Other Parts Discussed in Thread: ADS124S08, , ADS124S06
Hello,
I'm looking for an ADC to fit the following specifications.
- It has to measure a PT1000 RTD from a range of -50degC to 300 degC with a precision of 0.001 degC across the range.
- With the way the circuit is drawn below, the full temperature range comes out to be nearly 0.4 to 1.1V across the RTD using a 4.7kOhm resistor and 3.3V supply. The number of bits required to measure from -50.000 to 300.000 at that precision is log2(350000)=18.4. This puts us in the range of a 24 bit ADC.
- With a 24 bit ADC with the 3.3V excitation, the LSB is ~0.196 uV.
- The 350,000 bins over the range 0.4 to 1.1V gives a bin size of 2uV. So, the 24-bit ADC should be suitable.
- The above calculations ignore the non-linearity of the RTD, but there is some margin available.
- The RTD used to make this measurement will also be monitored by a some hardware comparators that are comparing against fixed voltages; so I cannot use internal excitation current sources to make the measurement.
- I'd like to make a ratio metric measurement of the RTDs using the stable 3.3V rail as the excitation current. Then measuring the signal across channel 0-1, I'd calculate the resistance with the knowledge that we have a precise reference resistor setting up the voltage divider.