From the review of PSL:2266 and the promises I made in PSL:2267, I think I have fixed the circuit. Corrected schematic is at DCC LIGO-D1800308-v1.
Main changes:
- I made GND pin of MAX6325 floating and hence controlled by the output of OPA827. OPA827 will drive the GND pin following voltage at RTD's non-grounded end. This will make sure 2.5 V is always applied across 20k resistor which will keep the current constant.
- The incoming Molex connector from RTD is now connected to AD8429 directly driving it differentially implementing the 4-wire topology.
- I added 100 Ohm series resistors to the outputs of OP27 for safety.
- Removed tantalum capacitors from the output of MAX63XX and added one to the input.
- Added 4-40 holes for mounting on a 1U chassis. The board is still small enough to be put in a 100mmx160mm Eurocard box if required.
Testing:
I also made a proof-of-principle circuit on a breadboard. It has only one channel (25  ) and uses different chips but same principle. Following were the replacements:
- MAX6325 (2.5V Ref) -> LT1021 (5V Ref)
- Current source resistance, 20k -> 39k
- OPA827 -> OP27
- MAX6350 (5V Ref) -> LT1021 (5V Ref)
- AD8429 (Gain set to 81) -> AD620 (Gain set to 79.68)
- Subtractor circuit resistors: R1, 5k -> 5.1k; R2, 25k -> 24k
- Everything else was the same.
The circuit produced meaningful and predictable node voltages. So hopefully, this circuit is correct. Only remaining thing is to characterize noise and drift properties which would be done once the actual parts arrive. |