I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC
PT1000 is pull up with a resistor
I would NOT do it like that, this is a design I have inhireted
Problem is the large offset voltage of the opamp is amplified producing large errors
We are contemplating production calibration, but I am worried that the offset isn't stable after the calibration has been done
In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?
By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then
I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects
I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...
Are you seeing resistor tempcos maybe? An RTD is about 4000 ppm/k, so you'd need some very good resistors to keep their error contribution down. If the pullup is 1K, that kills half of the gain too.
Would your RTD make enough voltage to drive the ADC directly? Too much excitation current could self-heat the RTD.
What about the RTD+resistor voltage reference? Is it the same as the ADC ref?
Can you auto-zero?
I would expect opamp offset to be pretty stable over time, so you could cal it out.
If the offset has a temperature dependence then you would not be able to take it out without doing calibrations at multiple temperatures. If the opamp and the RTD are not always at the same temperature then it may be impossible to fix by calibration.
Can you swap the opamp for a type that has less offset? There are ones that would have the same pinout.
The LT1013 is quite nice, but not all that cheap. You can run a Pt1000 bridge with more volts across the platinum sensor than you can with a nominally one hundred ohm sensor - those are normally rated for 1mA through the sensor, which is only 100mV and 0.3% of that per degree Celcius is only 30uV per degree.
At that level you start seeing thermocouple voltages in the copper to invar to alumium-on-silicon junctions you've got around the op amp, and you rapidily start thinking that the complexities of AC bridge excitation might be worth the effort.
It is the input bias current that is most temperature-sensitive. It can vary by quite a lot (even change sign at high temperature), so if the Thevenin input resistance of the two inputs mismatches by more than a few kOhms, you get thermal trends as a result. Figure 11 nA nominal, but plan for an order of magnitude more than that as range; a 20 mV change could result from 200k ohms disparity in the input feeds.
We jettisoned the amplifier and used a minimal component count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between the input and ground.
This will lose 90% of the range of the converter, but we still have more than 16 bits of resolution left. With suitable microprocessor linearization, it is more than needed for the sensor tolerances in a range of say, -50C to +200C.
LMV358 is not a precision op-amp- its a general purpose low-voltage part with Vos as much as +/-9mV. TCVos is not guaranteed and is 'typically' 5uV/°C. You can translate that into degrees error from whatever circuit you are using.
Not sure about your comment about the resistor, you need at least one precision resistor somewhere or you won't get a voltage as a function of the sensor resistance. Maybe you've got a bridge circuit given your "differential coupling" comment- in which case all the resistors affect the accuracy in general and the zero in particular.
If there is 2V across the sensor then the output at the sensor is about 7.7mV/°C ,The Vos of the LMV358 is a bit more than 1°C error worst-case, if there is 2V across the sensor. If you have a bridge configuration a 1% error in one of the resistors represents several degrees C error (20mV or so). So unless you're using precision (like
0.1% resistors) even the LMV358 is not your main issue.
With the series resistor, the excitation should be taken from the ADC reference voltage, obviously, so the ADC reading is ratiometric. Using the supply voltage as a reference for both can cause noise issues.
There are advantages and disadvantages to haveing an op-amp in there, I would probably stay with it in most situations because it allows a nice low pass filter. Sounds like your ADC has no PGA or buffer amplifier.
You can get op-amps that are pin-compatible and have offset voltages in the microvolts and TCVos in low tens of nV/°C. An old-fashioned non-zerodrift precision type may be more resistant to EMI, though. That said, you can probably count on the change in offset being within <100uV at the same temperature far off into the future, even with the crappy LMV358. If the voltage across the sensor is 2V, that's one or two hundredths of a °C. If the Pt1000 sees much in the way of temperature swing or mechanical stress that's in the wash.
We do that too. One current project has three thermistor Wheatstone bridges (on the bottom of an optical gadget) feeding a multiplexed differential-input delta-sigma ADC, using the ADC reference voltage to power the bridges. It turned out to be fairly linear from 25 to 45c, with microkelvin resolution and noise. It uses one entire PGA range.
Except the R1 and R3 is replaced by a fixed voltage reference. Gain is about 6 of the diff amp. Output feeds into an ADC with RC filter to deal with charge injection/sample capacitor
The pullup is 3kohm, and the amplifier is used to get full use of the ADC range (0 to 3.3V)
All resistor are 0.1%/5ppm, so it does account for some error, but does not explain it all
My point about this circuit is to shift to a direct input into the ADC, then only the pullup has temco. I would then use a pullup with the same resistance as the PT1000 (1kohm), to maximize the dynamic range. Use a FET to turn on the pullup to prevent self-heating of the PT1000
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