SN74AVC2T45: SN74AVC2T45 output oscillation

Alan Chang

Part Number: SN74AVC2T45
Other Parts Discussed in Thread: SN74LVC2G17, SN74AXC2T45, SN74AUP2G17

Hi,

Customer designs SN74AVC2T45 as 2nd source, schematic shown below. However, we find pin 2 output has oscillation at 1.8Vcc. Another side is 3.3Vcc.

Even, we change input signal slew rate at 3.3V side, 1.8V side still has oscillation. Can you please check what's wrong?

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 317370 points

The datasheet specifies the Δt/Δv limit as 5 ns/V. That signal is slower.

The SN74AXC2T45 could tolerate 10 ns/V (or 100 ns/V if only one input is switching). But for slower signals, you need a device with Schmitt-trigger inputs, e.g., SN74LVC2G17 or SN74AUP2G17.

0 Alan Chang over 3 years ago

TI__Intellectual 2320 points

Hi Ladisch,

Since the project has been MP over year, so the layout cannot be changed. NXP_74AVC2T45DP has no this kind of oscillation. Do we have workaround solution? or can you please explain why oscillation in case of slow slew rate?

0 Clemens Ladisch over 3 years ago in reply to Alan Chang

Guru 317370 points

See [FAQ] How does a slow or floating input affect a CMOS device?

NXP's 74AVC2T45 has the same 5 ns/V limit. There is no guarantee that it would not have the same problem.

If the board cannot be changed, then the easiest solution might be to add positive feedback to get hysteresis, i.e., to bodge a resistor of, e.g., 10 kΩ between the input and the output. (This requires that the 1.8 V supply can sink about 150 µA.)

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SN74AVC2T45: SN74AVC2T45 output oscillation