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| Precision RTD measurement - correct for tempco of ref resistor? |
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| max_torque:
I'm doing a device to read some precision RTD sensors. I've chosen a drive architecture for the 4 wire sensors that uses a simple current source to drive through the sensor, and then to drive through a reference resistor. The voltage across the sensor is fed into the differential inputs of a 20 bit ADC, and the voltage generated by the reference resistor is used as the ADCs reference voltage. In this way, the measurements digitised output is the ratiometric comparison of the resistance of the sensor to that of the "fixed" reference resistor. This therefore means the precision and drift of the current source are relatively unimportant. So far, so good (hopefully!) However, the tempco of that reference resistor is going to dominate things i think, unless i use some ridiculously trick resistor, which i want to avoid. As i have a micro on the device that is serialising the output of the ADC, then i can also use that micro to measure the actual temperature of the reference resistor, and apply a correction factor to the output of the ADC i think? Does this sound sensible? The actual temperature of the unit is likely to vary by a maximum of 55 degC from say 5 degC to 60 degC (it's an industrial device, but for indoor / lab use), so for a boggo resistor with a 100 ppm characteristic with a nominal 400 ohms resistance at 25 degC that means a change in resistance of 2.2 ohms, or a change in ADC counts of approx 1400 counts over that range. Those counts can be easily characterised and added to the ADC output to come up with a corrected output, which is then converted to the appropriate temperature value depending on the actual characteristic of the RTD being used. |
| DaJMasta:
Do you need the reference resistor? If your ADC has a reasonably low drift reference, the RTD's base construction and performance should be pretty good, and the 4 wire measurement basically avoids issues with cable length or quality. A standard RTD is going to be like 0.5% tolerance or better, a well established tempco that is very linear in the temperature range you're describing (unless it's not the temps you mentioned), and very good stability... so I'm not sure why you'd need a reference, but if you do, it's going to have to be a pretty fancy one. |
| max_torque:
If i use a fixed voltage reference for the ADC, then i need to ensure i excite the RTD with a precise current! By using the reference resistor to also create the reference voltage, i don't need a high quality current source, which is going to be both more complex and more expensive that a single decent resistor. I quick look shows 0.01% 5 ppm resistors for around the £1 mark (1.3$ US) and i doubt i can make a 5ppm current source for that! |
| Dr. Frank:
Hi, I have built a lot of precision current sources for temperature measurements with PT100 sensors @1mA, or 10uA for special temperature diodes, all going down to cryogenic temperatures, 20K for PT100. I also first used 5ppm/K reference resistors, and trimmed the current to precise nominal currents.. usually 0.01% total accuracy was good enough.. see Lake Shore sources, or do error calculation..PT100 have much higher T.C., so 5ppm/K is fine for 0.01°C I would estimate. The real problem you'll have are the thermo couple voltages and lead resistances which especially for PT100 will ruin your precision..at 100Ohm and 0.35Ohm/K. Therefore, you need a good 4 wire connection for the PT100 and the reference resistor, and a switched current source to eliminate these e.mf. That's the reason, why a switched current source is better. Frank |
| splin:
Correcting for the temperature coefficient of the reference resistor is perfectly reasonable but the TCRs of resistors vary non-linearly with temperature. Metal film resistors in particular can exhibit +ve or -ve TCR at room temperature. Ni-Cr resistors typically have parabolic characteristics where the TCR changes from +ve to -ve anywhere between, say, 0 and 100C. You can see some examples on page 181 here: https://core.ac.uk/download/pdf/4147785.pdf There is also quite good information here, but bear in mind it is pushing Vishay's expensive bulk metal foil products: http://www.vishaypg.com/doc?63519 Resistors of the same type may have differing TCRs both within a batch and especially between batches. Thus you would need to either: a) Calibrate every single resistor over the 5 to 55C range or b) Thoroughly characterise the TCR curve with temperature of sufficient samples of each batch of resistors. With 5ppm precision resistors however, the chances are that the resistance variation over your limited temperature range is rather better than the specified 5ppm such that temperature compensation is not worthwhile. Also bear in mind that the stability of the resistor over time may be a more significant factor than TCR, depending on how long a calibration period you need for your product. Metal film resistors, even the precision varieties costing $1 or more, rarely have any useful time drift specifications beyond a 1000 hour load life spec which typically may be 1000ppm or more. Some precision resistors have annual shelf life stability @ 25C specifications such as the NOMCA resistor network @100ppm/year, but they are few and far between. In reality you would probably see rather less than 200ppm change in the first year reducing to less than 100ppm subsequently, especially if the resistor spends most of its time at room temperature or less. Problem is that you can't guarantee the time drift without extensive characterisation. If this is a problem, you might be better using a precision wirewound or metal foil resistor but they are costly. Also, all non-hermetically sealed resistors are sensitive to humidity and might exhibit 25ppm shift with seasonal humidity changes if you're lucky or more likely 50ppm or more. Again the data sheets wont help you. Of course, if this is a one-off project then most of the above is irrelevant... :blah: :blah: |
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