LTC1408_15 Datasheet, PDF (12/20 Pages) Linear Technology – 6 Channel, 14-Bit, 600ksps Simultaneous Sampling ADC with Shutdown

LTC1408
APPLICATIO S I FOR ATIO
SELECT 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.
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 are easy to
drive. The inputs 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⁺
with CH0^–, CH1⁺ with CH1^–, CH2⁺ with CH2^–, CH3⁺ with
CH3^–, CH4⁺ with CH4^– and CH5⁺ with 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 charg-
ing the sample-and-hold capacitors at the end of conver-
sion. During conversion, the analog inputs draw only a
small leakage current. If the source impedance of the
driving circuit is low, then the LTC1408 inputs can be
driven directly. As source impedance increases, so will
acquisition time. For minimum acquisition time with high
source impedance, a buffer amplifier must be used. The
main requirement is that the amplifier driving the analog
input(s) must settle after the small current spike before the
next conversion starts (the time allowed for settling must
be at least 39ns for full throughput rate). Also keep in
mind, while choosing an input amplifier, the amount of
noise and harmonic distortion added by the amplifier.
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...
1408fa
12