What is the time duration required to clock out a conversion starting from power up of the MAX31865?

Because the MAX31865 powers up with the internal BIAS disabled and the Conversion Mode off, these control bits will first need to be set in the Configuration register before a temperature conversion can be executed. Once the MAX31865 has reached minimum VDD, the Configuration Register must be written to in order to enable the internal BIAS voltage (required for conversions). The user must also determine if they are going to operating in Continuous Conversion Mode or request conversions using the 1-Shot bit. Either way, once the desired mode is selected and a conversion is initiated, the user can expect a conversion result in 55ms if the 60Hz notch filter is selected, or in 66ms if the 50Hz notch filter is selected. After this conversion time, the user may clock out the conversion result over the SPI bus, the time required for that depending on the SPI clock speed.

Can I use the MAX31865 with a Nickle RTD instead of a Platinum RTD?

You may use a Nickle RTD in the MAX31865 in place of a Platinum RTD. Generally, we recommend using a Platinum RTD because it has the highest accuracy, but a Nickle RTD can work as well. It is important to note that you will not be able to use the same equation provided on page 10 of the datasheet to calculate the temperature. Generally, most temperature sensors will provide a lookup table in their datasheets that you can use to extrapolate the temperature from the measured resistance. It is also important to note that Nickle RTDs have a temperature range of -100°C to +300°C, whereas Platinum RTDs have a range of -260°C to +650°C. As long as you plan to stay within the appropriate temperature range, a Nickle RTD should work fine.

How does the resistor tolerance affect the accuracy of the MAX31865?

While there are no tolerance limits specified for the reference resistor in the data sheet, the tolerance does have an impact on the accuracy of the readings. The data sheet specifies a resistor value range - 350Ω(min) 10kΩ(max) - which is related to the reference voltage source. The tolerance of the resistor will be one of the sources of error that the designer needs to take into consideration when designing with the MAX31865. As indicated in the datasheet "The ADC therefore produces a digital output that is equal to the ratio of the RTD resistance to the reference resistance." The greater the value of the tolerance, the greater ADC output error due the error of the reference resistor's actual value. An smaller tolerance value of the resistor means a smaller error. Similarly, the temperature coefficient (TC) will come into play when concerned about error in the ADC result with respect to temperature fluctuations. Therefore using a resistor with a low TC is best.

My microcontroller only has 4 pins to communicate with MAX31865. Which one should I choose: 1. Not using the DRDY. 2. Use only 3 wires for SPI?

Since MAX31865 registers require setting up, it is not possible to use only 3 wires for SPI communication; Two wires are needed for chip select and clock, and the other two are required for setting up the registers and reading data back from MAX31865. However, it is possible if DRDY pin is not used. In this case, the microcontroller should wait maximum conversion time before reading each sample. The maximum conversion time depends on the mode the device is in. Refer to "Temperature Conversion Time" specification in Electrical Characteristics table of the datasheet. Note that in 1-shot mode, initiating each conversion will require writing to the register.

What are the limiting factors of the MAX31865's temperature conversion resolution ?

One limiting factor for the resolution on the MAX31865 is the internal ADC used to determine the resistance of the PT-RTD. The ADC has resolution of 15-bits, which creates a limit on the smallest change in resistance that can be measured. This value is inherent to the MAX31865 and cannot be changed. Another factor is that the relationship between a resistance and temperature for an PT-RTD is not linear. The resolution of the resistance is going to remain constant due to the ADC resolution, but the resolution of the temperature may vary slightly because of the non-linear relationship. This means that you may not see a consistent temperature resolution of 0.03125°C across the entire temperature range.

When using multiple MAX31865, is it possible to connect all RTDIN+ pins or all RTDIN- of different MAX31865 to a common point ?

The MAX31865 has a Fault Detection feature. The RTD wires cannot share a common connection across multiple MAX31865, else the Fault Detection feature will detect these additional connections as a fault condition and halt the conversion process. It is recommended to connect separate RTD lines to the different MAX31865 or use a mux to make those connections.