DS90LT012AQ-Q1: Output impedance , output matching

Rainer Baettig33

Part Number: DS90LT012AQ-Q1
Other Parts Discussed in Thread: SN74LVC1G04-Q1,

To whom it may concern.

I connect the output of the DS90LT012AQ-Q1 to an inverter gate SN74LVC1G04-Q1. The length of the connecting trace is on the order of 7cm. In order to reduce ringing I want to implement a series resistor on the trace. What can I assume for the output impedance of the DS90LT012AQ-Q1?

Any guidance will be greatly appreciated.

Best wishes,

Rainer

over 5 years ago

0 Yaser Ibrahim over 5 years ago

TI__Genius 10250 points

Hello,

Unfortunately, I don't have a spec on this parameter. Please notice that the output impedance would vary a lot with operating conditions (load, temp, supply voltage, ...) and also due process variations. There is an IBIS model for this device which you can look at to get an idea www.ti.com/.../snlm044 . Depending on the frequency or data rate and with this short distance, you may not need to worry about it.

Regards,
Yaser

0 Rainer Baettig33 over 5 years ago in reply to Yaser Ibrahim

Prodigy 130 points

Hi Yaser,
Thank you for the fast and thoughtful reply. Unfortunately I am not set up for working with IBIS models.

I am concerned because the DS90LT012AQ-Q1 has a fast rise time of a few hundred picoseconds. I thus think that a connection of 7cm has to be treated as a transmission line, and thus some sort of matching is necessary. In your experience of fast interface products, what is the range of the resistance of a series resistors I should look at, say if the transmission line is about 50 Ohm? Best wishes, Rainer

0 Yaser Ibrahim over 5 years ago in reply to Rainer Baettig33

TI__Genius 10250 points

Hi Rainer,

True; the edge rates are fairly fast and there is potential for issues. From the IBIS model, the output impedance estimate with typical values is around 20 Ω (rough estimate; can vary significantly, as mentioned before!). So, you can try adding 20-30 Ω series resistance and adjust up or down if needed.

Regards,
Yaser

0 Rainer Baettig33 over 5 years ago in reply to Yaser Ibrahim

Prodigy 130 points

Hi Yaser,
Thank you very much. Great feedback, very helpful. I have marked the issue as resolved. Best wishes, Rainer

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DS90LT012AQ-Q1: Output impedance , output matching