SN74LVC3G17: Signal ringing after long trace

Will Wang47

Part Number: SN74LVC3G17

Tool/software:

Hi team,

Customer use SN74LVC3G17 to buffer I2S signal; trace length is ~20cm; after the long trace, find the output is ring; Could you help check how to remove the ring?

Connect a termination resistor? How to determine the resistor value?

Thanks and Best regards,

Will

9 months ago

0 Clemens Ladisch 9 months ago

Guru 316440 points

It's possible that the ringing is in the loop formed by the oscilloscope probe's ground clip; use a ground spring instead.

The simplest termination is source termination; add a series resistor of about 33 Ω at the buffer's output pin.

0 Danny Han 9 months ago in reply to Clemens Ladisch

Prodigy 30 points

It's been confirmed that this has little to do with the test environment. In my design, there is no series - connected resistor at the LVC output. After a long path of about 20 cm, 0 - Ohm resistors are connected in series to five loads respectively. After discovering this problem, I adjusted the value of this 0 - Ohm resistor, but it had no effect. Is it necessary to connect a resistor in series at the LVC output to have an effect?

0 Clemens Ladisch 9 months ago in reply to Danny Han

Guru 316440 points

At the receiver, a series resistor has no effect (it is insignificant next to the GΩ impedance of the CMOS input).

See p. 47 (1–27) of the LVC Designer's Guide for other termination options, but you should really use source termination.

0 Danny Han 9 months ago in reply to Clemens Ladisch

Prodigy 30 points

noted, for a short term solution, I need to connect a resistor in parallel to the ground at the receiving end, which is Technique 1.

0 Clemens Ladisch 9 months ago in reply to Danny Han

Guru 316440 points

You will need a resistor of about 50 Ω, which can easily exceed the absolute maximum rating for the output current, and in any case will drop the output voltage.

0 Malcolm Lyn 9 months ago in reply to Danny Han

TI__Mastermind 25650 points

Hi Danny,

We really only recommend the resistor in series with the output nowadays: all of the other termination options are mostly inferior to a serial damping resistor.

The LVC has a very high drive strength which is part of why you are seeing that overshoot. Any 22-50 ohm resistor in series with the output will significantly reduce the overshoot without causing issue with current consumption or significant voltage drop.

Best,

Malcolm

0 Danny Han 9 months ago in reply to Malcolm Lyn

Prodigy 30 points

noted with thanks all!

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SN74LVC3G17: Signal ringing after long trace