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MAX31865

RTD-to-Digital Converter

General Description

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.

Applications

Industrial Equipment

Medical Equipment

Instrumentation

Benefits and Features

S Integration Lowers System Cost, Simplifies

Design Efforts, and Reduces Design Cycle Time

• Simple Conversion of Platinum RTD Resistance

to Digital Value

• Handles 100Ω to 1kΩ (at 0°C) Platinum RTDs

(PT100 to PT1000)

• Compatible with 2-, 3-, and 4-Wire Sensor Connections

• SPI-Compatible Interface

• 20-Pin TQFN Package

S High Accuracy Facilitates Meeting Error Budgets

• 15-Bit ADC Resolution; Nominal Temperature

Resolution 0.03125NC (Varies Due to RTD

Nonlinearity)

• Total Accuracy Over All Operating Conditions:

0.5NC (0.05% of Full Scale) max

• Fully Differential V

REF

Inputs

• 21ms (max) Conversion Time

S Integrated Fault Detection Increases System

Reliability

• ±50V Input Protection

• Fault Detection (Open RTD Element, RTD Shorted to

Out-of-Range Voltage, or Short Across RTD Element)

Typical Application Circuits

Typical Application Circuits continued at end of data sheet.

19-6478; Rev 1; 1/15

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.

EVALUATION KIT AVAILABLE

MAX31865

BIAS

REFIN+

DVDD

GND1

GND2

DGND

REFIN-DRDY

ISENSORSDI

SCLK

HOST

INTERFACE

SDO

N.C.

FORCE-

REF

RTD

0.1µF

FORCE+

FORCE2

RTDIN+

RTDIN-

= 10nF FOR 1kΩ RTD

100nF FOR 100Ω RTD

0.1µF

2-WIRE SENSOR CONNECTION

For pricing, delivery, and ordering information, please contact Maxim Direct

at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.

Summary of content (25 pages)

PAGE 1
EVALUATION KIT AVAILABLE MAX31865 RTD-to-Digital Converter General Description Benefits and 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.
PAGE 2
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.
PAGE 3
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.
PAGE 4
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.
PAGE 5
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.
PAGE 6
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.
PAGE 7
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.
PAGE 8
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.
PAGE 9
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.
PAGE 10
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.
PAGE 11
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 DAT
PAGE 12
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.
PAGE 13
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.
PAGE 14
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.
PAGE 15
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.
PAGE 16
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.
PAGE 17
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.
PAGE 18
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.
PAGE 19
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.
PAGE 20
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.
PAGE 21
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.
PAGE 22
MAX31865 RTD-to-Digital Converter Table 12.
PAGE 23
MAX31865 RTD-to-Digital Converter Typical Application Circuits (continued) VDD VDD 2-WIRE SENSOR CONNECTION 0.1µF VDD DVDD 0.1µF BIAS REFIN+ DRDY REFIN- SDI HOST INTERFACE RREF ISENSOR MAX31865 SCLK 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 VDD 0.1µF DVDD 0.
PAGE 24
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.
PAGE 25
MAX31865 RTD-to-Digital Converter Revision History REVISION NUMBER REVISION DATE 0 10/12 Initial release — 1 1/15 Revised Benefits and Features section 1 DESCRIPTION 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.