chinese chips to measure PT100

[ali_asadzadeh]

Hi,
I need to measure a RTD in the range of -10C to 120C with 0.1C accuracy, So the question is that do you know or suggest any Chinese chips? why Chinese Because I need them to be cheap, I want to know if we can do it with 1$ price on the main AFE.

[jbb]

Maybe you could do something with an HX711? They show up all over the place and might be cheap…
Or check out other offerings from whoever makes it?

[thm_w]

If you look on Alix they are using MAX31865 which is not cheap. So I don't think such an IC is easily available or they'd use it.
This board uses INA826: https://www.aliexpress.com/item/32829690782.html


LCSC doesn't seem to have many offbrand in-amps. Maybe roll your own circuit?

The HX711 is a good idea as well.

[moffy]

If you get 3 reasonably precise resistors better than 0.05% you can do a wheastone bridge measurement, with the PT100 in the bridge, with a differential input ADC and if you use the bridge excitation as your ADC reference you can get reasonable measurements. Beware of lead length to the PT100 as it will add to the sensors value.

[ali_asadzadeh]

Thanks for the feedback
moffy do you have any app notes or circuit and part recommendation for wheastone bridge measurement?

I found this microchip AN1154 and CS1237-SO ADC for  0.65$, so it this good to go!?

[moffy]

Looks good from what I can see as long as the REFIN and bridge excitation are the same. The wheastone bridge is pretty straight forward, simple to calculate. Just be careful not to put too much current through the PT100 to avoid self heating introducing an error. With the  excitation to the bridge being the same as the REFIN, it effectively cancels itself out, so that rather than doing an absolute measurement, you are doing a relative measurement. If REFIN goes up then the input goes up by the same proportion, if it goes down so the input drops proportionally.

[mikeselectricstuff]

Measuring PT100 is easy with a differential-input ADC - connect PTC in series with a stable reference resistor to a current source, and measure voltage across resistor and across PT100.
For less precise applications, you can measure single-ended with a simple 2-channel ADC setup. Linearisation can be done with a lookup table & interpolation - the PT100 characteristic isn't very nonlinear so you don't  need a particularly big table.

[Kleinstein]

With a high resolution ADC with external ref input, one can get away with only one resistor, so no ful bridge needed. Chances are the ADC is more stable than a pair of resistors. The differential input on the ADC even allows to use 4 wire sensing on the PT100.

[ali_asadzadeh]

Kleinstein thanks for the tips, Do you think the CS1237-SO is a good candidate as ADC?

[jbb]

The CS1237-SO looks interesting.

A couple of things I spotted in the data sheet…
- REFIN specified from 1.5V to VDD
- input range (differential) specified as +-Vref/Gain
- REFIN current / resistance isn’t specified (sadly)
- PGA gain of 1,2: inputs are rail-rail
- PGA gain of 64, 128: need 0.75V headroom

You could try using the Microchip AN method, with a regulated 3.3V supply, gain of 2 and a divider resistor Ra of around 500 Ohms, I guess. That would give about 5mA drive, and <4mW dissipation in the Pt100. Not sure if that’s enough to cause trouble with self heating.

I suggest you consider using input filtering and a 4-wire measurement method - refer TI SBAA336 circuit note - to remove lead resistance errors.  (The TI circuit also connects REF to the resistor, so you measure a direct ratio. But this relies on the REF input pin taking next to no current and I don’t know if that’s true for CS1237.)

If one of the force wires breaks, the voltage across the Pt100 will fall to 0V and the ADC can see that.  However, if a sense wire breaks, the ADC will measure some combination of input impedance and leakage currents and that might just look like a real reading.  This value is likely to change with temperature and chip to chip variation.  If you could connect a 1 Meg (?) resistor between AINP and AINN then a broken sense wire would make the ADC measure near 0V again.

[nfmax]

It is important that the value of the reference resistor is stable over temperature (and time), so you will likely need a very low tempco part. This may well cost more than your budget allows. For example, assuming a moderate 50ppm/˚C tempco, and only 25˚C range of operating temperature variation of the conditioning circuit, gives 0.125% change in reference resistance over the operating range. Since the resistance of a Pt100 changes approximately 0.385%/˚C in your measurement temperature range, this amounts to more than 0.3˚C measurement error from this source alone.

Assuming you do really mean 0.1˚C accuracy rather than resolution, I don't think your budget is realistic

[ali_asadzadeh]

nfmax thanks for the tips, I think I'm well with in the budget
Please forget about Digikey and Mouser, take a look at LCSC

6.8K  0805 1% 25ppm/˚C  resistor for  0.0114$
https://lcsc.com/product-detail/Chip-Resistor-Surface-Mount_Viking-Tech-ARG05FTC6801_C218677.html

6.8K  0805 0.1% 10ppm/˚C  resistor for   0.1399$
https://lcsc.com/product-detail/Chip-Resistor-Surface-Mount_Resistor-Today-PTFR0805B6K80N9_C351550.html

I would go with the 1% resistor personally

[ali_asadzadeh]

6.8K  1206 0.1% 25ppm/˚C  resistor for  $0.0235
https://lcsc.com/product-detail/Chip-Resistor-Surface-Mount_Viking-Tech-ARG06FTC6801_C2828908.html

Very close to 1%, so I would go with this one