Datasheet

MAX31865
RTD-to-Digital Converter
20Maxim Integrated
Table 9. Temperature Example for PT100 with 400I R
REF
(continued)
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 V
REFIN-
> 0.85 x V
BIAS
. 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 V
DD
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.
TEMPERATURE
(°C)
RTD RESISTANCE
(ω)
RTD DATA REG
(01h-02h) (hex)
ADC CODE (dec)
ADC CODE/32-256
(°C)
275 203.11 81FEh 16639 263.97
300 212.05 87B6h 17371 286.84
325 220.92 8D64h 18098 309.56
350 229.72 9304h 18818 332.06
375 238.44 989Ah 19533 354.41
400 247.09 9E24h 20242 376.56
425 255.67 A3A2h 20945 398.53
450 264.18 A914h 21642 420.31
475 272.61 AE7Ah 22333 441.91
500 280.98 B3D4h 23018 463.31
525 289.27 B922h 23697 484.53
550 297.49 BE64h 24370 505.56