LTC2430IGN datasheet(35/40 Pages) LINER

35 / 40 page

LTC2430/LTC2431

24301f

single variable element bridges, the nonlinearity of the half

bridge output can be eliminated completely; if the refer-

ence arm of the bridge is used as the reference to the ADC,

as shown in Figure 41. The LTC2430/LTC2431 can accept

inputs up to 1/2 VREF. Hence, the reference resistor R1

must be at least 2

× the highest value of the variable

resistor.

In the case of 100

Ω platinum RTD’s, this would suggest a

value of 800

Ω for R1. Such a low value for R1 is not

advisable due to self-heating effects. A value of 25.5k is

shown for R1, reducing self-heating effects to acceptable

levels for most sensors.

The basic circuit shown in Figure 41 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 RTD, a low pass filter is

recommended as shown in Figure 42. 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

reference 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

replaced 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

introduced 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.

APPLICATIO S I FOR ATIO

2431 F41

REF–

IN+

IN–

GND

VCC

2.7V TO 5.5V

PLATINUM

100

Ω

RTD

25.5k

0.1%

REF+

LTC2430/

LTC2431

Figure 41. Remote Half Bridge Interface

REF+

REF–

IN–

GND

VCC

2431 F42

–

LTC1050

PLATINUM

100

Ω

RTD

560

Ω

10k

10k, 5%

10k

0.1%

µF

IN+

10k

LTC2430/

LTC2431

Figure 42. Remote Half Bridge Sensing with Noise Supression on Reference

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