Klaus Vestergaard Kragelund
On 29/11/2021 02.52, Anthony William Sloman wrote:
The LT1013 is good. It was preferred when I did space stuff. But it\'sOn Monday, November 29, 2021 at 9:46:07 AM UTC+11, Chris Jones wrote:
On 29/11/2021 08:35, Klaus Kragelund wrote:
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 inherited .
Problem is the large offset voltage of the opamp is amplified producing
We are contemplating production calibration, but I am worried that the
offset isn\'t stable after the calibration has been done
In literature 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
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.