vne147
Member
For minimum current output, all the current of the 50uA source in the IC needs to pass through the FET. Given that the rated Vgs(thr) for the specified FET is ~1V, that means the opamp needs to drive the gate with at least 1.5V in theory. If for some reason either the opamp or the FET were out of spec that could account for the effect you are seeing. Can you try supplying the FET from 6V instead of 3.3V?
I already tried to do something similar to this (I think). Let me explain:
The MCP4726 DAC is using a reference voltage of 0.5V from the external ADR130. However, the reference voltage is software selectable. So in my troubleshooting I experimented with using the DAC supply voltage of 3.3V as the reference voltage. This had no affect on the behavior I described above. When the output of the DAC reached around 400 mV, I bottomed out at 0.9A. Going from 400 mV all the way up to a DAC output of 3.3V did nothing to decrease that 0.9A any further.
I'll admit though that when I tried this, I did not measure the op amp output connected to the gate of the MOSFET. But, does what I describe here accomplish the intent of what you're recommending. Or, do you think upping the supply voltage of the op amp is still worth a try. I assume you meant the op amp, not the FET when you wrote:
Can you try supplying the FET from 6V instead of 3.3V?
Thanks for your help.
EDIT #1: I just got home and decided to test the voltage output of the op amp connected to the gate of the MOSFET. When the output of the DAC is ~400 mV, the current output is ~0.9A, and the op amp output is around 1.2V. However, if I increase the DAC output ever so slightly, sat to 415 mV, the op amp output jumps to 3.3V, and the output current stays at 0.9A.
Does this help anyone narrow down my issue?
EDIT #2: Does anyone think lowering the value of the 10 kΩ resistor, R20 would help? I'm kind of grasping at straws for a solution.
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