LTC2414IGN datasheet(38/48 Pages) LINER

38 / 48 page

LTC2414/LTC2418

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 350

Ω

bridge is AV = (R1+ R2)/(R1+175Ω). Common mode gain

is half the differential gain. The maximum differential

signal that can be used is 1/4 VREF, as opposed to 1/2 VREF

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 VREF. Hence, the reference resistor R1

must be at least 2x the highest value of the variable

resistor.

In the case of 100

Ω platinum RTD’s, this would suggest a

value of 800

Ω 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

Figure 43. Bridge Amplification Using a Single Amplifier

0.1

µF

REF+

REF–

CH0

CH1

GND

VCC

350

Ω

BRIDGE

2410 F49

LTC2414/

LTC2418

–

LTC1050S8

0.1

µV

46.4k

20k

175

Ω

µF

4.99k

()

AV = 9.95 =

R1 + R2

R1 + 175

Ω

µF

2410 F50

REF+

REF–

CH0

CH1

GND

VCC

2.7V TO 5.5V

PLATINUM

100

Ω

RTD

25.5k

0.1%

LTC2414/

LTC2418

Figure 44. Remote Half Bridge Interface

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