
0
0
0
1
1
1
1
0
). The
IN
Bit 21
0
1
1
0
0
1
1
0
0
1
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
maximum operating range) a conversion result is gener-
ated for any differential input voltage V
• V
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. LTC2487 Status Bits
Input Range
V
0V ≤ V
–FS ≤ V
V
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
IN
IN
REF
≥ FS
< –FS
IN
Bit 20
IN
/Gain to +FS = 0.5 • V
–
< FS
0
1
0
1
0
1
0
1
0
1
) remains between –0.3V and V
< 0V
REF
/Gain.
Bit 19
0
1
0
1
0
1
0
1
0
1
…
…
…
…
…
…
…
…
…
…
…
REF
Bit 23
/Gain. For differential input
SIG
Bit 6
LSB
1
1
0
0
0
1
0
1
0
1
0
1
0
1
CC
IN
LTC2487
Always 0
Bits 5-0
000000
000000
000000
000000
000000
000000
000000
000000
000000
000000
+ 0.3V (absolute
from –FS = –0.5
Bit 22
MSB
1
0
1
0

2487fa
+