[SOLVED] Clamp Diode on High Speed Lines

[cks3976]

Hi,

Can I use Diode termination (Diode clamping) on a high speed CMOS line ? Speed of this line will be 250MHz. What will be the impact on the signal speed & switching ?

Secondly are there any other impacts such as power dissipation by the Diode on the clamp source voltage and radiation ?

I have attached the clamping diagram for your convenience.

Looking forward for suggestions from you all..

Thanks in Advance,
CKS

 

[jiripolivka]

One question is if the length of your line is long enough to generate transients to be clamped. Another question is what are the parameters of your clamping diode.

Diode connected according to the schematic opens when the line voltage exceeds 2.5 V Vcc plus ~ 0.7 V of diode P/N junction. Diodes have also a P/N junction capacitance when not open; at 250 MHz, this capacitance ( 0.5...2 pF) on a long line may affect signal spectrum and pulse shape.
Any inhomogeneity on a "clean" line will cause reflections, radiation, etc. A correct device to prevent this is a matched resistor, with a resistance equal to line impedance.
For signal like 250 MHz, I would suggest to check "www.hittite.com", for line-terminating circuits; those seem to be better than clamping diodes.