Improved PRTD circuit is product of EDN DI teamwork

Not too long ago I revealed 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 vital commentary of my design instantly adopted.

Reader Konstantin Kim urged that an AP4310A twin op-amp + voltage reference may be a superior substitute for the one amplifier and separate reference I used to be utilizing. It had the double benefits of decreasing each elements depend and price.

In the meantime VCF identified that >0.1^oC self-heating error is prone to consequence from the multi-milliamp excitation mandatory for 1 mV/^oC PRTD output from a passive bridge design. He urged lively output amplification due to the decrease excitation it could make doable. This might make for higher accuracy, significantly when measuring temperatures of nonetheless air.

Wow the engineering world together with your distinctive design: Design Concepts Submission Information

Determine 1 reveals the end 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 acquire permits decreased excitation that cuts self-heating error by 100x.

 A1’s built-in 2.5-V precision reference combines with the connected amplifier to kind a constant-current excitation suggestions loop (extra on this to comply with). 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 sweet, too.

The ensuing 100 µV/^oC PRTD sign is boosted by A2 to the unique multimeter-readout suitable 1 mV/^oC. R1 supplies a 0^oC bridge null adjustment, whereas R2 calibrates acquire at 100^oC. Nick’s DI features a nifty calibration writeup that ought to work as effectively 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 5^oC error) may appear to disqualify it for a precision software like this. However whenever you modify R1 to null the bridge, you’re concurrently nulling A2. So, it’s ok in spite of everything.

An surprising bonus good thing about the dual-amplifier topology was the simple implementation of a second-order Callendar-Van Dusen nonlinearity correction. Constructive 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 100^oC response to raised than +/-0.1^oC.

So, cheaper, less complicated, superior energy effectivity, and extra correct. Cool! Thanks for the options, guys!

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great 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

The publish Improved PRTD circuit is product of EDN DI teamwork appeared first on EDN.

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