MAXIM MAX31865

EVALUATION KIT AVAILABLE
MAX31865
RTD-to-Digital Converter
General Description
Features
The MAX31865 is an easy-to-use resistance-to-digital
converter optimized for platinum resistance temperature
detectors (RTDs). An external resistor sets the sensitivity
for the RTD being used and a precision delta-sigma ADC
converts the ratio of the RTD resistance to the reference
resistance into digital form. The MAX31865’s inputs are
protected against overvoltage faults as large as Q50V.
Programmable detection of RTD and cable open and
short conditions is included.
SSimple Conversion of Platinum RTD Resistance to
Digital Value
Applications
SHandles 100Ω to 1kΩ (at 0°C) Platinum RTDs
(PT100 to PT1000)
SCompatible with 2-, 3-, and 4-wire Sensor Connections
SConversion Time: 21ms max
S15-Bit ADC Resolution; Nominal Temperature
Resolution 0.03125NC (Varies Due to RTD
Nonlinearity)
Industrial Equipment
STotal Accuracy Over All Operating Conditions:
0.5NC (0.05% of Full Scale) max
Medical Equipment
S±50V Input Protection
Instrumentation
SFully Differential VREF Inputs
SFault Detection (Open RTD Element, RTD Shorted to
Out-of-Range Voltage, or Short Across RTD Element)
Ordering Information appears at end of data sheet.
For related parts and recommended products to use with this part,
refer to www.maximintegrated.com/MAX31865.related.
SSPI-Compatible Interface
S20-Pin TQFN Package
Typical Application Circuits
4-WIRE SENSOR CONNECTION
VDD
VDD
0.1µF
VDD
DVDD
0.1µF
BIAS
REFIN+
DRDY
REFIN-
SDI
HOST
INTERFACE
SCLK
RREF
ISENSOR
MAX31865
RCABLE
FORCE+
CS
FORCE2
SDO
RTDIN+
RCABLE
N.C.
CI*
RTD
RCABLE
DGND
GND2
GND1
RTDINFORCERCABLE
*CI = 10nF FOR 1kΩ RTD
100nF FOR 100Ω RTD
Typical Application Circuits continued at end of data sheet.
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
19-6478; Rev 0; 10/12
MAX31865
RTD-to-Digital Converter
ABSOLUTE MAXIMUM RATINGS
Voltage Range on VDD Relative to GND1.............-0.3V to +4.0V
Voltage Range on BIAS, REFIN+,
REFIN-, ISENSOR.................................. -0.3V to (VDD + 0.3V)
Voltage Range on FORCE+, FORCE2,
FORCE-, RTDIN+, RTDIN- Relative to GND1.....-50V to +50V
Voltage Range on DVDD Relative to DGND.........-0.3V to +4.0V
Voltage Range on All Digital Pins
Relative to DGND.............................. -0.3V to (VDVDD + 0.3V)
Continuous Power Dissipation (TA = +70NC)
TQFN (derate 34.5mW/NC above +70NC)...............2758.6mW
ESD Protection (all pins, Human Body Model)....................±2kV
Operating Temperature Range......................... -40NC to +125NC
Junction Temperature......................................................+150NC
Storage Temperature Range............................. -65NC to +150NC
Soldering Temperature (reflow).......................................+260NC
Lead Temperature (soldering,10s)..................................+300NC
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
PACKAGE THERMAL CHARACTERISTICS (Note 1)
TQFN
Junction-to-Ambient Thermal Resistance (qJA)...........29°C/W
Junction-to-Case Thermal Resistance (qJC)..................2°C/W
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer
board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial.
RECOMMENDED DC OPERATING CONDITIONS
(TA = -40NC to +125NC, unless otherwise noted.) (Notes 2 and 3)
PARAMETER
VDD
DVDD
SYMBOL
MIN
TYP
MAX
UNITS
VDD
CONDITIONS
3.0
3.3
3.6
V
VDVDD
3.0
3.3
3.6
V
V
Input Logic 0
VIL
CS, SDI, SCLK
-0.3
0.3 x
VDVDD
Input Logic 1
VIH
CS, SDI, SCLK
0.7 x
VDVDD
VDVDD
+ 0.3
V
0
VBIAS
V
350
10k
I
0
50
I
Analog Voltages
(FORCE+,FORCE2, FORCE-,
RTDIN+, RTDIN-)
Reference Resistor
Cable Resistance
Normal conversion results
RREF
RCABLE
Per lead
ELECTRICAL CHARACTERISTICS
(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are TA= +25NC, VDD = VDVDD = 3.3V.) (Notes 2
and 3)
PARAMETER
ADC Resolution
ADC Full-Scale Input Voltage
(RTDIN+ - RTDIN-)
Maxim Integrated
SYMBOL
CONDITIONS
No missing codes
MIN
TYP
MAX
UNITS
15
Bits
REFIN+ REFIN-
V
2
MAX31865
RTD-to-Digital Converter
ELECTRICAL CHARACTERISTICS (continued)
(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are TA= +25NC, VDD = VDVDD = 3.3V.) (Notes 2
and 3)
PARAMETER
SYMBOL
CONDITIONS
ADC Common-Mode Input
Range
MIN
0
Input Leakage Current
RTDIN+, RTDIN-, 0NC to +70NC, on-state
2
RTDIN+, RTDIN-, -40NC to +85NC, on-state
5
RTDIN+, RTDIN-, -40NC to 100NC, on-state
14
Bias Voltage
VBIAS
1.95
Bias Voltage Output Current
IOUT
0.2
Bias Voltage Load Regulation
IOUT P 5.75mA
Bias Voltage Startup Time
(Note 4)
ADC Offset Error
Input referred
Common-Mode Rejection
50/60Hz Noise Rejection
Fundamental and harmonics
Automatic Fault Detection Cycle
Time
mV/mA
IDD
Shutdown
±1
LSB
+3
FV RMS
90
dB
dB
55
Single conversion (50Hz notch)
62.5
66
Continuous conversion (50Hz notch)
20
21
From CS high to cycle complete
550
600
Bias on, active conversion
2
CIN
Logic inputs
IL
Logic inputs
Output High Voltage
