Details
https://www.eevblog.com/forum/beginners/application-test-question/
Test question
"Design and implement a pt1000, low noise, adc front-end with an output voltage of +2.5v at zero c and a resolution of 5mv per degree C"
This circuit is an amendment to the original question. I was shipped pt100 devices, and this circuit has been altered to work with these.
Using the lm324 quad opamp and +7.5Vdc, this setup produces from 25mv to 4.57v across 550 degrees, or 8.26mv per degree c. with 1V 50Hz input ripple Vcc error is less than 1/5 of a degree error applied to the resistor bridge (in simulation)
Beeadboard testing produces matching results only when resistor values around the inamp input and output stages are matched to within < 1%
TOP LEFT
opamp 1/4 is configured as a poor voltage regulator with 2nd order filtering @ 1Hz.
MIDDLE LEFT
Wheatstone bridge, the switches range from -50c~550c sinked by a constant current source.
MIDDLE TOP
1st order filtering @ 10Hz feeding opamp 2/4 and 3/4 used to buffer and amplify input from the resistor bridge. feeding into opamp 4/4 configured as an instrumental amplifier with a DC offset.
