I'm trying to find a way so I will always have close to
0V at the measurement point (above the diode), when I have
0V as input. The
-3.3V is a fixed voltage source.
0.64V at input, I have
-5.7mV at the measurement point. That's OK to me.
3.3V at input, I have
2.2V at the measurement point. That's OK to me.
But when I have
0V at input, then I have
-271.2mV at the measurement point. That's to much negative!
If the diode have at least 10 Ohm in resistance, then the measurement point would be very close to zero. I what it to happen. But I cannot use a diode. Is there any better way to solve this issue without using a diode, or using a better diode?
Try this with a 3.6V zener diode. Change the current source from 4-20mA and look at the output. At 4mA, then we have close to zero volt. At 20mA, then we have about 2.2V. OK! But at 0mA, then we have -225mV. That's not OK! It should be low as zero volt as well.
Here is my solution. Is it good? At 4mA, we have some voltage for the ADC and therefore, we can see that there is a sensor connected.