VOH
IOUT = -1.6mA
Output Low Voltage
VOL
IOUT = 1.6mA
ms
Fs
LSB/V
1.5
3
mA
2
3.5
mA
2.27
V
120
mV
6
-1
LSB
150
52
Bias off, ADC off
ms
LSB
Single conversion (60Hz notch)
Power-On Reset Voltage
Hysteresis
Maxim Integrated
V
mA
17.6
Power-On Reset Voltage
Threshold
Input Leakage Current
2.06
5.75
1
IDD
Input Capacitance
nA
16.7
Power-Supply Rejection
Power-Supply Current (Note 6)
V
82
Continuous conversion (60Hz notch)
tCONV
VBIAS
±1
-3
Noise (over Nyquist Bandwidth)
UNITS
10
Differential Input, endpoint fit, 0.3 x VBIAS
P VREF P VBIAS
ADC Integral Nonlinearity
2.00
MAX
30
ADC Full-Scale Error
Temperature Conversion Time
(Note 5)
TYP
pF
+1
VDVDD
- 0.4
FA
V
0.4
V
3
MAX31865
RTD-to-Digital Converter
AC ELECTRICAL CHARACTERISTICS: SPI INTERFACE
(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted. Typical values are TA= +25NC, VDD = VDVDD = 3.3V.) (Notes 3
and 7) (Figure 1 and Figure 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Data to SCLK Setup
tDC
(Notes 8, 9)
35
ns
SCLK to Data Hold
tCDH
(Notes 8, 9)
35
ns
SCLK to Data Valid
tCDD
(Notes 8, 9, 10)
SCLK Low Time
80
ns
tCL
(Note 9)
100
SCLK High Time
tCH
(Note 9)
100
SCLK Frequency
tCLK
(Note 9)
DC
SCLK Rise and Fall
tR, tF
(Note 9)
CS to SCLK Setup
tCC
(Note 9)
400
ns
SCLK to CS Hold
tCCH
(Note 9)
100
ns
CS Inactive Time
tCWH
(Note 9)
400
CS to Output High-Z
tCDZ
(Notes 8, 9)
Address 01h or 02h Decoded to
DRDY High
tDRDYH
After RTD register read access (Note 9)
ns
ns
5.0
MHz
200
ns
ns
40
50
ns
ns
Note 2: All voltages are referenced to ground when common. Currents entering the IC are specified positive.
Note 3: Limits are 100% production tested at TA= +25°C and/or TA= +85°C. Limits over the operating temperature range and relevant supply voltage range are guaranteed by design and characterization. Typical values are not guaranteed.
Note 4: For 15-bit settling, a wait of at least 10.5 time constants of the input RC network is required. Max startup time is calculated
with a 10kω reference resistor and a 0.1µF capacitor across the RTD inputs.
Note 5: The first conversion after enabling continuous conversion mode takes a time equal to the single conversion time for the
respective notch frequency.
Note 6: Specified with no load on the bias pin as the sum of analog and digital currents. No active communication. If the RTD
input voltage is greater than the input reference voltage, then an additional 400µA IDD can be expected.
Note 7: All timing specifications are guaranteed by design.
Note 8: Measured at VIH = 0.7V x VDVDD or VIL = 0.3 x VDVDD and 10ms maximum rise and fall times.
Note 9: Measured with 50pF load.
Note 10:Measured at VOH = 0.7 x VDVDD or VOL = 0.3 x VDVDD. Measured from the 50% point of SCLK to the VOH minimum of
SDO.
Maxim Integrated
4
MAX31865
RTD-to-Digital Converter
CS
tCC
SCLK
tCDD
tCDD
tCDH
tDC
SDI
A7
A6
A0
tCDZ
SDO
D7
D6
WRITE ADDRESS BYTE
D1
D0
READ DATA BYTE
NOTE: SCLK CAN BE EITHER POLARITY, TIMING SHOWN FOR CPOL = 1.
Figure 1. Timing Diagram: SPI Read Data Transfer
CS
tCWH
tCC
tR
tCL
tCCH
tF
SCLK
tCDH
tCH
tCDH
tDC
SDI
A7
A6
WRITE ADDRESS BYTE
A0
D7
D0
WRITE DATA BYTE
NOTE: SCLK CAN BE EITHER POLARITY, TIMING SHOWN FOR CPOL = 1.
Figure 2. Timing Diagram: SPI Write Data Transfer
Maxim Integrated
5
MAX31865
RTD-to-Digital Converter
Typical Operating Characteristics
(VDD = VDVDD = 3.3V, TA = +25°C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
(ADC AUTO CONVERSION MODE)
3
3
IDD (mA)
IDD (mA)
ANALOG IDD
(BIAS PIN UNLOADED)
2
1
MAX31865 toc02
4
MAX31865 toc01
4
SUPPLY CURRENT vs. TEMPERATURE
(ADC NORMALLY OFF MODE)
ANALOG IDD
(BIAS PIN UNLOADED)
2
1
DIGITAL IDD
0
DIGITAL IDD
0
0
-50
50
100
150
0
-50
50
100
150
TEMPERATURE (°C)
TEMPERATURE (°C)
LEAKAGE CURRENT PER PIN vs. TEMPERATURE
(1 VOLT APPLIED TO FORCE+, FORCE2, RTDIN+, RTDIN- PINS)
SINC FILTER OPERATION
INPUT FREQUENCY vs. NOISE RESPONSE
0
NOISE RESPONSE (dB)
120
CURRENT (nA)
100
80
60
40
0
50Hz
60Hz
-40
-60
-100
75
50
100
125
150
50
10
90
130
170
TEMPERATURE (°C)
INPUT NOISE FREQUENCY (Hz)
ADC CONVERSION ERROR vs. RTD RESISTANCE
(4kΩ RREF, 4-WIRE CONNECTION)
ADC CONVERSION ERROR vs. RTD RESISTANCE
(400Ω RREF, 4-WIRE CONNECTION)
MAX31865 toc05
-40°C
0.244
0.244
+25°C
˜ ±0.1°C
ERROR
0
+100°C
-0.244
MAX31865 toc06
0.488
ERROR (Ω)
0.488
ERROR (Ω)
-20
-80
20
+25°C
-40°C
˜ ±0.1°C
ERROR
0
+100°C
-0.244
-0.488
-0.488
0
500
1000 1500 2000 2500 3000 3500
RRTD (Ω)
Maxim Integrated
MAX31865 toc04
20
MAX31865 toc03
140
0
50
100
150
200
250
300
350
RRTD (Ω)
6
MAX31865
RTD-to-Digital Converter
SDO
CS
SCLK
SDI
TOP VIEW
DGND
Pin Configuration
15
14
13
12
11
GND1 16
10
GND2
N.C. 17
9
FORCE-
8
RTDIN-
7
RTDIN+
6
FORCE2
MAX31865
DRDY 18
DVDD 19
EP
2
3
4
5
ISENSOR
FORCE+
BIAS
1
REFIN-
+
REFIN+
VDD 20
TQFN
(5mm x 5mm)
Pin Description
PIN
NAME
1
BIAS
2
REFIN+
Positive Reference Voltage Input. Connect to BIAS. Connect the reference resistor between REFIN+
and REFIN-.
