Hello Kai,

Appreciate the input. That is quite a circuit!

The compliant voltage is from 12 - 28 VDC. So actually the circuit would need to protect from as much as 28 VDC input. I do like that circuit a lot since it doesn't make major changes to the original and can still be done surface mount.

The DN2540 datasheet from Supertex said the part is not recommended for new design but they recommened the DN3535 or DN3545. The newer part has lower RDS on so I am not sure how this effects matters.

I have a few questions about the circuit if you would be so kind to enhance my knowledge.

1)Consering the 100R trimmer how is that adjusted? How does someone know when it is properly adjusted? Perhaps I am missing something but it is not clear to me how someone would adjust this and know it is set correctly. How is the current limiting calculated is that dependant upon the Rds on value of the FET?

2)Is there a quantative value you can put on how much this circuit improves the common mode compared to the original circuit? Was the old one limited to 2.26V common mode because of the fixed inputs of Q2 and U8 and this one is higher? You have an LED when 24V is applied. Would it be possible if an LED would illuminate when the common mode voltage was getting above an area when the circuit would not function properly or errors would be expected?

3) CE compliance is a must for this product and I noticed that you did not include the ferrite beads in your schematic. Are they not necessary with the transorbs or should they be put on in the front end?

4)This is kind of a side question. I am wondering how is the voltage calculated to the INA128 when 0 - 5 and 0 - 10 Volts is applied? When it is 4 - 20 mA the 200 ohm shunted resistors would give an input of 0.4 V to 2.0 V. How is that calculated when using a voltage input?

5) How can common mode rejection ratio be tested on the bench? How is the circuit changed into a two pole active filter and why would you not want to do this?