ISO35: ISO35

Hi Divyanshu,

The low pass filter described in the document you linked is a simple external glitch filter. The ISO35 uses an edge based architecture which can potentially have some issues with very narrow pulses (on the order of nanoseconds). The maximum signaling rate allowed by the ISO35 is 1 Mbps, so a low pass filter can be designed so that it does not interfere with normal data (does not affect signals up to 1 Mbps) but filters out any narrow pulses. 

To put this in concrete terms, I'll derive some values for a maximum signaling rate of 1 Mbps. If the maximum signaling rate you are using in your system is slower, you can further enhance this filter. 

So as to not affect the 1Mpbs signals, we'll set the frequency of our low pass filter at 4 MHz. 

For a simple low pass filter f = 1/(2*pi*R*C).

R*C = 1/(2*pi*f) 

R*C = 1/(2*pi*(4MHz)) 

Let's set the capacitor to 10pF and solve for R 

C = 10pF

R = 3.92 k Ohm

Here is the response. We can see that signals up to 1Mpbs will be unaffected. 

Furthermore, the rise / fall time of the step response is now on the order of 100ns, so it will filter out any pulses shorter than that. 

This all applies to the filter on DE and D (RF1, CF1, and RF2, CF2). The filter on R and D is identical so that the same things that were considered invalid on the input (and are filtered out) should also be disregarded on the output (and thus filtered out). 

As I mentioned before, you can further enhance this filter by pulling in the frequency if your maximum signaling rate is less than 1 Mbps. 

Let me know if you have any questions!

Best regards,

Dan

Hi Divyanshu,

ISO3080 and ISO3086 are full duplex isolated RS-485 devices, but they are 5V devices. As for non-isolated RS-485 I would recommend THVD1452. It has a 3.3V-5V supply range, data rate up to 50Mbps, full duplex with enable. Here is a table showing the THVD14xx and THVD15xx family. 

Best regards, 

Dan