LTC2351-12
13
235112fa
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 6 channels. 101, 110 or
111 select all 6 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 4 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 4 channels. Later, you could convert 4 or more chan-
nels to read back the unread CH2 and CH3 data that was
converted in the fi rst set of 4 channels. These pins are
often hardwired to enable the right number of channels
for a particular application. Choosing to convert fewer
channels per conversion results in faster throughput of
those channels. For example, 6 channels can be converted
at 250ksps/ch, while 3 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 LTC2351-12 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
conversion, the analog inputs draw only a small leakage
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...
