I'm using a DAC to control the voltage at the source pin of BS170 N-Channel MOSFET. The purpose is to control the current (and therefore brightness) of a LED while the MOSFET has the gate and drain connected to act as a resistor.
The DAC operates as expected while the voltage is high (highest DAC voltage for the chip is 3.29V). However, the issue I'm facing is that the DAC is not driving the voltage at the transistor source below approximately 1.4V. Additional information: the board being used is the Nucleo-L432KC; the DAC is buffered so its driven by an output op-amp; max sink current for the DAC is 20mA (chip datasheet).
I'm using this circuit as an experiment, and I understand that the LED current can be controlled by connecting the DAC to the transistor gate, or even using PWM to control the LED brightness. I'm curious to know what might be causing the DAC to be unable to drive this source voltage below 1.4V.