LTC2494CUHF-PBF LINER [Linear Technology], LTC2494CUHF-PBF Datasheet - Page 17

• Manufacturer Part Number: LTC2494CUHF-PBF
• Description: 16-Bit 8-/16-Channel ?? ADC with PGA and Easy Drive Input Current Cancellation
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
INPUT DATA FORMAT
The LTC2494 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
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.
The fi rst three bits shifted into the device consist of two
preamble bits and an enable bit. These bits are used to
enable the device confi guration and input channel selec-
tion. Valid settings for these three bits are 000, 100 and
101. Other combinations should be avoided. If the fi rst
three bits are 000 or 100, the following data is ignored
(don’t care) and the previously selected input channel and
confi guration remain valid for the next conversion.
If the fi rst three bits shifted into the device are 101, then
the next fi ve bits select the input channel for the next
conversion cycle (see Table 3).
The fi rst input bit following the 101 sequence (SGL) de-
termines if the input selection is differential (SGL = 0) or
Table 2. LTC2494 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
= IN
BIT 23
EOC
+
0
0
0
0
0
0
0
0
0
0
– IN
–
. **The full-scale voltage FS = 0.5 • V
BIT 22
DMY
0
0
0
0
0
0
0
0
0
0
+
= CH0, IN
BIT 21
SIG
1
1
1
1
1
0
0
0
0
0
–
=
BIT 20
MSB
1
0
0
0
0
1
1
1
1
0
single-ended (SGL = 1). For SGL = 0, two adjacent channels
can be selected to form a differential input. For SGL = 1, one
of 16 channels is selected as the positive input. The negative
input is COM for all single ended operations. The remaining
4 bits (ODD, A2, A1, A0) determine which channel(s) is/are
selected and the polarity (for a differential input).
The next serial input bit immediately following the input
channel selection is the enable bit for the conversion
confi guration (EN2). If this bit is set to 0, then the next
conversion is performed using the previously selected
converter confi guration. This is useful in systems using
the same rejection/speed for all input channels and for
backward compatibility with the LTC2418/LTC2414 families
of delta sigma ADCs.
The second set of confi guration data can be loaded into the
device by setting EN2 = 1 (see Table 4). The fi rst bit (IM)
is used to select the internal temperature sensor. If IM = 1,
the following conversion will be performed on the internal
temperature sensor rather than the selected input channel.
The next 2 bits (FA and FB) are used to set the rejection
frequency. The next bit (SPD) is used to select either the
1x output rate if SPD = 0 (auto-calibration is enabled and
the offset is continuously calibrated and removed from
the fi nal conversion result) or the 2x output rate if SPD
= 1 (offset calibration disabled, multiplexing output rates
up to 15Hz with no latency). When IM = 1 (temperature
measurement), SPD, GS2, GS1, GS0 will be ignored and
the device will operate in 1x mode. The fi nal 3 bits (GS2,
BIT 19
0
1
1
0
0
1
1
0
0
1
REF
/Gain.
BIT 18
0
1
0
1
0
1
0
1
0
1
BIT 17
0
1
0
1
0
1
0
1
0
1
…
…
…
…
…
…
…
…
…
…
…
BIT 4
0
1
0
1
0
1
0
1
0
1
LTC2494
BITS 3 TO 0
0000
0000
0000
0000
0000
0000
0000
0000
0000
0000
17
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