日本語
Hello
I have a PCB that a part of one of the high speed lines is not placed over the ground plane. the length is 1300 mils
actually it's LSB line of a high speed DAC (200MHz, 3.3V logic, the input capacitance of the DAC is 5pF).
is there any solution to repair it?
as usual we have a 22 OHM resistor at the input of the line. can I replace 22 ohm with a 75 or 100 OHM resistor at the input of this line to reduce any ringing or overshoot? or can I scratch a part of this line and solder a 0402 resistor, for example here:
Thanks
- Comments(1)
A****min
May 13.2019, 14:43:14
I would measure the actual impedance with a TDR.
Then you can fix this to a fair degree by increasing the source termination resistor (your 22 Ohm) to match the line.
Make sure you subtract the output impedance of the driver chip.
Assuming you have sufficient setup time for the DAC, you should be OK. 1.3" is not all that long for a 200 MHz data feed.
To verify you would need a very low capacitance probe right at the DAC with minimal ground to tip separation and feed length, else the probe forms a stub and you are no longer measuring the circuit itself.
