Datasheet

LTC2414/LTC2418
38
241418fa
Figure 43 shows an example of a single amplifier used to
produce single-ended gain. This topology is best used in
applications where the gain setting resistor can be made
to match the temperature coefficient of the strain gauges.
If the bridge is composed of precision resistors, with only
one or two variable elements, the reference arm of the
bridge can be made to act in conjunction with the feedback
resistor to determine the gain. If the feedback resistor is
incorporated into the design of the load cell, using resis-
tors which match the temperature coefficient of the load-
cell elements, good results can be achieved without the
need for resistors with a high degree of absolute accuracy.
The common mode voltage in this case, is again a function
of the bridge output. Differential gain as used with a 350Ω
bridge is A
V
= (R1+ R2)/(R1+175Ω). Common mode gain
is half the differential gain. The maximum differential
signal that can be used is 1/4 V
REF
, as opposed to 1/2 V
REF
in the 2-amplifier topology above.
Remote Half Bridge Interface
As opposed to full bridge applications, typical half bridge
applications must contend with nonlinearity in the bridge
output, as signal swing is often much greater. Applications
include RTD’s, thermistors and other resistive elements
that undergo significant changes over their span. For
single variable element bridges, the nonlinearity of the half
bridge output can be eliminated completely; if the refer-
ence arm of the bridge is used as the reference to the ADC,
as shown in Figure 44. The LTC2414/LTC2418 can accept
inputs up to 1/2 V
REF
. Hence, the reference resistor R1
must be at least 2x the highest value of the variable
resistor.
In the case of 100Ω platinum RTD’s, this would suggest a
value of 800Ω for R1. Such a low value for R1 is not
advisable due to self-heating effects. A value of 25.5k is
shown for R1, reducing self-heating effects to acceptable
levels for most sensors.
APPLICATIO S I FOR ATIO
WUUU
Figure 43. Bridge Amplification Using a Single Amplifier
0.1µF
5V
REF
+
REF
–
CH0
CH1
GND
V
CC
11
3
2
4
6
7
12
350Ω
BRIDGE
21
22
2410 F49
15
9
LTC2414/
LTC2418
–
+
LTC1050S8
5V
0.1µV
R2
46.4k
20k
20k
175Ω
1µF
10µF
R1
4.99k
()
A
V
= 9.95 =
R1 + R2
R1 + 175Ω
+
+
1µF
+
2410 F50
REF
+
REF
–
CH0
CH1
GND
V
CC
V
S
2.7V TO 5.5V
11
12
21
22
PLATINUM
100Ω
RTD
R1
25.5k
0.1%
15
9
LTC2414/
LTC2418
Figure 44. Remote Half Bridge Interface