LTC2413IGN datasheet(37/44 Pages) LINER

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LTC2413

sn2413 2413fs

APPLICATIO S I FOR ATIO

has common mode rejection far beyond that of most

amplifiers. The LTC1051 is a dual autozero amplifier that

can be used to produce a gain of 15 before its input

referred noise dominates the LTC2413 noise. This ex-

ample shows a gain of 34, that is determined by a feedback

network built using a resistor array containing 8 individual

resistors. The resistors are organized to optimize tem-

perature tracking in the presence of thermal gradients. The

second LTC1051 buffers the low noise input stage from

the transient load steps produced during conversion.

The gain stability and accuracy of this approach is very

good, due to a statistical improvement in resistor match-

ing due to individual error contribution being reduced. A

gain of 34 may seem low, when compared to common

practice in earlier generations of load-cell interfaces, how-

ever the accuracy of the LTC2413 changes the rationale.

Achieving high gain accuracy and linearity at higher gains

may prove difficult, while providing little benefit in terms

of noise reduction.

At a gain of 100, the gain error that could result from

typical open-loop gain of 160dB is –1ppm, however,

worst-case is at the minimum gain of 116dB, giving a gain

error of –158ppm. Worst-case gain error at a gain of 34,

is –54ppm. The use of the LTC1051A reduces the worst-

case gain error to –33ppm. The advantage of gain higher

than 34, then becomes dubious, as the input referred

noise sees little improvement1 and gain accuracy is poten-

tially compromised.

Note that this 4-amplifier topology has advantages over

the typical integrated 3-amplifier instrumentation ampli-

fier in that it does not have the high noise level common in

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 45 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 46. The LTC2413 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.

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

0.048

µVRMS.

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