LTC2413 datasheet(39/44 Pages) LINER

39 / 44 page

LTC2413

sn2413 2413fs

APPLICATIO S I FOR ATIO

Figure 45. Bridge Amplification Using a Single Amplifier

0.1

µF

REF+

REF–

IN+

IN–

GND

VCC

350

Ω

BRIDGE

2413 F45

1, 7, 8, 9,

10, 15, 16

LTC2413

–

LTC1050S8

0.1

µV

46.4k

20k

175

Ω

µF

4.99k

()

AV = 9.95 =

R1 + R2

R1 + 175

Ω

µF

be a source of error. The fact that input offset voltage has

relatively little effect on overall error may lead one to use

low performance amplifiers for this application. Note that

the gain of a device such as an LF156, (25V/mV over

temperature) will produce a worst-case error of –180ppm

at a noise gain of 3, such as would be encountered in an

inverting gain of 2, to produce –10V from a 5V reference.

The error associated with the 10V excitation would be

–80ppm. Hence, overall reference error could be as high

as 130ppm, the average of the two.

Figure 50 shows a similar scheme to provide excitation

using resistor arrays to produce precise gain. The circuit

is configured to provide 10V and –5V excitation to the

bridge, producing a common mode voltage at the input to

the LTC2413 of 2.5V, maximizing the AC input range for

applications where induced 60Hz could reach amplitudes

up to 2VRMS.

The circuits in Figures 48 and 50 could be used where

multiple bridge circuits are involved and bridge output can

be multiplexed onto a single LTC2413, via an inexpensive

multiplexer such as the 74HC4052.

Figure 49 shows the use of an LTC2413 with a differential

multiplexer. This is an inexpensive multiplexer that will

contribute some error due to leakage if used directly with

the output from the bridge, or if resistors are inserted as

a protection mechanism from overvoltage. Although the

bridge output may be within the input range of the A/D and

multiplexer in normal operation, some thought should be

given to fault conditions that could result in full excitation

voltage at the inputs to the multiplexer or ADC. The use of

amplification prior to the multiplexer will largely eliminate

errors associated with channel leakage developing error

voltages in the source impedance.

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