HART-communication: smar HT2015 input-filter problems


Thread Starter

D. Eichmann

Hi everybody!

I've got a problem with my HART-circuit. I'm using the Smar HT2015 modem.

Almost everything works fine - only the HCF-Test 3.8 (Noise Sensitivity) fails. Without noise my device answers every request without any problem.

But during the Noise Sensitivity Test the device produces communication errors - it does not respond. The input filter seems to not work properly in my application.

My circuit is made of the information given in the datasheet and the application note. But even in these two documents there are differences concerning the input filter. The datasheet's input-capacitor's value is 470pF, while the application note says it should be 4.7nF.

I'm using 4n7 in my setup. Almost everything in my schematic is equal with exception of the reference voltage - I'm using 1,25V instead of 1,235V. Because of that I matched my resistor-divider for IAREF and ICDREF to have a voltage of 80mV difference between those two points.

The test produces errors in the out-of-band frequencies of 29Hz, 63Hz, 125Hz and 250Hz and even in the in-band-frequency of 1700Hz.

Is there anybody who had to get along with the same problem or does anyone know a solution for this problem?

I would be pleased, if you could help me.

With regards,
D. Eichmann