Hi,
We have made an amplifier to convert the sensor voltage to a 4-20 mA output current. (with use of the XTR115).
The design works without temperature correction rather well < 0.5% error over 80 degrees. But we need better.
We use the Fine Offset to make the 4.0 mA. the Vex is used to power the bridge sensor.
So we are now using the temperature table in the Eeprom. With a climat chamber we make sweeps and measure
with a vary stable (outside the chamber) reference of 0.0 and 0.8xfullscale. So a measurement around 4.0 mA, and one
arround 18.0 mA. The difference is the span.
I expected to correct the gain with the gaindrift I measure on the span. And correct the offset with the drift on the 4.0mA measurment.
But when I calculate the GMx and ZMx this gives no good correction. I checked the actual fine gain and fine offset, these are both right,
so my Eeprom has correct values.
I ve not been able to find out the reason for not good output values. But one of the main reasons I'm beginning the think of is the drift
of the internal Reference. This one is used for powering the sensor, so the sensitivity. And it powers the Fine Offset.
Even if I change the GM1 an leave the ZM1 to 0x0000; My 4.0mA measurement changes due to the Fine gain. Much more than
I expect from calculating the tranfer function of the PGA309 as given in the user manual.
Does some one have experience with temperature correction on a PGA309.
Is the a callcualtion methode to calculate GMx and ZMx? a spreadsheet?
How should I callibrate my design the best way?
I'm gathering more chamber measurements and gone try to get a good overview.
Marien.