3
REFIN-
Negative Reference Voltage Input. Connect the reference resistor between REFIN+ and REFIN-.
4
ISENSOR
5
FORCE+
6
FORCE2
Positive Input Used in 3-Wire Only. When in the 3-wire connection configuration, connect to FORCE+.
When in the 2-wire or 4-wire connection configuration, connect to ground. Protected to Q50V.
7
RTDIN+
Positive RTD Input. Protected to Q50V.
8
RTDIN-
Negative RTD Input. Protected to Q50V.
9
FORCE-
Low-Side RTD Return. Protected to Q50V.
10
GND2
11
SDI
12
SCLK
13
CS
14
SDO
Maxim Integrated
FUNCTION
Bias Voltage Output (VBIAS)
Low Side of RREF. Connect to REFIN-.
High-Side RTD Drive. Connect to FORCE2 when using the 3-wire connection configuration. Protected to
Q50V.
Analog Ground. Connect to GND1.
Serial-Data Input
Serial-Data Clock Input
Active-Low Chip Select. Set CS low to enable the serial interface.
Serial-Data Output
7
MAX31865
RTD-to-Digital Converter
Pin Description (continued)
PIN
NAME
FUNCTION
15
DGND
Digital Ground
16
GND1
Analog Ground. Connect to GND2.
17
N.C.
18
DRDY
Active-Low Push-Pull Data-Ready Output. DRDY goes low when a new conversion result is available in
the data register. When a read-operation of an RTD resistance data register occurs, DRDY returns high.
19
DVDD
Digital Supply Voltage Input. Connect to a 3.3V power supply. Bypass to DGND with a 0.1FF bypass
capacitor.
20
VDD
Analog Supply Voltage Input. Connect to a 3.3V power supply. Bypass to GND1 with a 0.1FF bypass
capacitor.
—
EP
Do Not Connect
Exposed Pad (Bottom Side of Package). Connect to GND1.
Block Diagram
VDD
BIAS
DVDD
VDD
VBIAS
GENERATOR
VDVDD
REFIN+
SCLK
SDO
REFINSERIAL
LOGIC
DATA REGISTERS
ISENSOR
SDI
CS
FORCE+
DIGITAL LOGIC
FORCE2
3-WIRE
ONLY
RTDIN+
15-BIT
Σ∆ ADC
±50V PROTECTION
RTDIN-
50/60Hz DIGITAL
SINC FILTER
DIGITAL
COMPARATOR
FOR
FAULT DETECTION
MASTER-INITIATED
FAULT-DETECTION
CYCLE
ADC STATE
MACHINE
DRDY
FORCE-
MAX31865
GND2
Maxim Integrated
GND1
DGND
8
MAX31865
RTD-to-Digital Converter
Detailed Description
Temperature Conversion
Resistance temperature detectors (RTDs) are sensors
whose resistance varies with temperature. Platinum
is the most common, most accurate wire material;
platinum RTDs are referred to as PT-RTDs. Nickel,
copper, and other metals may also be used to make
RTDs. Characteristics of platinum RTDs include a wide
temperature range (to over +800NC), excellent accuracy
and repeatability, and reasonable linearity.
For PT-RTDs, the most common values for nominal
resistance at 0NC are 100I and 1kI, though other
values are available. The average slope between 0NC
and +100NC is called alpha (α). This value depends on
the impurities and their concentrations in the platinum.
The two most widely used values for alpha are 0.00385
and 0.00392, corresponding to the IEC 751 (PT100) and
SAMA standards.
The resistance vs. temperature curve is reasonably
linear, but has some curvature, as described by the
Callendar-Van Dusen equation:
R(T) = R0(1 + aT + bT2 + c(T - 100)T3)
where:
T = temperature (NC)
R(T) = resistance at T
R0 = resistance at T = 0NC
IEC 751 specifies α = 0.00385055 and the following
Callendar-Van Dusen coefficient values:
a = 3.90830 x 10-3
b = -5.77500 x 10-7
c = -4.18301 x 10-12 for -200NC P T P 0NC, 0 for 0NC P T
P +850NC
Figure 3 shows the curve of resistance vs. temperature
for a PT100 RTD along with a straight-line approximation
based on the slope between 0NC and +100NC.
To measure the RTD’s resistance, connect a reference
resistor (RREF) and RTD in series and apply the bias
voltage to the top of RREF as shown in the Typical
Maxim Integrated
450
400
350
RESISTANCE (Ω)
The MAX31865 is a sophisticated RTD-to-digital converter
with a built-in 15-bit analog-to-digital converter (ADC),
input protection, a digital controller, an SPI-compatible
interface, and associated control logic. The signal
conditioning circuitry is optimized to work with PT100
through PT1000 RTDs. Thermistors are also supported.
