12
LTC1408-12
140812f
APPLICATIO S I FOR ATIO
WUUU
SELECTING THE NUMBER OF CONVERTED CHANNELS
(SEL2, SEL1, SEL0)
These three control pins select the number of channels
being converted (see Table 1). 000 selects only the first
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 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. During
the conversion of the first set of two channels you will be
able to read the data from the same two channels con-
verted 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 channels per conversion re-
sults in faster throughput of those channels. For example,
6 channels can be converted at 100ksps/ch, while 3
channels can be converted at 200ksps/ch.
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 LTC1408-12 may be
driven differentially or as a single-ended input (i.e., the
CHx
–
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 instant. 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 current. If the
source impedance of the driving circuit is low, then the
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...
