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Altera_Forum
Honored Contributor I
788 Views

Max V: Voltage overshoot is too high ?

Hi 

Can someone explain to me why the voltage overshoot of Max V is so high? and how to improve this voltage overshoot ? 

Here is an example I captured from an oscilloscope. In this case the undershoot of rising edge and overshoot of falling edge are almost at the same voltage level. 

 

http://www.alteraforum.com/forum/attachment.php?attachmentid=12389&stc=1  

 

Thanks. 

Bien
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4 Replies
Altera_Forum
Honored Contributor I
30 Views

Hi, 

 

how are you probing this? The screenshot looks like a typical case of probe-induced ringing. If you have a second, equal probe, connect the second probe to the same point. The measured signal must not change, otherwise you know your probe load is too high, and you'll need a probe with less load (e.g. an active one). 

 

Then it could be ringing due to impedance mismatch. This should only be a problem if the signal propagates through a long trace. However, maybe some termination would help, even if it's not perfectly matched (if I decipher your tiny screenshot correctly, the signal has only 10 MBit/s). The simplest termination would be to add an e.g. 50 Ohm series resistor near the Tx end. 

 

 

Best regards, 

GooGooCluster
Altera_Forum
Honored Contributor I
30 Views

It looks the the ground wire on your oscilloscope probe is either disconnected, or too long.

Altera_Forum
Honored Contributor I
30 Views

Hi 

I made a simple 50 Ohm termination as you suggested and the voltage overshoot becomes much better. Thank alot GooGooCluster.
Altera_Forum
Honored Contributor I
30 Views

 

--- Quote Start ---  

It looks the the ground wire on your oscilloscope probe is either disconnected, or too long. 

--- Quote End ---  

 

I'll check again the ground wires. Thank gj_leeson.
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