PT100 RTD RESISTANCE
vs. TEMPERATURE
STRAIGHT-LINE
APPROXIMATION
300
250
200
RTD RESISTANCE
150
100
50
0
-200 -100 0
100 200 300 400 500 600 700
TEMPERATURE (°C)
Figure 3. PT100 RTD resistance vs. temperature.
Application Circuits. The reference resistor current also
flows through the RTD. The voltage across the reference
resistor is the reference voltage for the ADC. The voltage
across the RTD is applied to the ADC’s differential inputs
(RTDIN+ and RTDIN-). The ADC therefore produces
a digital output that is equal to the ratio of the RTD
resistance to the reference resistance. A reference
resistor equal to four times the RTD’s 0NC resistance is
optimum for a platinum RTD. Therefore, a PT100 uses
a 400I reference resistor, and a PT1000 uses a 4kI
reference resistor.
A 2-wire connection (see the Typical Application Circuits)
can give acceptable results when the RTD is located
close to the MAX31865. Note that, for a PT100, series
resistance of 0.4I causes an error of approximately 1NC.
Therefore, as the cable length increases, the error due to
cable resistance can become excessive.
The 4-wire connection eliminates errors due to cable
resistance by using separate force and sense leads.
A 3-wire connection is a compromise approach that
uses one less conductor than the 4-wire approach. To
compensate for the voltage drop across the return wire,
the voltage between FORCE+ and RTDIN+ is subtracted
from (RTDIN+ - RTDIN-). This is accomplished using
the FORCE2 sampling input. If the cable resistances
are well-matched, the error due to cable resistance is
cancelled. Select 3-wire operation by setting the 3-wire
bit in the Configuration register to 1.
9
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:
requested by the master. During a fault detection cycle
the MAX31865 has the ability to disconnect the FORCEinput from its GND2 return path by means of and internal
analog switch.
Temperature (NC) ≈ (ADC code/32) – 256
The conditions that generate a fault are listed below, see
Figure 4 for a fault detection flowchart.
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
Maxim Integrated
• Detected at any point in time
Overvoltage (> VDD) or undervoltage (< GND1) condition 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 faultinduced 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.
10
MAX31865
RTD-to-Digital Converter
FAULT DETECTION
ALWAYS ACTIVE FAULT DETECTION
MONITOR PINS
IS FORCE+,
FORCE2, FORCE-,
RTDIN+, RTDIN-,
PINS > VDD OR <
GND
EVERY CONVERSION FAULT DETECTION
N
CONVERSION
INITIATED
PERFORM
CONVERSION
IS
RTD RESISTANCE
VALUE > HIGH
THRESHOLD
REGISTER
Y
IS
RTD RESISTANCE
VALUE < LOW
THRESHOLD
REGISTER
N
Y
Y
PROTECT PINS AGAINST
±50V
N
SET BIT D6 OF FAULT
STATUS REGISTER
SET BIT D7 OF FAULT
STATUS REGISTER
SET BIT D2 OF FAULT
STATUS REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
ADC HALTS UPDATES
SET BIT D0 OF RTD DATA
LSB REGISTER
MASTER-INITIATED FAULT-DETECTION CYCLE - AUTOMATIC MODE
MASTER WRITES
100X010Xb TO
CONFIGURATION
REGISTER
FORCE-INPUT
SWITCH
REMAINS
CLOSED
IS VREFIN>
0.85 x VBIAS
100µs
DELAY
N
100µs
DELAY
OPEN
FORCEINPUT
SWITCH
IS VREFIN<
0.85 x VBIAS
210µs
DELAY
Y
N
Y
SET BIT D5 OF FAULT
STATUS REGISTER
SET BIT D4 OF FAULT
STATUS REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
CONFIGURATION
REGISTER SET TO
100X000Xb TO
END FAULT
DETECTION
CYCLE
FORCE-INPUT
SWITCH
CLOSED
N
IS RTDIN<
0.85 x VBIAS
100µs
DELAY
Y
SET BIT D3 OF FAULT
STATUS REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
MASTER-INITIATED FAULT-DETECTION CYCLE - MANUAL MODE
MASTER WRITES
100X100Xb TO
CONFIGURATION
REGISTER
FORCE-INPUT
SWITCH
REMAINS
CLOSED
100µs
DELAY
IS VREFIN>
0.85 x VBIAS
N
100µs
DELAY
OPEN
FORCEINPUT
SWITCH
Y
DID
MASTER WRITE
100X110Xb TO
CONFIGURATION
REGISTER
Y
IS VREFIN<
0.85 x VBIAS
100µs
DELAY
Y
N
SET BIT D5 OF FAULT
STATUS REGISTER
SET BIT D4 OF FAULT
STATUS REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
END FAULT
DETECTION
CYCLE
CONFIGURATION
REGISTER SET TO
100X000Xb TO
FORCE-INPUT
SWITCH
CLOSED
N
N
IS RTDIN<
0.85 x VBIAS
100µs
DELAY
Y
SET BIT D3 OF FAULT
STATUS REGISTER
SET BIT D0 OF RTD DATA
LSB REGISTER
Figure 4. Fault Detection Flowcharts
Maxim Integrated
11
MAX31865
RTD-to-Digital Converter
Internal Registers
Conversion Mode (D6)
Write 1 to this bit to select automatic conversion mode, in
which conversions occur continuously at a 50/60Hz rate.
Write 0 to this bit to exit automatic conversion mode and
enter the “Normally Off” mode. 1-shot conversions may
be initiated from this mode.
The registers are accessed using the 0Xh addresses
for reads and the 8Xh addresses for writes. Data is read
from or written to the registers MSB first.
