LTC2400CS8 datasheet(36/40 Pages) LINER

36 / 40 page

LTC2400

TYPICAL APPLICATIONS

The 12.1k resistor should be a precision resistor such as

a Vishay S102 series, or must be temperature stabilized.

The excitation current is low enough for most sensors that

the self-heating effect is near the noise floor of the LTC2400.

The use of a bipolar amplifier configuration shown in

Figure 39 offers a potential resolution of 0.001

°C

In order to achieve these results, the following effects

must be considered. Variation in the self-heating of the

RTD element due to air currents is the most difficult

challenge. If the RTD is mounted in a sealed glass enclo-

sure and painted black, the LTC2400 can detect the arrival

of a person in the room. This is also true of infrared

thermocouple sensors (thermopiles) that can also be used

directly with the LTC2400. A variation of this circuit with

two RTDs can detect small differential temperatures in

order to determine heat inflow or outflow from a process.

In order for this circuit to be practical, the ambient tem-

perature of the amplifier and resistors must be controlled

or the resistors must exhibit very low temperature coeffi-

cients. Precision resistor networks are always a good

alternative and are available from Vishay or Caddock.

Half-Bridge Strain Gauge

The circuit in Figure 40 is a ratiometric half-bridge circuit

with direct connection to the LTC2400. The use of two

thin-film strain gauges in a half-bridge configuration can

produce 2mV/V output and approximately 12-bit resolu-

tion. The 175

Ω source impedance seen by the LTC2400

does not compromise operation.

The optional resistor shown can be up to 5k and will

provide surge and transient protection for the LTC2400

if the strain gauges are located some distance from the

LTC2400, or if the strain bearing member is not well

grounded and may be subject to ESD discharge. Thin-

film strain elements form coupling capacitance to the

strain bearing member to which they are bonded. If noise

–

LT1028

LTC2400

300

Ω

–5V

VREF

R3*

9.09k

R4**

100

Ω

R2*

9.09k

Pt RTD

100

Ω

MUST BE 5ppm/

°C OR BETTER,

AN ARRAY IS RECOMMENDED

MUST BE VERY STABLE < 5ppm/

°C

R1*

9.09k

0.1

µF

2400 F38

VIN

SDO

SCK

350

Ω

STRAIN

ELEMENT

OPTIONAL

350

Ω

STRAIN

ELEMENT

VREF

VCC

0.1

µF

GND

LTC2400

2400 F39

10k

60Hz

50Hz

Figure 39. Extremely High Resolution RTD Interface

Figure 40. Half-Bridge Connection for Strain Gauges

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