MAX31865 データシートの表示（PDF） - Maxim Integrated

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MAX31865

RTD-to-Digital Converter

Maxim Integrated 

MAX31865

RTD-to-Digital Converter

Linearizing Temperature Data

For a temperature range of -100NC to +100NC, a good

approximation of temperature can be made by simply

using the RTD data as shown below:

Temperature (NC) ≈ (ADC code/32) – 256

This equation gives 0NC error at 0NC, -1.75NC error

at -100NC, and -1.4NC error at +100NC (assuming an

IEC751 RTD and RREF equal to four times the 0 NC RTD

resistance). For high precision, use the Callendar-Van

Dusen equation (in the Temperature Conversion section)

or a lookup table to correct the RTD’s predictable

nonlinearity.

Using Thermistors

Other resistive sensors, such as thermistors (NTCs or

PTCs) may be used. Select an RREF that is greater than

or equal to the sensor’s maximum resistance over the

temperature range of interest. The output data is the ratio

of the sensor resistance to the reference resistance.

Analog-to-Digital Converter (ADC)

The ADC has fully differential analog inputs, RTDIN+

and RTDIN-, and fully differential reference inputs,

REFIN+ and REFIN-. The output code represents the

ratio between the analog input voltage and the reference

voltage. A negative input voltage produces an output

code of 0. An input voltage greater than the reference

voltage produces a full-scale output.

Input noise is attenuated by a third-order digital “sinc”

filter. Noise from 50Hz or 60Hz power sources (including

harmonics of the ac power’s fundamental frequency) is

attenuated by 82dB.

Fault Detection and Input Protection

The MAX31865 detects a variety of faults that can

occur with the external RTD and 2-, 3-, or 4-wire cables.

Some faults are detected on every conversion, while

others are detected only when a fault detection cycle is

requested by the master. During a fault detection cycle

the MAX31865 has the ability to disconnect the FORCE-

input from its GND2 return path by means of and internal

analog switch.

The conditions that generate a fault are listed below, see

Figure 4 for a fault detection flowchart.

• Detected at any point in time

Overvoltage (> VDD) or undervoltage (< GND1) con-

dition on FORCE+, FORCE2, RTDIN+, RTDIN-, or

FORCE- pins

• Detected every ADC conversion

Greater than or equal to threshold high conversion result

Less than or equal to threshold low conversion result

• Detected on demand by initiating a Fault Detection

Cycle (Configuration Register bits (D[3:2])

VREFIN- > 0.85 x VBIAS

VREFIN- < 0.85 x VBIAS when FORCE- input switch is

open

VRTDIN- < 0.85 x VBIAS when FORCE- input switch is

open

FORCE+, FORCE2, FORCE-, RTDIN+, and RTDIN- are

protected against input voltages up to Q50V. Signals

applied to these pins are gated by analog switches that

open when the applied voltage is typically greater than

VDD + 100mV or less than GND1 - 400mV. Note that

when a voltage fault occurs, the protection circuits may

allow approximately 350FA of current flow. This fault-

induced leakage current does not cause any damage to

the MAX31865.

When an overvoltage or undervoltage condition is

detected, bit D2 of the Fault Status register is set and the

ADC halts conversion updates until the fault is no longer

detected, at which point conversions resume.

Maxim Integrated

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