DS25BR440: Unexpected output waveform when power down is set to LOW and input signals are pull down

Ee Wen Quek

Part Number: DS25BR440

Hi,

Link: https://e2e.ti.com/support/interface/f/138/t/809895?tisearch=e2e-sitesearch&keymatch=DS25BR440%2520powerdown%2520mode

I saw in previous post in above link mentioned about no signal at output during power down mode. However, in my board when the differential input signals are 0V and power down mode is activate, I get a waveform at the output. Is this something expected? I thought during power down, nothing should be obtained on the output. I need make sure when the input no signal, the output should not toggle or provide any output. Can someone provide me some information about this? Thanks.

over 6 years ago

0 Lee Sledjeski over 6 years ago

TI__Mastermind 43795 points

Hi,

What is the output loading when this waveform is observed?

What is the probe model used to take the measurement?

Thanks and Regards,

Lee

0 Ee Wen Quek over 6 years ago in reply to Lee Sledjeski

Intellectual 300 points

Hi,

The output is not loaded when this waveform is probed. The probe model used for measurement is a Tetronix TPP1000 1GHz passive probe.

Thanks

0 Lee Sledjeski over 6 years ago in reply to Ee Wen Quek

TI__Mastermind 43795 points

Hi,

Is there a 100 ohm termination connected to the output?

Regards.

Lee

0 Ee Wen Quek over 6 years ago in reply to Lee Sledjeski

Intellectual 300 points

Hi,

there is no 100 ohm termination connected at output. The previous waveform is probe on one side of signal. So I did tried to probe the signals differentially using differential probe. Here is the waveform I get. Is this something expected where for power off mode, OUT+ and OUT- will have signal respectively if probe on one side. But when probe differentially, it actually cancelled each other and get 0V? Power off mode didnt not prevent signal out to the OUT pin? Thanks.

0 Lee Sledjeski over 6 years ago in reply to Ee Wen Quek

TI__Mastermind 43795 points

Hi,

The devices outputs are expecting to see a 100 ohm termination. Without the termination the common mode voltage will vary as you have measured.

Regards,

Lee

0 Ee Wen Quek over 6 years ago in reply to Lee Sledjeski

Intellectual 300 points

Hi,

I thought the IC itself internally already terminate the output with 100ohm. Why I still need terminate the output with 100ohm? I have try probe the signal using active probe, the signal seems ok right now, do not have the periodic waveform produced. It is because passive probe is 1MHohm impedance which cause impedance mismatched in such case? Thanks

0 Lee Sledjeski over 6 years ago in reply to Ee Wen Quek

TI__Mastermind 43795 points

Hi,

The output does have 100 ohm internal impedance, but it also requires and external load to develop the normal output common mode voltage of ~ 1.2V

I am not sure why using an active probe would stop this output common mode movement. My active probe does not stop the common mode signal.

Regards,

Lee

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DS25BR440: Unexpected output waveform when power down is set to LOW and input signals are pull down