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?

8 months ago

0 Clemens Ladisch 8 months ago

Guru 295240 points

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

0 qing fang 8 months 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 8 months ago in reply to qing fang

Guru 295240 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 8 months 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 8 months ago in reply to qing fang

Guru 295240 points

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

Logic

Logic forum

SN74AVC4T774: SN74AVC4T774RSVR