LTC2410IGN datasheet(33/44 Pages) LINER

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LTC2410

APPLICATIO S I FOR ATIO

techniques is used to produce a precision divide operation

on the reference signal. Another option is the use of a

reference within the 5V input range of the LTC2410 and

developing excitation via fixed gain, or LTC1043 based

voltage multiplication, along with remote feedback in the

excitation amplifiers, as shown in Figures 34 and 35.

Figure 46 shows an example of a simple bridge connec-

tion. Note that it is suitable for any bridge application

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 LTC2410’s provides the benefit of

a root square reduction in noise. The low power consump-

tion of the LTC2410 makes 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 LTC2410 exhib-

its extremely low temperature dependent drift. As a result,

exposure to external ambient temperature ranges does

not compromise performance. The incorporation of any

amplification considerably complicates thermal stability,

as input offset voltages and currents, temperature coeffi-

cient of gain settling resistors all become factors.

The circuit in Figure 47 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 LTC2410

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 LTC2410 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 LTC2410 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

Figure 46. Simple Bridge Connection

REF+

REF–

SDO

SCK

IN+

IN–

GND

VREF

350

Ω

BRIDGE

2410 F46

1, 7, 8, 9,

10, 15, 16

LTC2410

R1 AND R2 CAN BE USED TO INCREASE TOLERABLE AC COMPONENT ON REF SIGNALS

LT1019

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