LTC2413IGN datasheet(38/44 Pages) LINER

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LTC2413

sn2413 2413fs

APPLICATIO S I FOR ATIO

Figure 44. Using Autozero Amplifiers to Reduce Input Referred Noise

0.1

µF

0.1

µF

0.1

µF

REF+

REF–

SDO

SCK

IN+

IN–

GND

VCC

312

5VREF

350

Ω

BRIDGE

2413 F44

1, 7, 8, 9,

10, 15, 16

LTC2413

RN1 = 5k

× 8 RESISTOR ARRAY

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

–

U1A

–

RN1

611

710

512

U1B

–

U2A

–

U2B

The basic circuit shown in Figure 46 shows connections

for a full 4-wire connection to the sensor, which may be

located remotely. The differential input connections will

reject induced or coupled 60Hz interference, however, the

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 47. 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, but may result in errors if used with a long

cable. For short cable applications, the errors may be

acceptably 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 47 shows a more rigorous

example of Figure 46, with increased noise suppression

and more protection for remote applications.

Figure 48 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, hence, introduce a very little error due to

gain error or due to offset voltages. A 1

µV/°Coffsetvoltage

drift translates into 0.05ppm/

°C gain error. Simpler alter-

natives, 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

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