1-Shot (D5)
When the conversion mode is set to “Normally Off”, write
1 to this bit to start a conversion. This causes a single
resistance conversion to take place. The conversion
is triggered when CS goes high after writing a 1 to
this bit. Note that if a multibyte write is performed, the
conversion is triggered when CS goes high at the end
of the transaction. If VBIAS is on (as selected by the
Configuration Register), the RTD voltage is sampled
when CS goes high and the conversion begins. Note
that if VBIAS is off (to reduce supply current between
conversions), any filter capacitors at the RTDIN inputs
need to charge before an accurate conversion can be
performed. Therefore, enable VBIAS and wait at least
10.5 time constants of the input RC network plus an
additional 1ms before initiating the conversion. Note that
a single conversion requires approximately 52ms in 60Hz
filter mode or 62.5ms in 50Hz filter mode to complete.
1-Shot is a self-clearing bit.
Communication is through eight 8-bit registers that
contain conversion, status, and configuration data. All
programming is done by selecting the appropriate
address of the desired register location. Table 1 illustrates
the addresses for the registers.
Configuration Register (00h)
The configuration register selects the conversion mode
(automatic or triggered by the 1-shot command), enables
and disables BIAS pin output voltage VBIAS, initiates
1-shot conversions, selects the RTD connection (either
3-wire or 2-wire/4-wire), initiates a full fault detection
cycle, clears the Fault Status register, and selects the
filter notch frequencies. The effects of the configuration
bits are described below.
BIAS (D7)
When no conversions are being performed, VBIAS may
be disabled to reduce power dissipation. Write 1 to this
bit to enable VBIAS before beginning a single (1-Shot)
conversion. When automatic (continuous) conversion
mode is selected, VBIAS remains on continuously.
Table 1. Register Addresses and POR State
READ ADDRESS (HEX)
WRITE ADDRESS (HEX)
POR STATE
READ/WRITE
Configuration
REGISTER NAME
00h
80h
00h
R/W
RTD MSBs
01h
—
00h
R
RTD LSBs
02h
—
00h
R
High Fault Threshold MSB
03h
83h
FFh
R/W
High Fault Threshold LSB
04h
84h
FFh
R/W
Low Fault Threshold MSB
05h
85h
00h
R/W
Low Fault Threshold LSB
06h
86h
00h
R/W
Fault Status
07h
—
00h
R
Table 2. Configuration Register Definition
D7
D6
D5
D4
VBIAS
1 = ON
0 = OFF
Conversion
mode
1 = Auto
0 = Normally off
1-shot
1 = 1-shot
(auto-clear)
3-wire
1 = 3-wire RTD
0 = 2-wire or
4-wire
Maxim Integrated
D3
D2
Fault Detection
Cycle Control
(see Table 3)
D1
D0
Fault Status
Clear
1 = Clear
(auto-clear)
50/60Hz filter
select
1 = 50Hz
0 = 60Hz
12
MAX31865
RTD-to-Digital Converter
The Fault Detect Cycle bits (D[3:2]) self-clear to 00b
upon completion.
3-Wire (D4)
Write 1 to this bit when using a 3-wire RTD connection.
In this mode the voltage between FORCE+ and RTDIN+
is subtracted from (RTDIN+ - RTDIN-) to compensate
for the IR errors caused by using a single wire for the
FORCE- and RTDIN- connections. When using 2-wire or
4-wire connections, write 0 to this bit.
To enter the manual fault detection cycle, first ensure that
VBIAS has been on for at least 5 time constants. Next,
write 100X100Xb to the Configuration register. The ADC
is now in “Normally Off” mode. The MAX31865 checks
for faults while the FORCE- input switch is closed, and
when the check completes, the FORCE-input switch
opens. The Fault Detect Cycle bits (D[3:2]), remain set to
10b. Again, wait at least 5 time constants, and then write
100X110Xb to the Configuration register. The MAX31865
now checks for faults while the FORCE- inputs switch
is open; when the check completes, the FORCE- input
switch closes and the Fault Detect Cycle bits (D[3:2])
self-clear to 00b. Note that if 1 is written to D5 (1-Shot)
and D2 or D3 in a single write, both commands are
ignored. If 100X110Xb is set without a prior initiation of
the first manual step (setting 100X100Xb), the automatic
fault detection mode is run instead.
Fault Detection Cycle (D3:D2)
The master initiated fault detection cycle has two modes
of operation, manual and automatic mode timing. If the
external RTD interface circuitry includes an input filter with
a time constant greater than 100Fs, the fault detection
cycle timing should be controlled in the manual mode
operation. The fault detection cycle checks for three
faults by making the following voltage comparisons and
setting the associated bits in the Fault Status Register:
1) Is the voltage at REFIN- greater than 85% x VBIAS?
(Fault Status Register bit D5)
2) Is the voltage at REFIN- less than 85% x VBIAS when
FORCE- input switch is open? (Fault Status Register
bit D4)
Fault Status Clear (D1)
Write a 1 to this bit while writing 0 to bits D5, D3, and D2
to return all fault status bits (D[7:2]) in the Fault Status
Register to 0. Note that bit D2 in the Fault Register, and
subsequently bit D0 in the RTD LSB register may be set
again immediately after resetting if an over/undervoltage
fault persists. The fault status clear bit D1, self-clears to
0.
3) Is the voltage at RTDIN- less than 85% x VBIAS when
FORCE- input switch is open? (Fault Status Register
bit D3)
Note: All voltages are referenced to GND1.
The Applications Information provides tables for decoding
possible causes of set fault status bits.
50/60Hz (D0)
This bit selects the notch frequencies for the noise
rejection filter. Write 0 to this bit to reject 60Hz and
its harmonics; write 1 to this bit to reject 50Hz and its
harmonics. Note: Do not change the notch frequency
while in auto conversion mode.
To enter the automatic fault detection cycle, write
100X010Xb to the Configuration register. The ADC
is now in “Normally Off” mode. The automatic fault
detection cycle inserts 100Fs delays before checking for
faults, thereby allowing the external input filter to settle.
