LTC2351-14
12
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Table 1. Conversion Sequence Control
(“acquire” Represents Simultaneous Sampling of All Channels; CHx Represents Conversion of Channels)
SEL2 SEL1 SEL0 CHANNEL ACQUISITION AND CONVERSION SEQUENCE
0 0 0 acquire, CH0, acquire, CH0...
0 0 1 acquire, CH0, CH1, acquire, CH0, CH1...
0 1 0 acquire, CH0, CH1, CH2, acquire, CH0, CH1, CH2...
0 1 1 acquire, CH0, CH1, CH2, CH3, acquire, CH0, CH1, CH2, CH3...
1 0 0 acquire, CH0, CH1, CH2, CH3, CH4, acquire, CH0,CH1,CH2, CH3, CH4...
1 0 1 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
1 1 0 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
1 1 1 acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
APPLICATIONS INFORMATION
SELECTING THE NUMBER OF CONVERTED CHANNELS
(SEL2, SEL1, SEL0)
These three control pins select the number of channels
being converted. 000 selects only the fi rst channel (CH0)
for conversion. Incrementing SELx selects additional
channels for conversion, up to six channels. 101, 110
or 111 select all six channels for conversion. These pins
must be kept in a fi xed state during conversion and during
the subsequent conversion to read data. When changing
modes between conversions, keep in mind that the output
data of a particular channel will remain unchanged until
after that channel is converted again. For example: convert
a sequence of four channels (CH0, CH1, CH2, CH3) with
SELx = 011, then, after these channels are converted change
SELx to 001 to convert just CH0 and CH1. See Table 1.
During the conversion of the fi rst set of two channels you
will be able to read the data from the same two channels
converted as part of the previous group of four channels.
Later, you could convert four or more channels to read
back the unread CH2 and CH3 data that was converted
in the fi rst set of four channels. These pins are often
hardwired to enable the right number of channels for a
particular application. Choosing to convert fewer chan-
nels per conversion results in faster throughput of those
channels. For example, six channels can be converted at
250ksps/ch, while three channels can be converted at
500ksps/ch.
BIPOLAR/UNIPOLAR MODE
The input voltage range for each of the CHx input differ-
ential pairs is UNIPOLAR 0V – 2.5V when BIP is LOW, and
BIPOLAR ±1.25V when BIP is HIGH. This pin must be kept
in fi xed state during conversion and during subsequent
conversion to read data. When changing BIP between con-
versions the full acquisition time must be allowed before
starting the next conversion. After changing modes from
BIPOLAR to UNIPOLAR, or from UNIPOLAR to BIPOLAR,
you can still read the fi rst set of channels in the new mode,
by inverting the MSB to read these channels in the mode
that they were converted in.
DRIVING THE ANALOG INPUT
The differential analog inputs of the LTC2351-14 may be
driven differentially or as a single-ended input (i.e., the
CH0
–
input is grounded). All twelve analog inputs of all
six differential analog input pairs, CH0
+
and CH0
–
, CH1
+
and CH1
–
, CH2
+
and CH2
–
, CH3
+
and CH3
–
, CH4
+
and
CH4
–
and CH5
+
and CH5
–
, are sampled at the same in-
stant. Any unwanted signal that is common to both inputs
of each input pair will be reduced by the common mode
rejection of the sample-and-hold circuit. The inputs draw
only one small current spike while charging the sample-
and-hold capacitors at the end of conversion. During
