ltc2428 Linear Technology Corporation, ltc2428 Datasheet - Page 30

• Manufacturer Part Number: ltc2428
• Description: 4-/8-channel 20-bit ?power No Latency Delta-sigma Adc
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC2424/LTC2428
The LTC2428’s Resolution and Accuracy Allows You
to Measure Points in a Ladder of Sensors
In many industrial processes, for example, cracking tow-
ers in petroleum refineries, a group of temperature mea-
surements must be related to one another. A series of
platinum RTDs that sense slow changing temperatures
can be configured into a resistive ladder, using the LTC2428
to sense each node. This approach allows a single excita-
tion current passed through the entire ladder, reducing
total supply current consumption. In addition, this ap-
proach requires only one high precision resistor, thereby
reducing cost. A group of up to seven temperatures can be
measured as a group by a single LTC2428 in a loop-pow-
ered remote acquisition unit. In the example shown in
Figure 33, the excitation current is 240 A at 0 C. The
LTC2428 requires 300 A, leaving nearly 3.5mA for the
remainder of the remote transmitter.
30
PLATINUM
Figure 33. Measuring Up to Seven RTD Temperatures with One Reference Resistor and One Reference Current
100
RTD
PT1
PT2
PT7
UP TO SEVERAL
HUNDRED FEET.
U
WIRE TYPE
ALL SAME
TO PT3-PT6
INFORMATION
U
R1
20.1k
0.1%
+
W
47 F
300 A
LTC1634-2.5
PROTECTION
RESISTORS
OPTIONAL
5k MAX
5V
6
4
R2
U
5
10
11
12
13
14
15
17
9
5 ZS
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
SET
3
2
R2
+
–
8-CHANNEL
LTC1050
The resistance of any of the RTDs (PT1 to PT7) is deter-
mined from the voltage across it, as compared to the
voltage drop across the reference resistor (R1). This is a
ratiometric implementation where the voltage drop across
R1 is given by V
the voltage on a representative length of wire. If the same
type and length of wire is used for all connections, then
errors associated with the voltage drops across all wiring
can be removed in software. The contribution of wiring
drop can be scaled if wire lengths are not equal.
Gain can be added to this circuit as the total voltage drop
across all the RTDs is small compared to ADC full-scale
range. The maximum recommended gain is 50, as limited
by both amplifier noise contribution, as well as the maxi-
mum voltage developed at CH0 when all sensors are at the
maximum temperature specified for platinum RTDs.
MUX
5V
7
4
MUXOUT
1, 6, 16, 18, 22, 27, 28
R3
OPTIONAL
GAIN
BLOCK
6
7
4
ADCIN
+
REF
–
20-BIT
GND
LTC2428
– V
ADC
3
FS
CH1
SET
V
. Channel 7 is used to measure
0.1 F
CC
2, 8
CSMUX
CSADC
24248 F33
SDO
SCK
CLK
D
IN
F
O
23
20
25
19
21
24
26
5V
V
1 F
CC