LTC2431_15 datasheet(34/40 Pages) LINER

No de pièce	LTC2431

Description	20-Bit No Latency ADCs with Differential Input and Differential Reference

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LTC2430/LTC2431

34

24301f

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

ever the accuracy of the LTC2430/LTC2431 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 improvement 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 0.28

µVRMS. The buffer stages

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

gain stability and linearity.

Figure 40 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:

A

RR

R

V ==

+

+Ω

995

12

1 175

.

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

APPLICATIO S I FOR ATIO

0.1

µF

5V

REF+

REF–

IN+

IN–

GND

VCC

3

2

4

6

7

350

Ω

BRIDGE

2431 F40

–

+

LTC1050

5V

0.1

µV

R2

46.4k

20k

175

Ω

1

µF

10

µF

R1

4.99k

AV = 9.95 =

R1 + R2

R1 + 175

Ω

+

1

µF

+

LTC2430/

LTC2431

Figure 40. Bridge Amplification Using a Single Amplifier

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