LTC2410IGN datasheet(34/44 Pages) LINER

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LTC2410

APPLICATIO S I FOR ATIO

the output stage that usually dominates when an instru-

mentation amplifier is used at low gain. If this amplifier is

used at a gain of 10, the gain error is only 10ppm and input

referred noise is reduced to 0.1

µVRMS. The buffer stages

can also be configured to provide gain of up to 50 with high

gain stability and linearity.

Figure 48 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 = 1+ 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 49. The LTC2410 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.

The basic circuit shown in Figure 49 shows connections

for a full 4-wire connection to the sensor, which may be

located remotely. The differential input connections will

reject induced or coupled 60Hz interference, however, the

1Input referred noise for AV = 34 for approximately 0.05µVRMS, whereas at a gain of 50, it would be

0.048

µVRMS.

Figure 47. Using Autozero Amplifiers to Reduce Input Referred Noise

0.1

µF

0.1

µF

0.1

µF

REF+

REF–

SDO

SCK

IN+

IN–

GND

VCC

312

5VREF

350

Ω

BRIDGE

2410 F47

1, 7, 8, 9,

10, 15, 16

LTC2410

RN1 = 5k

× 8 RESISTOR ARRAY

U1A, U1B, U2A, U2B = 1/2 LTC1051

–

U1A

–

RN1

611

710

512

U1B

–

U2A

–

U2B

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