LTC2410IGN datasheet(39/48 Pages) LINER

39 / 48 page

LTC2410

APPLICATIO S I FOR ATIO

reference inputs do not have the same rejection. If 60Hz or

other noise is present on the reference input, a low pass

filter is recommended as shown in Figure 51. Note that you

cannot place a large capacitor directly at the junction of R1

and R2, as it will store charge from the sampling process.

A better approach is to produce a low pass filter decoupled

from the input lines with a high value resistor (R3).

The use of a third resistor in the half bridge, between the

variable and fixed elements gives essentially the same

result as the two resistor version, but has a few benefits.

If, for example, a 25k reference resistor is used to set the

excitation current with a 100

Ω RTD, the negative refer-

ence input is sampling the same external node as the

positive input and may result in errors if used with a long

cable. For short cable applications, the errors may be

acceptalby low. If instead the single 25k resistor is re-

placed with a 10k 5% and a 10k 0.1% reference resistor,

the noise level introduced at the reference, at least at

higher frequencies, will be reduced. A filter can be intro-

duced into the network, in the form of one or more

capacitors, or ferrite beads, as long as the sampling pulses

are not translated into an error. The reference voltage is

also reduced, but this is not undesirable, as it will decrease

the value of the LSB, although, not the input referred noise

level.

The circuit shown in Figure 51 shows a more rigorous

example of Figure 50, with increased noise suppression

and more protection for remote applications.

Figure 52 shows an example of gain in the excitation circuit

and remote feedback from the bridge. The LTC1043’s

provide voltage multiplication, providing

±10V from a 5V

reference with only 1ppm error. The amplifiers are used at

unity gain and introduce very little error due to gain error

or due to offset voltages. A 1

µV/°C offset voltage drift

translates into 0.05ppm/

°C gain error. Simpler alterna-

tives, with the amplifiers providing gain using resistor

arrays for feedback, can produce results that are similar to

bridge sensing schemes via attenuators. Note that the

amplifiers must have high open-loop gain or gain error will

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.

Figure 49. Bridge Amplification Using a Single Amplifier

0.1

µF

REF+

REF–

IN+

IN–

GND

VCC

350

Ω

BRIDGE

2410 F49

1, 7, 8, 9,

10, 15, 16

LTC2410

–

LTC1050S8

0.1

µV

46.4k

20k

175

Ω

µF

4.99k

()

AV = 9.95 =

R1 + R2

R1 + 175

Ω

µF

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