Table 3. Fault-Detection Cycle Control Bits
D3
D2
CONFIGURATION REGISTER
WRITE (BINARY)
0
0
XXXX00XXb
No action
Fault detection finished
0
1
100X010Xb
Fault detection with automatic delay
Automatic fault detection still running
1
0
100X100Xb
Run fault detection with manual delay
(cycle 1)
Manual cycle 1 still running; waiting for
user to write 11
1
1
100X110Xb
Finish fault detection with manual delay
(cycle 2)
Manual cycle 2 still running
WRITE ACTION
READ MEANING
X = Don’t care
Maxim Integrated
13
MAX31865
RTD-to-Digital Converter
RTD Resistance Registers (01h−02h)
Two 8-bit registers, RTD MSBs and RTD LSBs, contain
the RTD resistance data. The data format is shown in
Table 4. The data format is simply the 15-bit ratio of RTD
resistance to reference resistance. D0 of the RTD LSBs
register is a Fault bit that indicates whether any RTD
faults have been detected.
Table 4. RTD Resistance Registers Definition
REGISTER
Bit
RTD MSBS (01h) REGISTER
RTD LSBS (02h) REGISTER
D7
D6
D5
D4
D3
D2
D1
D0
D7
D6
D5
D4
D3
D2
D1
D0
RTD
Resistance
Data
MSB
—
—
—
—
—
—
—
—
—
—
—
—
—
LSB
Fault
Bit
Weighting
214
213
212
211
210
29
28
27
26
25
24
23
22
21
20
—
4096
2048
1024
512
256
128
64
32
16
8
4
2
1
—
Decimal
Value
16384 8192
Table 5. RTD Resistance-Data Relationship
RRTD/RREF
BINARY
HEX
DECIMAL
RTD MSBs (01h)
RTD LSBs (02h)
RTD MSBs (01h)
RTD LSBs (02h)
0.025
0000 0110
0110 0110b
06h
66h
819
0.125
0010 0000
0000 0000b
20h
00h
4096
0.25
0100 0000
0000 0000b
40h
00h
8192
0.50
1000 0000
0000 0000b
80h
00h
16,384
0.75
1100 0000
0000 0000b
C0h
00h
24,576
0.999
1111 1111
1111 1110b
FFh
FEh
32,767
Note: D0 (“Fault”) is assumed to be 0.
Maxim Integrated
14
MAX31865
RTD-to-Digital Converter
Fault Threshold Registers (03h–06h)
The High Fault Threshold and Low Fault Threshold
registers select the trip thresholds for RTD fault detection.
The results of RTD conversions are compared with the
values in these registers to generate the “Fault” (D[7:6])
bits in the Fault Status register. The RTD Data Registers,
High Fault Threshold Registers, and Low Fault Threshold
Registers all have the same format.
The RTD High bit in the Fault Status Register is set if the
RTD resistance register value is greater than or equal to
the value in the High Fault Threshold register. The POR
value of the High Fault Threshold register is FFFFh.
The RTD Low bit in the Fault Status Register is set if the
RTD resistance value is less than or equal to the value in
the Low Fault Threshold register. The POR value of the
Low Fault Threshold register is 0000h.
Fault Status Register (07h)
The Fault Status register latches any detected fault bits;
writing a 1 to the Fault Status Clear bit in the Configuration
Register returns all fault status bits to 0.
Serial Interface
The MAX31865 supports SPI modes 1 and 3. Four pins
are used for SPI-compatible communications: SDO
(serial-data out), SDI (serial-data in), CS (chip select),
and SCLK (serial clock). SDI and SDO are the serialdata input and output pins for the devices, respectively.
The CS input initiates and terminates a data transfer.
SCLK synchronizes data movement between the master
(microcontroller) and the slave (MAX31865).
The serial clock (SCLK), which is generated by the
microcontroller, is active only when CS is low and dur­
ing address and data transfer to any device on the
Table 6. Fault Threshold Registers Definition
REGISTER
Bit
HIGH FAULT THRESHOLD MSB (03h) REGISTER
HIGH FAULT THRESHOLD LSB (04h) REGISTER
LOW FAULT THRESHOLD MSB (05h) REGISTER
LOW FAULT THRESHOLD MSB (06h) REGISTER
D7
D6
D5
D4
D3
D2
D1
D0
D7
D6
D5
D4
D3
D2
D1
D0
RTD
Resistance
Data
MSB
—
—
—
—
—
—
—
—
—
—
—
—
—
LSB
X
Bit
Weighting
214
213
212
211
210
29
28
27
26
25
24
23
22
21
20
—
16384
8192
4096
2048
1024
512
256
128
64
32
16
8
4
2
1
—
Decimal
Value
X = Don’t care
Table 7. Fault Status Register Definition
D7
D6
D5
D4
D3
D2
D1
D0
RTD High
Threshold
RTD Low
Threshold
REFIN- >
0.85 x VBIAS
REFIN- < 0.85 x VBIAS
(FORCE- open)
RTDIN- < 0.85 x
VBIAS (FORCE- open)
Overvoltage/
undervoltage fault
x
x
X = Don’t care
Maxim Integrated
15
MAX31865
RTD-to-Digital Converter
SPI bus. The inactive clock polarity is programmable
in some microcontrollers. The MAX31865 automatically
accommodates either clock polarity by sampling SCLK
when CS becomes active to determine the polarity of the
inactive clock. Input data (SDI) is latched on the internal
strobe edge and output data (SDO) is shifted out on
the shift edge (see Table 8 and Figure 5). There is one
clock for each bit transferred. Address and data bits are
transferred in groups of eight, MSB first.
Address and Data Bytes
Address and data bytes are shifted MSB-first into the
serial-data input (SDI) and out of the serial-data output
(SDO). Any transfer requires the address of the byte to
specify a write or a read, followed by one or more bytes
of data. Data is transferred out of the SDO for a read
opera­
tion and into the SDI for a write operation. The
address byte is always the first byte transferred after
CS is driven low. The MSB (A7) of this byte determines
whether the following byte is written or read. If A7 is 0,
one or more byte reads follow the address byte. If A7 is
1, one or more byte writes follow the address byte.
For a single-byte transfer, 1 byte is read or written and
then CS is driven high (see Figure 6 and Figure 7). For
a multiple-byte transfer, multiple bytes can be read or
written after the address has been written (see Figure 8).
