I am working on designing a control board which contains HART communication. My problem is that in my board there should be some zener diodes.
Let's assume this diodes are 12 & 16 volts, so the difference is 4 volts. In case of loop current = 20 mA, we can have maximum 4/20=200 ohm of resistance in series, but I need minimum of 450 ohm resistance to make my HART signal sensible.
There is where I have trouble. I need to have ac resistance in order of 400 ohm (frequency is around 1200-2200 Hz) but with dc voltage drop less than 4 volts in 20 mA.
Can you help me?