I am trying to re-design the TI Precision Design - Low Level (5uA) V-to-I Converter. My design goals are:
- Increase bandwidth from around 1kHz to 100kHz (ideally 1MHz in the future)
- Increase output range from 0-5uA to 0-50uA
To increase the output range I simply decreased Rset by a factor of 10 down to 10kOhm. To increase the bandwidth I needed to select a new op amp and instrumentation amp; while selecting these amplifiers I prioritized GBWP > input offset voltage > offset drift. My reason for this is the TI app note says that the key design specifications for the amplifiers are offset voltage and drift, but with my goal of increasing bandwidth I moved GBWP to the top.
I settled on replacing the OPA333 with the OPA277 and the INA326 with the INA118. Note, I switched from singled ended to +/- 15V power, this was necessary to find amplifiers with the bandwidth that I wanted. My schematic of the entire redesigned setup is at the very end of this post.
I was having troubles simulating with various SPICE tools (I couldn't for the life of me get the TI models to work in OrCad PSPICE and TI's TINA kept freezing on me) so I did a quick turn PCB with the circuit after getting some vaguely similar simulations running. What I found in the lab is that my circuit oscillates at around 25kHz and is just flat out unusable. I am positive that it is my compensation network, as I didn't change any of the values from the TI design so it makes perfect sense that my bandwidth isn't my desired 100kHz. I am struggling to understand how the compensation network values are picked and how I can design something that is stable. Can anyone point me in the right direction or offer some advice?
I should note I'll be attempting to drive this current in to a capacitive load (approximately 10uF after several feet of cabling). For the first level bench top testing I was attempting to drive the current in to a 50kOhm load (no large capacitance yet).