LTC2351-14 Linear Technology, LTC2351-14 Datasheet - Page 12

• Manufacturer Part Number: LTC2351-14
• Description: 1.5Msps Simultaneous Sampling ADC
• Manufacturer: Linear Technology

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LTC2351-14
APPLICATIO S I FOR ATIO
SELECTING THE NUMBER OF CONVERTED CHANNELS
(SEL2, SEL1, SEL0)
These three control pins select the number of channels
being converted. 000 selects only the first channel (CH0)
for conversion. Incrementing SELx selects additional chan-
nels for conversion, up to 6 channels. 101, 110 or 111
select all 6 channels for conversion. These pins must be
kept in a fixed 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 first 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 channels
to read back the unread CH2 and CH3 data that was
converted in the first set of 4 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, 6 channels can be converted at
250ksps/ch, while 3 channels can be converted at
500ksps/ch.
12
Table 1. Conversion Sequence Control
(“acquire” represents simultaneous sampling of all channels; CHx represents conversion of channels)
SEL2
0
0
0
0
1
1
1
1
SEL1
0
0
1
1
0
0
1
1
U
SEL0
0
1
0
1
0
1
0
1
U
CHANNEL ACQUISITION AND CONVERSION SEQUENCE
acquire, CH0, acquire, CH0...
acquire, CH0, CH1, acquire, CH0, CH1...
acquire, CH0, CH1, CH2, acquire, CH0, CH1, CH2...
acquire, CH0, CH1, CH2, CH3, acquire, CH0, CH1, CH2, CH3...
acquire, CH0, CH1, CH2, CH3, CH4, acquire, CH0,CH1,CH2, CH3, CH4...
acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
acquire, CH0, CH1, CH2, CH3, CH4, CH5, acquire, CH0, CH1, CH2, CH3, CH4, CH5...
W
U
BIPOLAR/UNIPOLAR MODE
The input voltage range for each of the CHx input differen-
tial pairs is UNIPOLAR 0V – 2.5V when BIP is LOW, and
BIPOLAR ±1.25V when BIP is HIGH. This pin must be kept
in fixed state during conversion and during subsequent
conversion to read data. When changing BIP between
conversions 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 first 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
differential analog input pairs, CH0
CH1
and CH5
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 current. If the
–
–
, CH2
input is grounded). All twelve analog inputs of all six
+
and CH5
+
and CH2
–
, are sampled at the same instant. Any
–
, CH3
+
and CH3
+
and CH0
–
, CH4
–
+
, CH1
and CH4
+
235114f
and
–