LTC2414IGN datasheet(37/48 Pages) LINER

37 / 48 page

LTC2414/LTC2418

241418fa

The circuit in Figure 42 shows an example of a simple

amplification scheme. This example produces a differen-

tial output with a common mode voltage of 2.5V, as

determined by the bridge. The use of a true three amplifier

instrumentation amplifier is not necessary, as the LTC2414/

LTC2418 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 LTC2414/LTC2418 noise.

This example 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 opti-

mize temperature 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. A gain of 34 may seem low, when compared to

common practice in earlier generations of load-cell inter-

faces, however the accuracy of the LTC2414/LTC2418

changes the rationale. Achieving high gain accuracy and

linearity at higher gains may prove difficult, while provid-

ing 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 and 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.

0.1

µF

0.1

µF

0.1

µF

REF+

REF–

SDI

SCK

SD0

CH0

CH1

GND

VCC

5VREF

350

Ω

BRIDGE

2414/18 F42

LTC2414/

LTC2418

RN1 = 5k

× 8 RESISTOR ARRAY

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

–

U1A

–

RN1

611

710

512

U1B

–

U2A

–

U2B

Figure 42. Using Autozero Amplifiers to Reduce Input Referred Noise

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