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

Table 9. Temperature Example for PT100 with 400I RREF (continued)

TEMPERATURE

(°C)

275

300

325

350

375

400

425

450

475

500

525

550

RTD RESISTANCE

(ω)

203.11

212.05

220.92

229.72

238.44

247.09

255.67

264.18

272.61

280.98

289.27

297.49

RTD DATA REG

(01h-02h) (hex)

81FEh

87B6h

8D64h

9304h

989Ah

9E24h

A3A2h

A914h

AE7Ah

B3D4h

B922h

BE64h

ADC CODE (dec)

16639

17371

18098

18818

19533

20242

20945

21642

22333

23018

23697

24370

ADC CODE/32-256

(°C)

263.97

286.84

309.56

332.06

354.41

376.56

398.53

420.31

441.91

463.31

484.53

505.56

Detecting RTDIN+ Cable Faults

In the 3- and 4-wire RTD connection configuration, a

broken or disconnected RTDIN+ cable results in an

unbiased ADC+ input into the MAX31865. This causes

unpredictable ADC conversion results, which can be

influenced by PCB layout, external circuit noise, and

ambient temperature. This cable fault condition can go

undetected depending upon the values set in the fault

threshold registers. If this condition is of interest, add a

10Mω resistor from the RTDIN+ pin to the BIAS pin. Doing

so results in a full-scale RTD resistance measurement if

the RTDIN+ lead is broken or disconnected.

Decoding RTD and Cable Fault Conditions

An open RTD element or a short across the RTD

element are detected on every conversion based on the

resistance data. An open RTD element results in a full-

scale reading. Set the threshold for open RTD element

detection using the High Fault Threshold registers. If the

conversion result is greater than or equal to the threshold

value, the RTD High bit in the Fault Status register is

set at the end of the conversion. An open RTD element

can also be detected on demand by testing for VREFIN-

> 0.85 x VBIAS. A shorted RTD element produces a

conversion result near zero. Set the threshold for shorted

RTD detection in the Low Fault Threshold registers.

Table 10, Table 11, and Table 12 summarize how RTD and

cable faults are detected for 2-, 3-, and 4-wire setups and

provide a description for the most common cause.

Fault Status bits are latched until the Fault Clear bit in the

Configuration register is set. This allows intermittent faults

to be captured.

Power-Supply Decoupling

To achieve the best results when using the device,

decouple the VDD and DVDD power supplies with a

0.1µF capacitor. Use a high-quality, ceramic, surface-

mount capacitor if possible. Surface-mount components

minimize lead inductance, which improves performance,

and ceramic capacitors tend to have adequate high-

frequency response for decoupling applications.

Maxim Integrated

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