MAX31865
RTD-to-Digital Converter
Table 9. Temperature Example for PT100 with 400I RREF (continued)
TEMPERATURE
(°C)
RTD RESꢀSTANCE
RTD DATA REG
(01h-02h) (hex)
ADC CODE/32-256
(°C)
ADC CODE (dec)
(ω)
275
300
325
350
375
400
425
450
475
500
525
550
203.11
212.05
220.92
229.72
238.44
247.09
255.67
264.18
272.61
280.98
289.27
297.49
81FEh
87B6h
8D64h
9304h
989Ah
9E24h
A3A2h
A914h
AE7Ah
B3D4h
B922h
BE64h
16639
17371
18098
18818
19533
20242
20945
21642
22333
23018
23697
24370
263.97
286.84
309.56
332.06
354.41
376.56
398.53
420.31
441.91
463.31
484.53
505.56
set at the end of the conversion. An open RTD element
Detecting RTDIN+ Cable Faults
In the 3- and 4-wire RTD connection configuration, a
broꢀen 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 broꢀen or disconnected.
can also be detected on demand by testing for V
REFIN-
> 0.85 x V . A shorted RTD element produces a
BIAS
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
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
To achieve the best results when using the device,
decouple the V
and DVDD power supplies with a
DD
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.
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