LTC2430IGN datasheet(33/40 Pages) LINER

33 / 40 page

LTC2430/LTC2431

24301f

where measurement speed is not of the utmost impor-

tance. For many applications where large vessels are

weighed, the average weight over an extended period of

time is of concern and short term weight is not readily

determined due to movement of contents, or mechanical

resonance. Often, large weighing applications involve load

cells located at each load bearing point, the output of

which can be summed passively prior to the signal pro-

cessing circuitry, actively with amplification prior to the

ADC, or can be digitized via multiple ADC channels and

summed mathematically. The mathematical summation

of the output of multiple LTC2430/LTC2431’s provide the

benefit of a root square reduction in noise. The low power

consumption of the LTC2430/LTC2431 make it attractive

for multidrop communication schemes where the ADC is

located within the load-cell housing.

A direct connection to a load cell is perhaps best incorpo-

rated into the load-cell body, as minimizing the distance to

the sensor largely eliminates the need for protection

devices, RFI suppression and wiring. The LTC2430/

LTC2431 exhibit extremely low temperature dependent

drift. As a result, exposure to external ambient tempera-

ture ranges does not compromise performance. The in-

corporation of any amplification considerably complicates

thermal stability, as input offset voltages and currents,

temperature coefficient of gain settling resistors all be-

come factors.

The circuit in Figure 39 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 LTC2430/

LTC2431 have 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 10 before its input

referred noise dominates the LTC2430/LTC2431 noise.

This example shows a gain of 34, that is determined by a

feedback network built using a resistor array containing

eight individual resistors. The resistors are organized to

optimize 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 due to individual error contribution being reduced. A

gain of 34 may seem low, when compared to common

APPLICATIO S I FOR ATIO

0.1

µF

0.1

µF

0.1

µF

REF+

REF–

SDO

SCK

IN+

IN–

GND

VCC

5VREF

350

Ω

BRIDGE

2431 F39

RN1 = 5k

× 8 RESISTOR ARRAY

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

–

U1A

–

RN1

611

710

512

U1B

–

U2A

–

U2B

LTC2430/

LTC2431

Figure 39. Using Autozero Amplifiers to Reduce Input Referred Noise

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