Can I get a more detailed schematic/ design description focusing on the TRIM pin and it's interaction with the output?
I'm looking at driving the TRIM pin with an Op-Amp and would like to get a better understanding of acceptable voltage ranges, Vout/Vtrim transfer functions, and overall behavior in this configuration.
I am happy to help you understand ADR445 user trim capability. After reviewing ADR445 design schematic the expected trim output voltage range is 5.05 V to 4.74 V. Within trim configuration the R1 was set to 100K Ohm. Please note the expected trim output voltage range is based on nominal resistance value, in other words it does not include worst case variation of trim range caused by resistance tolerance and temperature.
As for transfer function and understanding ADR445 Vref trim configuration, I have designed ADR445 user trim LTSpice model. Please click on link below to download .asc file. With this model you can connect your Op-Amp configuration to trim pin and execute spice model and observe ref Vout. Please see picture below.
What trim range are you trying to achieve ?
Are you driving Op-Amp with DAC ?
ADR445 User Trim Model LTSpice
ADR445 user trim LT Spice model: