FR4 base material resistance

Hi all,

I'm designing a PCB for a lowish-current application and I'm wondering how much I can rely on the FR4's isolation properties. This is a transimpedance amplifier with the summing junction at virtual ground, and I'm going to use guard traces. The input amp is an LT1793, DIP-8 package. I can tolerate a 10pA leakage current, which with a worst-case input offset voltage of 2mV translates to a maximum tolerable leakage resistance of 200 Megohms.

Will I have to use teflon standoffs, or will standard PCB do? Will a solder stopmask make a difference? The summing junction copper area will be small and will only connect five pads.

Thanks, robert

The teflon standoff is only for vibrationally sensitive parts. The isolation from not touching anywhere at all is equally good.

Rene

Yeah, that's how I usually do it without thinking much about it. But it's a pain production-wise, so I wondered if I can get away with normal PCB insulation. Looks like I can. I have already tried it out and it worked fine, but I didn't know if I was just lucky that one time.

Thanks, robert

FR4's pretty good. You'll find that performance related leakage currents will be determined by soldering residues if you don't clear these up.

You already know about guard traces - which is good.

I'm sure it'll be humidity sensitive at *very* low currents though. If you really need to get down that low you should have the pcb cleaned after assembly thoroughly ending up with a de-ionised water rinse and then apply a conformal coating.

Graham

Hello Graham,

That's exactly the point. After all, you don't want this circuit only to function properly in the Sahara desert at high noon.

Potting the whole thing up is another option.

Regards, Joerg

Lol ! I missed the part about baking the pcb after the de-ionised water rinse for brevity ! I expect the Sahara would do the equivalent just fine ! ;-)

If you like using chisels to fix it ! ;-)

Graham

just found this bit of peaseology

martin