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Have you got a product model for that?
Yes, of course. But true even with your beloved bridge.
The AC bridge doesn't "notch" without syncing to the interference.
I recall a time when you were mostly helpful, Chuck. What happened?
Have you actually tried any of these modern ADCs?
Try reading what I wrote.
It's a holdover from the bad old days when DC accuracy was hard to achieve. An Pt RTD is a relatively high-level device (about 10x that of a thermocouple), so AC is seldom used in modern designs requiring ordinary accuracy/resolution levels.
(I have done a rather special RTD design that uses an FPGA-based ultra-precision AC bridge to measure temperatures to resolutions maybe
4-5 orders of magnitude better than you require.. but for you.. DC works fine for your kind of application and it's simple and easy to do. Your sensor and how closely it tracks the variable to be measured is likely going to limit the accuracy).LM4040-2.5 + 0.1% resistor + precision op-amp (low Vos) with a few passives for filtering and gain control will work fine with a Pt 1000 sensor. Correct for the nonlinearity with a LUT or fit to a curve (just calculate it with a C program or whatever from the Calendar Van Dusen equations , calibrate in firmware/EEPROM or use a trimpot or two. Preferably, use a PIC with a 12-bit ADC and use the LM4040 for the ADC reference.
Best regards, Spehro Pefhany
The op doesnt want 0.04c, youve made his project much more difficult/ expensive for nothing in return. The standard bridge arrangement can be calibrated with a decent multimeter.
Its synced to its own excitation, rejects everything but its own signal
You read it again, it doesnt make sence. You do not need a current sence resistor to detect missing bridge/excitation. Not to mention your 10k resistor adding errors in.
You still havent mention the product name.
And as was pointed out earlier, you can just use an Analog Devices 77xx device and follow the datasheet. The Delta-Sigma A/D will give you a beautiful array of speed/resolution tradeoffs and it will just work.
Consider that, each connection from your system to the RTD forms a small battery (this is especially so for cryogenic measurements but applies equally to other temperature ranges). The Square-wave AC drive allows you to average out all the "battery junctions" from the real measurement.
Still needed to eliminate "battery junction" effects in the wiring. I know that near room temperature you won't see too much of a difference but when you are making temperature measurements in the cryogenic or very hot ranges you will see the differences unless you do drive with square waves and synchronise the read-back for when you are driving the bridge. One read while driving positive, one read while driving negative and then average out the difference (in magnitude) between the two readings to eliminate the errors.
Did I write "bridge"? No, I wrote "chain". It's obvious I'm referring to to the simple 10k +
5k + Pt100 R chain directly driving differential ref and sig inputs of a delta sigma ADC.Did you not notice "ratiometric conversion"? The ADC count is
2^N * sig / ref. If ref==0, then the output is undefined. The 10k adds *no* error. It's only ill effect is reduction of the current through the R chain which effectively increases the temperature-referred noise by less than a factor of 2.That's because you didn't ask me. You asked Mike Harrison. The product I'm talking about is the MPA160. It's errors are dominated by the variability of the thermal losses (room air currents). I could go through the whole error budget, but the smallest contribution comes from the ADC. BTW: Every unit is calibrated traceable to WHO melting point standard compounds.
Sometimes it's hard to know who's replying to what. I was looking for the guy who was claiming 0.005C. Obviously that was not you with this product.
"Battery junction"? That sounds like you're talking about electrolytic effects, but I assume you really mean Thompson/Seebeck (thermocouple) effects caused by dissimilar metals.
One would have to try very, very hard to make those significant in this application. At (say) 0.22mA and (say) 1385.1 ohms you have over
800 uV per degree C change in temperature.. that's huge.. about 15x the output of a type K thermocouple. The thermal EMFs cancel out when things are symmetrical even in the presence of a symmetrical gradient. Without a gradient they don't appear at all. With ordinary care, they are not significant at all, and modern commercial designs in this class (including mine) all use DC.What stuff exists in physics labs is another story.. there's lots of really bad and/or outdated analog electronic designs in physics labs.
Best regards, Spehro Pefhany
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