Just lately I printed a easy platinum resistance temperature detector (PRTD) design concept that was largely impressed by a deviously intelligent earlier DI by Nick Cornford.
Outstanding and persistently constructive crucial commentary of my design instantly adopted.
Reader Konstantin Kim instructed that an AP4310A twin op-amp + voltage reference may be a superior substitute for the only amplifier and separate reference I used to be utilizing. It had the double benefits of decreasing each elements rely and price.
In the meantime VCF identified that >0.1oC self-heating error is more likely to outcome from the multi-milliamp excitation crucial for 1 mV/oC PRTD output from a passive bridge design. He instructed lively output amplification due to the decrease excitation it will make potential. This may make for higher accuracy, significantly when measuring temperatures of nonetheless air.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
Determine 1 exhibits the result of some critical consideration and quiet contemplation of these, as they turned out to be, terrific concepts.
Determine 1 Nonlinearity is cancelled by constructive suggestions to PRTD fixed excitation present suggestions loop by way of R8. A2’s 10x achieve permits decreased excitation that cuts self-heating error by 100x.
A1’s built-in 2.5-V precision reference combines with the hooked up amplifier to type a constant-current excitation suggestions loop (extra on this to observe). Observe-on amplification permits a tenfold excitation discount from ~2.5 mA to 250 µA with an related hundredfold discount in self-heating from ~1 mW to ~10 µW and a proportionate discount within the related measurement error.
The sixfold enchancment in anticipated battery life from the decreased present consumption is good, too.
The ensuing 100 µV/oC PRTD sign is boosted by A2 to the unique multimeter-readout suitable 1 mV/oC. R1 supplies a 0oC bridge null adjustment, whereas R2 calibrates achieve at 100oC. Nick’s DI features a nifty calibration writeup that ought to work as nicely right here as in his unique.
Admittedly the 4310’s general-purpose-grade specs like its 500-µV typical enter offset (equal if uncompensated to a 5oC error) may appear to disqualify it for a precision utility like this. However once you regulate R1 to null the bridge, you’re concurrently nulling A2. So, it’s ok in any case.
An sudden bonus advantage of the dual-amplifier topology was the straightforward implementation of a second-order Callendar-Van Dusen nonlinearity correction. Optimistic suggestions by way of R8 to the excitation loop will increase bias by 150 ppm/oC. That’s all that’s wanted to linearize the 0 oC to 100oC response to raised than +/-0.1oC.
So, cheaper, easier, superior energy effectivity, and extra correct. Cool! Thanks for the strategies, guys!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- DIY RTD for a DMM
- The facility of sensible constructive suggestions to excellent PRTDs
- Reduce measurement errors in RTD circuits
- Designing with temperature sensors, half three: RTDs
- RTDs present differential temperature measurement
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