Datasheet

27
LTC2404/LTC2408
quently, these resistors should be low temperature coef-
ficient devices. The use of higher resistance RTDs is not
recommended in this topology, although the inclusion of
one 1000Ω RTD at the top on the ladder will have minimal
impact on the lower elements. The same caveat applies to
fast changing temperatures. Any fast changing sensors
should be at the top of the ladder.
The LTC2408’s Uncommitted Multiplexer Finds Use in
a Programmable Gain Scheme
If the multiplexer in the LTC2408 is not committed to
channel selection, it can be used to select various signal-
processing options such as different gains, filters or at-
tenuator characteristics. In Figure 25, the multiplexer is
shown selecting different taps on an R/2R ladder in the
feedback loop of an amplifier. This example allows selec-
tion of gain from 1 to 128 in binary steps. Other feedback
networks could be used to provide gains tailored for
specific purposes. (For example, 1x, 1.1x, 1.41x, 2x,
2.028x, 5x, 10x, 40x, etc.) Alternatively, different bandpass
APPLICATIONS INFORMATION
WUU
U
characteristics or signal inversion/noninversion could be
selected. The R/2R ladder can be purchased as a network
to ensure tight temperature tracking. Alternatively, resis-
tors in a ladder or as separate dividers can be assembled
from discrete resistors. In the configuration shown, the
channel resistance of the multiplexer does not contribute
much to the error budget, as only input op amp current
flows through the switch. The LTC1050 was chosen for
its low input current and offset voltage, as well as its
ability to drive the input of a ∆Σ ADC.
Insert Gain or Buffering After the Multiplexer
Separate MUXOUT and ADCIN terminals permit insertion
of a gain stage between the MUX and the ADC. If passive
filtering is used at the input to the ADC, a buffer amplifier
is strongly recommended to avoid errors resulting from
the dynamic ADC input current. If antialiasing is required,
it should be placed at the input to the MUX. If bandwidth
limiting is required to improve noise performance, a filter
with a –3dB point at 1500Hz will reduce the effective total
Figure 24. Measuring Up to Seven RTD Temperatures with One Reference Resistor and One Reference Current
2404/08 F24
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
9
10
11
12
13
14
15
17
8-CHANNEL
MUX
GND
1, 5, 6, 16, 18, 22, 27, 28
23
20
25
19
21
24
CSADC
CSMUX
SCK
CLK
D
IN
SDO
26
F
O
LTC2408
ADCINMUXOUT
74
7
0.1µF
6
3
6
47µF
R2300µA
5V
45
OPTIONAL
GAIN
BLOCK
2
TO PT3-PT6
PT1
100Ω
PLATINUM
RTD
OPTIONAL
PROTECTION
RESISTORS
5k MAX
UP TO SEVERAL
HUNDRED FEET.
ALL SAME
WIRE TYPE
PT2
PT7
4
R3
5V
R2
R1
20.1k
0.1%
3 2, 8
1µF
5V
V
REF
V
CC
V
CC
+
–
+
LTC1050
LTC1634-2.5
24-BIT
∆Σ ADC