SX inputs pulled below ground....?

ChrisP

I just assembled my spiffy new batch of miniboards, well the first one anyway. Looking at the board layout I realized that I made a horrible mistake. The optoisolator and resonator are both grounded to the incoming ground to the board while the vreg is grounded through a 1n4004 diode.. This would put the opto and resonator ground at -.6V...... Can I get away with this or is it time to cut traces and manualy move the ground lines to match the SX?

All other traces are fine, no other mixed grounds. Just the resonator and opto....

Silly question but I really dont know how it will effect the resonator or input lines...

Mike Green

Read the datasheet. For most CMOS ICs, there are protective diodes on the input pins and any voltage more than one diode drop below ground will be clamped by the diode. The diode will conduct current until it burns out or one of the leads burns out. -0.6V is approximately one diode drop below ground and may conduct enough current to damage the chip. I'm not quite sure how you have wired this all up and what the actual voltage could be on the I/O pin. You probably have to fix this.

ChrisP

Fair enough, I just pulled the ground diode on the vreg, the data sheet isnt clear to me anyway on how the SX would respond to that kind of treatment and these being prototypes of a new board I'll just sacrifice some filtering. The way I read the data sheet at best its edgey putting things at their max limits.

I asked because sometimes there is knowledge that isnt in data sheets, all things look good on paper, reality has a way of changing paper specs though as I'm sure you already know.