The address continues to increment through all memory
locations as long as CS remains low. If data continues to
be clocked in or out, the address loops from 7Fh/FFh to
00h/80h. Invalid memory addresses report an FFh value.
Attempting to write to a read-only register results in no
change to that register’s contents.
Table 8. Function Table
MODE
Disable Reset
CS
High
Write
Low
Read
Low
SCLK
SDI
SDO
Input disabled
Input disabled
High impedance
Data bit latch
High impedance
X
Next data bit shift**
CPOL = 1*, SCLK rising
CPOL = 0, SCLK falling
CPOL = 1, SCLK falling
CPOL = 0, SCLK rising
Note: CPHA bit polarity must be set to 1.
*CPOL is the clock polarity bit that is set in the control register of the microcontroller.
**SDO remains at high impedance until 8 bits of data are ready to be shifted out during a read.
CS
CPOL = 1
SHIFT
INTERNAL STROBE
SHIFT
INTERNAL STROBE
SCLK
CS
CPOL = 0
SCLK
NOTE: CPOL IS A BIT THAT IS SET IN THE MICROCONTROLLER’S CONTROL REGISTER.
Figure 5. Serial Clock as a Function of Microcontroller Clock Polarity (CPOL)
Maxim Integrated
16
MAX31865
RTD-to-Digital Converter
CS
SCLK
SDI
A7
SDO
A6
A5
A4
A3
A2
A1
A0
HIGH-Z
D7
D6
D5
D4
D3
D2
D1
D0
Figure 6. SPI Single-Byte Read
CS
SCLK
SDI
A7
SDO
A6
A5
A4
A3
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
HIGH-Z
Figure 7. SPI Single-Byte Write
CS
SCLK
WRITE
SDI
ADDRESS
BYTE
SDI
ADDRESS
BYTE
DATA
BYTE 0
DATA
BYTE 1
DATA
BYTE N
DATA
BYTE 0
DATA
BYTE 1
DATA
BYTE N
READ
SDO
Figure 8. SPI Multibyte Transfer
Maxim Integrated
17
MAX31865
RTD-to-Digital Converter
DRDY
The DRDY output goes low when a new conversion result
is available in the RTD Data Registers. When a readoperation of the RTD Data Registers completes, DRDY
returns high.
Applications Information
For operation in noisy environments, a filter capacitor
may be placed across the RTDIN+ and RTDIN- inputs.
After an overvoltage or undervoltage fault, after a fault
detection cycle, or after enabling VBIAS, always allow for
the settling time of the input filter before restarting the
ADC. A delay time of at least five time constants plus an
additional 1ms (for the protection devices to stabilize) is
recommended to achieve specified accuracy.
Converting RTD Data Register
Values to Temperature
The ratiometric ADC conversion results found in the RTD
Data Registers can be converted to temperature with a
few calculations.
First, the Resistance of the RTD needs to be determined
with the following equation:
RRTD = (ADC Code x RREF)/215
DRDY
RTD REGISTER
CONTENTS
CONVERSION n
CONVERSION n+1
CONVERSION n+2
SDI
RTD DATA
ADDRESS
SDO
RTD DATA
CS
Figure 9. DRDY Operation
Maxim Integrated
18
MAX31865
RTD-to-Digital Converter
ADC Code = 15-bit ADC results from RTD Data registers
(01h–02h)
used to determine temperature by either calculations or
lookup tables.
RREF = Resistance of the reference resistor
In the typical case of a PT100 RTD with a 400I high
precision low drift reference resistor, Table 9 shows
examples of temperature and resistance values with the
corresponding ADC code results.
Once the resistance of the RTD is known, the welldefined resistive properties of the selected RTD can be
Table 9. Temperature Example for PT100 with 400I RREF
TEMPERATURE
(°C)
RTD RESISTANCE
(ω)
RTD DATA REG
(01h-02h) (hex)
ADC CODE (dec)
ADC CODE/32-256
(°C)
-200
-175
-150
-125
-100
-75
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
225
250
18.52
29.22
39.72
50.06
60.26
70.33
80.31
84.27
88.22
92.16
96.09
100.00
103.90
107.79
111.67
115.54
119.40
123.24
127.08
130.90
134.71
138.51
142.29
146.07
149.83
153.58
157.33
161.05
164.77
168.48
172.17
175.86
185.01
194.10
0BDAh
12B4h
196Ch
200Ah
2690h
2D04h
3366h
35EEh
3876h
3AFCh
3D7Eh
4000h
4280h
44FCh
4778h
49F2h
4C6Ah
4EE0h
5154h
53C6h
5636h
58A4h
5B12h
5D7Ch
5FE4h
624Ch
64B0h
6714h
6974h
6BD4h
6E30h
708Ch
7668h
7C3Ah
1517
2394
3254
4101
4936
5762
6579
6903
7227
7550
7871
8192
8512
8830
9148
9465
9781
10096
10410
10723
11035
11346
11657
11966
12274
12582
12888
13194
13498
13802
14104
14406
15156
15901
-208.59
-181.19
-154.31
-127.84
-101.75
-75.94
-50.41
-40.28
-30.16
-20.06
-10.03
0.00
10.00
19.94
29.88
39.78
49.66
59.50
69.31
79.09
88.84
98.56
108.28
117.94
127.56
137.19
146.75
156.31
165.81
175.31
184.75
194.19
217.63
240.91
Maxim Integrated
19
MAX31865
RTD-to-Digital Converter
Table 9. Temperature Example for PT100 with 400I RREF (continued)
TEMPERATURE
(°C)
RTD RESISTANCE
(ω)
RTD DATA REG
(01h-02h) (hex)
ADC CODE (dec)
ADC CODE/32-256
(°C)
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
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 fullscale 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
Maxim Integrated
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, surfacemount capacitor if possible. Surface-mount components
minimize lead inductance, which improves performance,
and ceramic capacitors tend to have adequate highfrequency response for decoupling applications.
