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# ISO35: ISO35

Part Number: ISO35

Hello Sir,

I am using ISO 35  the full duplex transceiver for my application. I got this linked from website and i want to know about the values for the low pass filter to get the best signal frequency for it. http://www.ti.com/lit/an/slla299/slla299.pdf.

thank you

• 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

• In reply to Dan Kisling:

hello Sir,

Sorry for the late reply. I got all the concept clearly. but one doubt is there for the values of RF2,CF2. will it be the same as for RF1,CF1...?? for 1mpbs speed.

and can u tell if there any relation with the package . I am using 0603 capacitor and 0402 resistor for all its requirements

THANK YOU

• In reply to divyanshu bawa:

Hi Divyanshu,

I'm glad you were able to understand the concept. RF2 and CF2 can use the same values. If you know that the signals on DE are going to be significantly slower than those on D, you can alter the filter further. Otherwise I would just use the same values.

The resistor will not be burning a significant amount of power, so 0402 resistors should be fine. 0603 capacitors are fine as well.

Best regards,
Dan
• HI SIR,
Hope you are doing good.

Sir i want to ask u something regarding this full duplex. Can there is alternete option for this IC . its not cost effective for our product.is there any way we can reduce the cost even if its not isolated or better in isolated.

thank you
• In reply to divyanshu bawa:

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.

 Part Number Data Rate (Mbps) Duplex Enable Pkg THVD1510 0.5 Half Yes SOIC-8 (D); VSSOP-8 (DGK) THVD1550 50 Half Yes SOIC-8 (D); VSSOP-8 (DGK) THVD1551 Full No VSSOP-8 (DGK) THVD1552 Full Yes SOIC-14 (D), VSSOP-10 (DGS)

 Part Number Data Rate (Mbps) Duplex Enable Pkg THVD1410 0.5 Half Yes SOIC-8 (D); VSSOP-8 (DGK) THVD1450 50 Half Yes SOIC-8 (D); VSSOP-8 (DGK); VSON-8(DRB) THVD1451 Full No VSON-8(DRB) THVD1452 Full Yes SOIC-14 (D); VSSOP-10 (DGS)

Best regards,

Dan

• In reply to Dan Kisling:

hello sir,

THVD1452 is fine for me. But how much does it cost . Its not showing on the ti store for its price. and will it be active for longer duration.
• In reply to divyanshu bawa:

Hi Divyanshu,

My apologizes. I did not realize that pricing is not online for THVD1452. I am not able to discuss pricing on the public forum. May I send you an email at the address you listed when you signed up for E2E to discuss pricing with you offline?

Best regards,
Dan
• In reply to Dan Kisling:

hi sir,
ya sure.
E-mail ID :- divyanshubawa1@gmail.com

Thank you