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LTC2439-1 Datasheet(PDF) 11 Page - Linear Technology

Part # LTC2439-1
Description  8-/16-Channel 16-Bit No Latency Delta-Sigma ADC
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC2439-1 Datasheet(HTML) 11 Page - Linear Technology

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LTC2439-1
11
24391f
Input Data Format
When the LTC2439-1 is powered up, the default selection
used for the first conversion is IN+ = CH0 and IN= CH1
(Address = 00000). In the data input/output mode follow-
ing the first conversion, a channel selection can be up-
dated using an 8-bit word. The LTC2439-1 serial input data
is clocked into the SDI pin on the rising edge of SCK (see
Figure 3a). The input is composed of an 8-bit word with the
first 3 bits acting as control bits and the remaining 5 bits
as the channel address bits.
The first 2 bits are always 10 for proper updating opera-
tion. The third bit is EN. For EN = 1, the following 5 bits are
used to update the input channel selection. For EN = 0,
previous channel selection is kept and the following bits
are ignored. Therefore, the address is updated when the 3
control bits are 101 and kept for 100. Alternatively, the 3
control bits can be all zero to keep the previous address.
This alternation is intended to simplify the SDI interface
allowing the user to simply connect SDI to ground if no
update is needed. Combinations other than 101, 100 and
000 of the 3 control bits should be avoided.
When update operation is set (101), the following 5 bits
are the channel address. The first bit, SGL, decides if the
differential selection mode (SGL = 0) or the single-ended
selection mode is used (SGL = 1). For SGL = 0, two
adjacent channels can be selected to form a differential
input; for SGL = 1, one of the 16 channels (CH0-CH15) is
selected as the positive input and the COM pin is used as
the negative input. For a given channel selection, the
converter will measure the voltage between the two chan-
nels indicated by IN+ and INin the selected row of Table 1.
Output Data Format
The LTC2439-1 serial output data stream is 19 bits long.
The first 3 bits represent status information indicating the
conversion state and sign. The next 16 bits are the conver-
sion result, MSB first. The third and fourth bit together are
also used to indicate an underrange condition (both bits low
means the differential input voltage is below –FS) or an
overrange condition (both bits high means the differential
input voltage is above +FS).
Bit 18 (first output bit) is the end of conversion (EOC)
indicator. This bit is available at the SDO pin during the
conversion and sleep states whenever the CS pin is LOW.
This bit is HIGH during the conversion and goes LOW
when the conversion is complete.
Bit 17 (second output bit) is a dummy bit (DMY) and is
always LOW.
Bit 16 (third output bit) is the conversion result sign indi-
cator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this
bit is LOW.
Bit 15 (fourth output bit) is the most significant bit (MSB)
of the result. This bit in conjunction with Bit 16 also
provides the underrange or overrange indication. If both
Bit 16 and Bit 15 are HIGH, the differential input voltage is
above +FS. If both Bit 16 and Bit 15 are LOW, the
differential input voltage is below –FS.
The function of these bits is summarized in Table 2.
Table 2. LTC2439-1 Status Bits
Bit 18
Bit 17
Bit 16
Bit 15
Input Range
EOC
DMY
SIG
MSB
VIN ≥ 0.5 • VREF
00
1
1
0V
≤ VIN < 0.5 • VREF
00
1
0
–0.5 • VREF ≤ VIN < 0V
0
0
0
1
VIN < – 0.5 • VREF
00
0
0
Bits 15-0 are the 16-Bit conversion result MSB first.
Bit 0 is the least significant bit (LSB).
Data is shifted out of the SDO pin under control of the serial
clock (SCK), see Figure 3a. Whenever CS is HIGH, SDO
remains high impedance and any externally generated
SCK clock pulses are ignored by the internal data out shift
register.
In order to shift the conversion result out of the device, CS
must first be driven LOW. EOC is seen at the SDO pin of the
device once CS is pulled LOW. EOC changes real time from
HIGH to LOW at the completion of a conversion. This
signal may be used as an interrupt for an external micro-
controller. Bit 18 (EOC) can be captured on the first rising
edge of SCK. Bit 17 is shifted out of the device on the first
falling edge of SCK. The final data bit (Bit 0) is shifted out
on the falling edge of the 18th SCK and may be latched on
the rising edge of the 19th SCK pulse. On the falling edge
of the 19th SCK pulse, SDO goes HIGH indicating the
initiation of a new conversion cycle. This bit serves as EOC
APPLICATIO S I FOR ATIO


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