20
MAX31865
RTD-to-Digital Converter
Table 10. Decoding RTD Faults for 2-Wire Setups When Fault Bit in RTD Data LSB
Register = 1
FAULT
STATUS
BIT SET
D7
D6
DESCRIPTION OF POSSIBLE
CAUSE
Open RTD element
Shorted RTD element
RTDIN+ shorted low
CONDITION DETECTED
Measured resistance greater than High Fault
Threshold value
Full scale
Measured resistance less than Low Fault
Threshold value
Near zero
Open RTD
D5
RTDIN+ shorted high
Full scale
VREFIN- > 0.85 x VBIAS
RTDIN- shorted high
D4
D3
D2
RTDIN- shorted low
RTDIN- shorted low
RTDIN+ shorted low
Overvoltage or undervoltage fault
DESCRIPTION OF
RESULTING DATA
Indeterminate
Indeterminate
VREFIN- < 0.85 x VBIAS (FORCE- open)
VRTDIN- < 0.85 x VBIAS (FORCE- open)
Any protected input voltage >VDD or <GND1
Appear to be valid
Appear to be valid
Near zero
Indeterminate
Table 11. Decoding RTD Faults for 3-Wire Setups When Fault Bit in RTD Data LSB
Register = 1
FAULT
STATUS
BIT SET
DESCRIPTION OF POSSIBLE
CAUSE
CONDITION DETECTED
DESCRIPTION OF
RESULTING DATA
Open RTD element
D7
RTDIN+ shorted high and not
connected to RTD
Force+ shorted high and
connected to RTD
Measured resistance greater than High Fault
Threshold value
Full scale
Measured resistance less than Low Fault
Threshold value
Near zero
RTDIN+ shorted to RTDIND6
RTDIN+ shorted low and not
connected to RTD
Force+ shorted low
Open RTD element
Full scale
Force+ shorted high and
connected to RTD
D5
Force+ unconnected
VREFIN- > 0.85 x VBIAS
Force+ shorted high and not
connected to RTD
Indeterminate
RTDIN- shorted high
D4
RTDIN- shorted low
VREFIN- < 0.85 x VBIAS (FORCE- open)
Appear to be valid
Force+ shorted low
D3
RTDIN+ shorted low and
connected to RTD
VRTDIN- < 0.85 x VBIAS (FORCE- open)
RTDIN- shorted low
D2
Overvoltage or undervoltage fault
Maxim Integrated
Near zero
Appear to be valid
Any protected input voltage >VDD or < GND1
Indeterminate
21
MAX31865
RTD-to-Digital Converter
Table 12. Decoding RTD Faults for 4-Wire Setups When Fault Bit in RTD Data LSB
Register = 1
FAULT
STATUS
BIT SET
DESCRIPTION OF POSSIBLE
CAUSE
CONDITION DETECTED
DESCRIPTION OF
RESULTING DATA
Open RTD element
D7
RTDIN+ shorted high and not
connected to RTD
Force+ shorted high and
connected to RTD
Measured resistance greater than High Fault
Threshold value
Full scale
Measured resistance less than Low Fault Threshold
value
Near zero
RTDIN+ shorted to RTDIND6
RTDIN+ shorted low and not
connected to RTD
RTDIN- shorted high and not
connected to RTD
Force+ shorted low
Open RTD element
Full scale
Force+ shorted high and
connected to RTD
Force- unconnected
Force+ unconnected
D5
Force+ shorted high and not
connected to RTD
VREFIN- > 0.85 x VBIAS
Force- shorted high and not
connected to RTD
Indeterminate
Force- shorted high and connected
to RTD
Force- shorted low and not
connected to RTD
D4
Force- shorted low and connected
to RTD
RTDIN- shorted low and connected
to RTD
Indeterminate
VREFIN- < 0.85 x VBIAS (FORCE- open)
Appear to be valid
Force+ shorted low
Near zero
RTDIN+ shorted low and
connected to RTD
D3
RTDIN- shorted low and connected
to RTD
VRTDIN- < 0.85 x VBIAS (FORCE- open)
Appear to be valid
RTDIN- shorted low and not
connected to RTD
Force- shorted low
D2
Overvoltage or undervoltage fault
Maxim Integrated
Any protected input voltage >VDD or < GND1
Indeterminate
22
MAX31865
RTD-to-Digital Converter
Typical Application Circuits (continued)
2-WIRE SENSOR CONNECTION
VDD
VDD
0.1µF
VDD
DVDD
0.1µF
BIAS
REFIN+
DRDY
REFIN-
SDI
HOST
INTERFACE
SCLK
RREF
ISENSOR
MAX31865
FORCE+
CS
FORCE2
SDO
RTDIN+
N.C.
CI*
RTD
DGND
GND2
GND1
RTDINFORCE*CI = 10nF FOR 1kΩ RTD
100nF FOR 100Ω RTD
3-WIRE SENSOR CONNECTION
VDD
VDD
0.1µF
VDD
DVDD
0.1µF
BIAS
REFIN+
DRDY
REFIN-
SDI
HOST
INTERFACE
SCLK
RREF
ISENSOR
MAX31865
RCABLE
FORCE+
CS
FORCE2
SDO
RTDIN+
N.C.
RCABLE
CI*
RTD
RCABLE
Maxim Integrated
DGND
GND2
GND1
RTDINFORCE-
23
MAX31865
RTD-to-Digital Converter
Package Information
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX31865ATP+
-40°C to +125°C
20 TQFN-EP*
MAX31865ATP+T
-40°C to +125°C
20 TQFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
T = Tape and reel.
*EP = Exposed pad.
Maxim Integrated
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that
a “+”, “#”, or “-” in the package code indicates RoHS status only.
Package drawings may show a different suffix character, but the
drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
20 TQFN-EP
T2055+5
21-0140
90-0010
24
MAX31865
RTD-to-Digital Converter
Revision History
REVISION
NUMBER
REVISION
DATE
0
10/12
DESCRIPTION
Initial release
PAGES
CHANGED
—
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and
max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
© 2012 Maxim Integrated Products, Inc.
25
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.