SN74LVC16T245: Interference in ground isolate

Hello Samuel, 

I am familiar with the SN74LVC16T245DGGR, so I may be able to help, however, I am not sure I fully understand the setup. 

Are you going from MCU --> Translator --> RS485 Transceiver --> DRV135?

If so, it seems like the signals are all in the right range. 

Note that the SN74LVC16T245 does not isolate the gnds between input and output, so there could be noise coupling. 

The only thing that comes to mind from the SN74LVC16T245 that might be causing some issues is the fact that it has very strong output drivers, so if it is driving the SN65HVD part directly with very light loads, then there may be a bit of ringing/reflections on that line. Do you have a scope shot of the output of the LVC16T245 device?

If you notice ringing on the line then you can add output series damping resistors (resistors in series with the output) to help reduce the overshoots and undershoots at the output.

Hope that helps.

Best,
Michael

Hi Michael,

Thank you for your reply., In order to clarify my design, see below block diagram:

MCU receives control command from several remote stations via RS485 channels to generate tone for every command type, 3.3v and 5V are power supplies and those share the same ground reference and +/-15V and 5V are isolated power supplies with another ground reference.

In idle conditions and no commands sent, the output at audio-balanced driver pins have a low noise of 20mV. We desoldered IC´s one by one  and found that SN74LVC16T245DGGR generate noise at output balanced audio driver. When removed SN74LVC16T245DGGR, noise was extinguished.

I don´t know What´s wrong.

Please, any advice.

Best Regards,

SAM

Hello Sam,

Ok, so the presence of the SN74LVC16T245 is causing noise in the system, but its not because its driving the audio signal, it is doing something else in the system.

As mentioned previously, the most likely cause is that the LVC16T deviceis driving the SN65HVD part directly with very light loads. There may be a bit of ringing/reflections on that line that are causing system noise. Do you have a scope shot of the output of the LVC16T245 device to check this?

If you notice ringing on the line then you can add output series damping resistors (resistors in series with the output) to help reduce the overshoots and undershoots at the output.

Best,
Michael

Hi Michael,

In datasheet of SN74LVC16T245, on page 7, output current is -32mA and 32 mA, SN74LVC16T245 and SN65HVD09IDGGREP are interconnected directly, Is there problem? 

Best Regards,

SAM

Sam,

The problem does not have to do with whether or not the SN74LVC16T245 is capable of driving the inputs of the SN65HVD device. It has to do with the fact that the output drivers are very strong for driving the high-impedance inputs of the SN65HVD device. 

Essentially, its a low impedance driver (say 10Ohms give or take) turning on and driving into a high Z load. There is a significant impedance mismatch between the load and the driver which has a tendency of causing reflections or overshoot. 

There is an FAQ which goes into this in detail here: https://e2e.ti.com/support/logic-group/logic/f/logic-forum/763609/faq-what-happens-when-i-connect-a-logic-device-s-output-to-a-50-ohm-transmission-line 

Check the section about "What can I do to keep transmission lines from affecting my logic signals?"

I can't say 100% for sure that this is what is causing the problem, as I don't have enough information, but it seems to be the most likely culprit at this point in time.

If you could share a scope shot of the output of the LVC16T switching, then it would help to confirm. And if you are seeing a lot of line reflection and overshoot on the line, the best way to reduce this is to put a resistor in series with the output of the LVC16T.

Best,
Michael

Hi Michael,

You are right, the problem is due to ringing/reflections, attach scope capture:

I never considered to match impedances, but I want to know what is value or range and type of resistors that I should connect. Is there any compatible voltage translator with SN65HVD device ?

Thanks a lot.

Best Regards,

SAM

Hi Michael,

Proposed solution doesn´t solve it , I start with 120 ohm and decrease the value by resistors in parallel at 60 ohm and them at 40 ohm but  only reduce peaks and oscillation is still present  as shown below

 SN74LVC16T245 Output before place resistor  in serie between SN74LVC and SN65HVD without generated signal by MCU

Comparative signals  before (yellow) and after (blue) leads resistor at 60ohm

Comparative signals  before (yellow) and after (blue) leads resistor at 40ohm

When output sn74 and isolated ground were measured with its own reference, respectively, I noted that both waves have a similar sequence as shown below

I will implement a new solution with Shottky diodes to translate voltage level.

Best Regards,

SAM

Hi SAM,

You might consider using a passive voltage translator like the LSF0108. The device doesn't include any drivers like the LVC family translators, so you won't have issues with overshoot.

Hi Emrys,

I implemented a solution with a B340A-E3/61T diode in serie and 44.2kohm pull-up resistor were on hand and these were connected between MCU and SN65HVD09DGGR in order to remove SN74LVC16T245. For more detail connection, please see image below where it was extracted from SN65HVD82 datasheet.

I´m testing with 3 communication channel USART at 38400,None,8,1, I have excelent result, no more noise or overshoot Only have a problem with MCU about Framming Error when interchanging input signal between ports, That could be because of SN65HVD09DGGR doesn't has fail-safe input.

I want to know if can use SN74CBTD3384 according to the report SCDA003B by Nalin Yogasundram

Best Regards,

SAM