SN74AVC4T774: SN74AVC4T774RSVR

qing fang

Part Number: SN74AVC4T774

Tool/software:

Hello

We have such a issue. LDAC signals have a lot of clutter signals during each cold boot from -5℃ before FPGA loading complete . It will cause the DAC work abnormal.

LDAC is always high at room temperature. the issue can't be reproduce at room temperature.

Our design block is like this:

Here is our circuit:

The pull up resistor of LDAC_B is not populated at BOM.

If we populate the pull up resistor of LDAC_B, issue can't be reproduce at -5℃.

Here are abnormal waveform:

Can you help to explain why this issue happens at low temperatures and not at room or high temperatures?

1 month ago

0 Clemens Ladisch 1 month ago

Guru 267040 points

Where exactly did you measure this waveform, on the FPGA side, or on the DAC side?

0 qing fang 1 month ago in reply to Clemens Ladisch

Prodigy 30 points

I measured at DAC side. The buffer input cannot be connected to an oscilloscope, otherwise it will affect the buffer output. The oscilloscope access terminal has a resistance and capacitance to ground, which is equivalent to a weak pull-down. If we connect buffer input to oscilloscope, the issue can't be reproduce anymore.

0 Clemens Ladisch 1 month ago in reply to qing fang

Guru 267040 points

The problem is that the buffer's inputs are floating. This is forbidden; see footnote (3) in section 5.3.

You must add the pull-up resistors. (If you care about power usage, use a higher value, up to 100 kΩ.)

0 qing fang 1 month ago in reply to Clemens Ladisch

Prodigy 30 points

Thank you.I just want to know why this issue happened at low temperature, not at room or high temperatures

0 Clemens Ladisch 1 month ago in reply to qing fang

Guru 267040 points

Apparently, at low temperatures, the leakage currents of your board keep the inputs near the switching threshold.

Logic

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SN74AVC4T774: SN74AVC4T774RSVR