An example device comprises a digital-to-analog converter (DAC) comprising first and second transistors coupled to a first amplifier, the second transistor coupled to a first output of the DAC and to an output of the first amplifier, and third and fourth transistors coupled to the first amplifier and to a second output of the DAC, the third and fourth transistors switchably coupled to a voltage supply and to the first transistor. The device also comprises a first node coupled to the first output of the DAC and to a resistor. The device further includes a second node coupled to the second output of the DAC, and a second amplifier coupled to the second node and to the first transistor and switchably coupled to the third and fourth transistors. The device also comprises a comparator coupled to the first node.