LMV7219: LMV7219: output

Hello Lei,

Make sure you bypass the supplies like it was a high speed gate - because it is!

"Oscillations" with high speed comparators are usually attributed to poor layout and/or supply bypassing or coupling between the input and output.

Keep the leads short. Do not use wires on the GND and supply pins. The bypass cap (0.1uF ceramic) should be *directly* across the V+ and V- pins. See the section 10 layout guidelines in the datasheet.

Keep the input traces away from the output trace. There should be a shield or ground between them. The output trace should be constructed as a transmission line as the rise/fall times are in the nanoseconds.

Can you provide a schematic and scope photo? Maybe a photo of the setup?

pls ref the PCB and schematiic and Output waveform

Hello Lei,

Your circuit and layout look good. Traces are short with bypass caps close. I do not see any major issues. I cannot see the input and output trances and their relation to each other - make sure they are separated.

Try changing R25 and/or R26 to 10 to 20 ohms. Some high-speed devices can have their input devices oscillate at high frequencies if driven by low impedance or inductive sources.

You can also terminate the end of the output trace to dampen any ringing. Ideally, you would create a back-terminated transmission line by changing R28 to the transmission line impedance and terminate the end of the trace in the same impedance as R28.

Also, you also do not need R27, the LMV7219 is a push-pull output and does not need a pull-up resistor.

Hi Lei,

the LMV7219 is so fast, that it doesn't forgive even the least mistake. So, use a solid ground plane for AGND and introduce a
hysteresis as explained in the datasheet. Treat the LMV7219 as you would treat an ultra fast OPAmp and use the well known HF techniques.

Can you show the whole schematic and the whole layout (with all layers)?

Kai

Hi Lei,

I would introduce an additional hysteresis as shown in section 8.2 of datasheet. And you should provide a 50 Ohm input impedance in your circuit, provided your signal generator has 50 Ohm output impedance and is connected via a 50 Ohm cable.

Kai